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Age-dependent modulation of parathyroid hormone action
Abstract
The effects of aging on the calcemic action of parathyroid hormone were evaluated. In the first and second studies, 1-, 8-, 12-, and 22-month-old male Wistar rats were parathyroidectomized (PTX) or thyroparathyroidectomized (TPTX), injected with parathyroid hormone and bled at 0, 3, 6, 9, and 12 h and plasma calcium determined. In the third study, 2-and 15-month-old TPTX rats were injected with parathyroid hormone and bled at 0, 1, 2, 5, 15, 30 and 120 minutes and plasma immunoreactive parathyroid hormone measured. Since the rats were prestarved, subsequent rise in plasma calcium is due primarily to the effect of parathyroid hormone on bone.
The greatest increase in plasma calcium in PTX rats occurred 12 h after the initial injection of parathyroid hormone and was (mean ± S.E.) 2.98±0.48, 1.90±0.24, 0.45±0.19 and 0.2±0.20 mg/100 ml for 1-, 8-, 12-and 22-month-old rats respectively. The peak levels of plasma immunoreactive parathyroid hormone in young and mature rats injected with the hormone were not statistically different. The kidneys of the oldest rats (22-month-old) showed no histological evidence of overt renal disease, and the plasma creatinine and urea of the rats did not increase progressively with age. In TPTX rats, the oldest animals were again much less responsive to the calcemic action of parathyroid hormone than the younger animals. However, the inhibitory effect of aging on the calcemic action of the hormone was more marked in PTX than in TPTX rats.
... Administration of a single subcutaneous dose of PTH to rats results in detectable plasma levels of immunoreactive PTH 15 min later, peaking at 30-60 min, before becoming undetectable 4 h after the injection (19,25). In our experiments, the mechanical stimulus was timed to coincide with peak PTH levels. ...
... It is unlikely that the augmentation of bone formation response was due to raised serum calcium levels, since bone formation in the nonloaded sixth caudal vertebra was unaffected by a single PTH administration. Moreover, PTH administration causes a transient hypocalcemia in rats (30), and the dose that effectively restores and enhances mechanical responsiveness (60 µg/kg) has been shown not to cause hypercalcemia in rats (13,15,19). ...
We investigated the relationship between parathyroid hormone (PTH) and mechanical stimulation in mechanically induced osteogenesis. In normal rats, mechanical stimulation of the eight caudal vertebra induced an osteogenic response. This was augmented by a single injection of human PTH-(1-34) 30-45 min before loading. No osteogenic response was seen in thyroparathyroidectomized (TPTX) rats; the osteogenic response was restored by a single injection of PTH before stimulation, suggesting that physiological levels of PTH are necessary for the mechanical responsiveness of bone. c-fos expression was detected only in the osteocytes of those rats that were both mechanically stimulated and given PTH. This suggests that PTH supports mechanically induced osteogenesis by sensitizing either the strain-sensing mechanism itself or early responses of bone to strain-generated signals. The osteogenic response was not augmented by two further daily injections of PTH and was not seen in TPTX rats in which PTH administration was started 3 days after loading. These results reveal a major role for PTH in the mechanical responsiveness of rat bone.
We investigated the relationship between parathyroid hormone (PTH) and mechanical stimulation in mechanically induced osteogenesis. In normal rats, mechanical stimulation of the eighth caudal vertebra induced an osteogenic response. This was augmented by a single injection of human PTH-(1-34) 30-45 min before loading. No osteogenic response was seen in thyroparathyroidectomized (TPTX) rats; the osteogenic response was restored by a single injection of PTH before stimulation, suggesting that physiological levels of PTH are necessary for the mechanical responsiveness of bone. c-fos expression was detected only in the osteocytes of those rats that were both mechanically stimulated and given PTH. This suggests that PTH supports mechanically induced osteogenesis by sensitizing either the strain-sensing mechanism itself or early responses of bone to strain-generated signals. The osteogenic response was not augmented by two further daily injections of PTH and was not seen in TPTX rats in which PTH administration was started 3 days after loading. These results reveal a major role for PTH in the mechanical responsiveness of rat bone.
Calcium plays a key role in many fundamental biologic processes and is also an essential structural component of the skeleton. These processes include neuromuscular excitability, membrane permeability, muscle contraction, enzyme activity, hormone release, blood coagulation. The precise control of calcium in extracellular fluids is vital to health. To maintain a constant concentration of calcium, despite marked variations in intake and excretion, endocrine control mechanisms have evolved that consist primarily of the interactions of three major hormones: parathyroid hormone (PTH), calcitonin (CT), and cholecalciferol (vitamin D). This chapter summarizes recent advances in the pathobiology of parathyroid glands and selected disease problems in animals associated with perturbation of parathyroid structure and function in animals.
To determine whether exposure to magnetic fields might have chronopharmacological effects on bone growth in rats, 150-g male animals were exposed to a 60-Hz magnetic field (average field strength, 10.2 Gauss) either during the day or during the night for 30 days. Serial tetracycline studies revealed a trend toward impaired periosteal growth/mineralization rates with both day and night exposures, but the data were not statistically different. Body weight gain, bone length, ratios of ash wt./dry fat-free wt., and metaphyseal trabecular volume were comparable In each of the two groups. This preliminary study suggested that a period of exposure longer than 30 days to a 60-Hz, 10.2 Gauss magnetic field might be required to produce significant skeletal effects.
Four- and 21-month old female Sprague-Dawley rats were sacrificed and their tibiae and femurs isolated for histology and initiation of bone marrow and trabecular bone cultures. The bone loss observed in 21-month old rats was associated with a markedly decreased osteoblastic index. The percentages of mineralizing trabecular surfaces were only slightly decreased in aged rats, whereas the percentages of mineralizing endocortical surfaces were strikingly decreased. Diaphyseal femoral marrows from 21-month old rats were less cellular than those from four-month old rats, and developed in culture fewer fibroblast colony forming units (FCFU) and fewer adherent cells with phenotypic characteristics of osteoblast-like cells. Trabecular bone cultures from 21-month old rats produced as many cells as cultures from four-month old rats, whereas the amount of trabeculae put into culture was much less in aged rats. Moreover, the proliferation rate in secondary culture was significantly increased in 21-month old rats. Similar responses to calcitriol were observed in bone marrow and trabecular bone cells from aged and younger mature rats, while cAMP responses to PTH were decreased in cells from aged rats. Our data confirm the age-related decrease in the FCFU efficiency of the bone marrow and show a stimulated proliferation of trabecular bone cultures from 21-month old rats that could be seen either as the result of the inhibition in vivo of the response to a signal to proliferate, or as a rebound response to factors present in the serum-enriched medium and lacking in vivo.
The effect of age on the biomechanical properties of healing tibial fractures was studied by comparing the fracture healing in 2-year-old male Wistar rats with the fracture healing in 3-month-old male Wistar rats after 40 and 80 days of healing. There were no significant differences in the mechanical parameters after 40 days of healing, but after 80 days, a considerable delay in the fracture healing process was noted in the old rats compared with the young adult rats when evaluated by maximum load, maximum stress, stiffness, and energy absorption in a three-point bending procedure. In the contralateral, nonfractured bones, the tibiae from the old animals sustained higher loads and had higher stiffness than the bones from the young adult animals, but stress values, elastic modulus, and capacity for energy absorption was much lower in the old animals.
The purpose of these studies was to determine whether the responsiveness of the kidney to parathyroid hormone (PTH) and calcitonin changed with age. Experiments were performed in young (3 months old), adult (12–14 months old) and old (22–24 months old) male Fischer 344 rats fed normal diets and thyroparathyroidectomized. Parathyroid hormone was administered i.p. at 24, 12 and 2 h before death and calcitonin was given i.p. at 12 and 2 h before death. Parathyroid hormone significantly increased the conversion of 25-hydroxyvitamin D 3 to 1,25-dihydroxyvitamin D 3 (1,25-(OH) 2 D 3 ) by renal slices from young but not adult or old animals. A similar age-related decline in the capacity of PTH to raise serum 1,25-dihydroxyvitamin D (1,25-(OH) 2 D) levels was also seen. Parathyroid hormone significantly decreased tubular reabsorption of phosphorus, increased concentrations of urinary cyclic AMP (cAMP) and increased serum concentrations of calcium in all age groups. In contrast, calcitonin significantly increased 1,25-(OH) 2 D 3 production by renal slices from both young and adult animals. Calcitonin decreased serum concentrations of calcium in young but not in adult rats. These results suggest that there are maturational changes in the PTH- and cAMP-dependent pathways in the kidney but not in the calcitonin- and cAMP-independent pathways. The changes in the PTH- and cAMP-dependent pathways affect the stimulation of 1,25-(OH) 2 D production but not the inhibition of phosphate transport.
J. Endocr. (1987) 114, 173–178
The bone status of male rats 6, 12, and 24 months of age (n = 10) was examined. Femur calcium (Ca), phosphorus (P), and osteocalcin contents; serum chemistry; and mechanical properties of the bone were measured and correlated. Diaphyseal Ca, P, and osteocalcin contents were not different in animals 6 and 12 months of age but decreased significantly at 24 months: -7.4% for Ca, -4.2% for P, and -24% for osteocalcin compared to 12 months. Femurs from 24-month-old (senescent) rats were characterized by a scalloped appearance of the midfemoral endosteal surface and by cortical porosities. These age-associated changes coincided with nearly two-fold increases in serum immunoreactive parathyroid hormone (PTH) and osteocalcin. Serum Ca did not change with age, whereas serum P decreased (-14.8%) from 6 to 24 months. Maximum breaking force required to fracture femurs at midshaft did not change with age. Hence, the strength of the femur as an intact organ was not compromised with age despite the loss of diaphyseal Ca and P in the senescent animal. However, ultimate stress, a parameter that normalizes for differences in bone geometry and size, decreased 35% in femurs from 12- compared with 24-month-old animals. These mechanical results might be explained by the morphometric finding that, in contrast to the small but progressive age-associated increases in femur weight and length, the cortical and medullary areas increased at least two-fold. Therefore, the strength of the intact femur was maintained by architectural compensations, even though normalized tissue strength decreased with age. These findings suggest that bone status was compromised in the aged male rat.
In order to differentiate the relative effects of age and estrogen on rates of calcium absorption and serum levels of parathyroid hormone and calcitonin, the effect of oophorectomy with and without estrogen replacement (2 weeks) was studied in rats for 30- or 120-day periods. Whereas oophorectomy for 30 days resulted in a significant decrease in serum calcium and an increase in serum phosphate, no change in either calcium or phosphate was observed in the 120-day oophorectomized animals. iPTH decreased in both the 30- and 120-day oophorectomized animals although these changes were not significant at the .05 level. Whereas no significant change in basal circulating calcitonin occurred in the 30-day oophorectomized rats, it decreased significantly in the older animals following the ablative procedure. Forty-five days following estrogen deprivation, calcitonin release to a calcium secretagogue was significantly blunted. Intestinal calcium absorption decreased with age, and unlike the increments in calcium absorption observed in the younger estrogen-repleted, 30-day oophorectomized rat, no change in calcium absorption was observed when estrogens were administered to the older, 120-day oophorectomized rat. The accumulated data suggest that the effects of estrogen loss on the hormonal control of bone metabolism and calcium absorption are age dependent, and that estrogen contributes significantly to changes in calcium homeostasis observed in the maturing rat.
Serum immunoreactive parathyroid hormone (iPTH) and calcitonin (iCT) levels are higher in young than aged rats. However, serum calcium concentration does not change with age suggesting that the calcium regulation of PTH and CT secretion may be affected by aging. We compared iPTH and iCT secretion in vitro at low and high calcium concentrations using thyroparathyroid glands removed from young (2-3 months), adult (12-13 months), and old (24-27 months) F-344 male rats fed regular rat chow. Glands from each animal were incubated for 3 h in serum-free culture media containing 1.0 mM calcium and then transferred to media containing 2.5 mM calcium for another 3 h. Immunoreactive PTH and iCT concentrations of the media after each incubation period were determined by radioimmunoassay. Immunoreactive PTH and iCT secretion per pair of glands was significantly higher in glands from older animals regardless of calcium concentration. The decrease in iPTH, and increment in iCT, secretion in response to 2.5 mM calcium by glands from old rats was smaller than that observed for glands from young animals. These age-related changes in the regulation of secretion by calcium may contribute to the increased iPTH and iCT secretion and serum levels seen in older animals.
Young, growing rats which had been chronically (2 weeks) adrenalectomized or parathyroidectomized were used to define the roles of the adrenal and parathyroid glands on the maintenance of normal circadian rhythms of DNA, collagen and non-collagen protein synthesis in the skeleton. The animals were conditioned to food being available ad libitum and to 12 h light: 12 h darkness (lights on from 08.00 to 20.00 h). The pace of DNA, collagen and non-collagen protein synthesis in different regions of the tibia (tibial growth cartilage, metaphysial bone and diaphysial bone) was measured by the in-vivo incorporation of tritiated thymidine (1 h) and radioactive proline (48 h). In intact rats there were no regional differences in the phasing of the circadian profiles; peak DNA and non-collagen protein synthesis occurred at the onset of the dark period while peak collagen synthesis occurred during the middle of the period of light. Adrenalectomy selectively abolished the regional DNA synthesis rhythms without altering the phases of the serum Ca and phosphorus (P) rhythms, which peak at mid-day and at the onset of darkness respectively. Parathyroidectomy abolished the regional rhythms for collagen and non-collagen protein synthesis and serum Ca rhythms, without altering the phase of the serum P and corticosterone rhythms. Dietary Ca-lactate supplements, which raised serum Ca levels towards normal in parathyroidectomized rats, were able to correct serum corticosterone values but did not normalize bone collagen and non-collagen protein synthesis values. These data indicate that the adrenal rhythm governs the proliferative activities of bone and cartilage cells, and that parathyroid hormone is essential to maintain normal collagen and non-collagen protein synthesis rhythms.
J. Endocr. (1984) 103 , 49–57
Parathyroid hormone (PTH) dependent cyclic AMP (cAMP) accumulation was evaluated in renal cortex from 2- and 12-month-old rats. Basal cAMP was lower, and responses to PTH were greater at all concentrations of hormone in kidney from 2-month-old rats. This difference was obliterated by prior removal of parathyroid glands. cAMP responses to calcitonin and both basal and hormone-stimulated adenylate cyclase activity were the same at both ages. The results suggest progressive loss of responsiveness to PTH with age, but at a site other than the receptor-adenylate cyclase complex. Blunted cAMP accumulation in year-old rats most likely reflects agonist-specific desensitization.
Morphometric parameters of bone formation are markedly depressed in senescent, 21-month old rats and even in middle-aged, 12-month-old animals when compared with mature, 4-month old adults. However, osteoblast-like cells obtained from the metaphyseal trabeculae of the distal femur of 21-month-old female and male rats proliferate more rapidly in primary and secondary cultures than cells from 4-month-old donors. In females the increase in proliferation is significant for donor ages from 4 to 12 months and from 12 to 21 months. Ex vivo cell proliferation is inversely correlated with trabecular bone volume and bone surface in females and with bone surface in males. The relationships are being maintained in females (not tested in males) when cells are grown in serum-free medium. We interpret age and bone loss-dependent stimulated cell proliferation as the in vitro response to an in vivo signal to proliferate resulting from higher strains on less trabeculae. The absence of response in vivo could result from the local deficiency of factors brought back to the cells by the serum-enriched culture medium, or from proliferation inhibitors developing with age.
Parathyroid hormone (PTH) has multiple effects on water and electrolyte transport along the nephron. However, the influences of PTH and calcium on the urinary concentration ability are not fully understood. In this study, clearance and microperfusion studies were performed in thyroparathyroidectomized (TPTX) rats either supplemented (TPTX+Ca2+) or not with calcium added to the ingested food as CaCl2 (1.6 g/100 g). Acid-base data and renal functional parameters were measured in TPTX and TPTX+Ca2+ rats. Additional studies were performed in the isolated inner medullary collecting tubules of intact and TPTX rats to evaluate the osmotic permeability of this segment in the presence of 10–6M PTH added to the bath. In these experiments the possible influence of PTH on antidiuretic hormone induced changes of the osmotic permeability in TPTX and TPTX+Ca2+ rats was also investigated. In the TPTX+Ca+ group, the glomerular filtration rate increased significantly when compared to the TPTX group (6.04 ± 0.42 vs. 4.88 ± 0.20 ml·min–1·kg–1; p –1·kg–1) significantly decreased the urinary flow and increased the renal plasma flow, results that agree with the vasomodulator action of this hormone on the renal vasculature. A significant increase in the fractional K+ excretion observed in the TPTX+Ca2+ group as compared with both control and TPTX, groups suggests that the excreted load of Ca2+ may interfere with tubular K+ handling in the absence of PTH. PTH (10–6M) added to the bath of the isolated inner medullary collecting tubules did not change the osmotic permeability, of intact, TPTX, and TPTX+Ca2+ rats. Furthermore, it did not modify the antidiuretic hormone induced changes in the osmotic permeability. These results suggest that this segment of the nephron is PTH insensitive as far as water and ion transport are concerned.
Intestinal calcium absorption assessed by a double-isotope method, decreased significantly with aging in 94 normal subjects (r = -0.22, P < 0.025). In 52 untreated patients with postmenopausal osteoporosis, calcium absorption was significantly lower than normal when either age or habitual calcium intake was used as a covariable (P < 0.001). Serum 25-hydroxyvitamin D (25-OH-D) and 1,25-dihydroxyvitamin D (1,25(OH)(2)D) were measured in 44 normal subjects and 27 osteoporotic patients. For all normals, calcium absorption and serum 1,25(OH)(2)D were positively correlated (r = 0.50, P < 0.001). In nonelderly normal subjects (ages 30-65 yr), dietary calcium intake correlated inversely with both calcium absorption (r = -0.39, P < 0.01) and with serum 1,25(OH)(2)D (r = -0.50, P < 0.01). Both osteoporotic patients and elderly normal subjects (ages 65-90 yr) differed from nonelderly normals in that these correlations were not present. In addition although serum 25-OH-D was normal, serum 1,25(OH)(2)D was significantly decreased in both osteoporotic patients and elderly normals (P < 0.001). In osteoporotic patients, calcium absorption increased significantly (P < 0.001) after 7 d administration of a small dose (0.4 mug/d) of synthetic 1,25(OH)(2)D(3). In osteoporotics mean serum immunoreactive parathyroid hormone was either normal (COOH-terminal assay) or low (NH(2)-terminal assay) relative to age-matched controls, and mean serum phosphate was increased. The data suggest that inadequate metabolism of 25-OH-D to 1,25(OH)(2)D contributes significantly to decreased calcium absorption and adaptation in both osteoporotics and elderly normal subjects. In patients with osteoporosis this abnormality could result from a decrease in factors that normally stimulate 1,25(OH)(2)D production, such as the decreased parathyroid hormone secretion and increased serum phosphate demonstrated in this group. In elderly subjects a primary abnormality in metabolism of 25-OH-D to 1,25(OH)(2)D, analagous to that seen in aging rats, cannot be excluded.
1. Tetany occurs spontaneously in many forms and may also be produced by the destruction of the parathyroid glands. Recent researches tend to demonstrate an intimate relation between the various forms of tetany and relative or absolute insufficiency of the parathyroid gland. 2. The parathyroid glands are independent organs with definite specific function. Whether or not this function is intimately related to that of other organs of internal secretion is not as yet proven. 3. The number and distribution of the parathyroid glands varies. Failure to produce tetany experimentally is probably due to the fact that some parathyroid tissue remains after an apparently complete extirpation. When extirpation is complete tetany appears, even in herbivora. Only a very small amount of parathyroid tissue is required to prevent this. 4. The effect of the extirpation of the parathyroid glands may be annulled by the reintroduction of an extract of these glands even from an animal of widely different character. The active principle is associated with a nucleo-proteid in the extract and may be separated with this nucleo-proteid from the remaining inert albuminous substances. Its effect in counteracting tetany appears some hours after injection and lasts several days. 5. The parathyroid glands contain no considerable amount of iodine. The parathyroid extract is not an iodine containing compound. 6. In tetany there is apparently some disturbance of the composition of the circulating fluids ordinarily prevented by the secretion of the parathyroid, which disarranges the balance of the mineral constituents of the tissues. Possibly this consists in the appearance of an injurious substance of an acid nature for such tetany may he relieved by extensive bleeding with replacement of the blood by salt solution. No actual poisonous material has, however, been demonstrated by the transference of the blood of a tetanic animal to the veins of a normal one. 7. Numerous researches have shown the important relation of the calcium salts to the excitability of the central nervous system. Their withdrawal leaves the nerve cells in a state of hyperexcitability which can be made to disappear by supplying them with a solution of a calcium salt. 8. Tetany may be regarded as an expression of hyperexcitability of the nerve cells from some such cause. 9. The injection of a solution of a salt of calcium into the circulation of an animal in tetany promptly checks all the symptoms and restores the animal to an apparently normal condition. 10. Injections of magnesium salts probably have a similar effect but these effects are masked by the toxic action of the salt. 11. The injection of sodium or potassium salts has no such beneficial effect but rather tends to intensify the symptoms. This is true also of the alkaline salts of sodium which were studied especially in respect to their basic properties. 12. The effect of calcium is of value in human therapeutics in combating the symptoms of spontaneous forms of tetany and in relieving the symptoms in cases of operative tetany and thus tiding over the period of acute parathyroid insufficiency until remnants of parathyroid tissue can recover their function or new parathyroid tissue can be transplanted. It is in this way an important and convenient ally of the method of injecting parathyroid extract. 13. Studies of the metabolism in parathyroidectomized animals show: 1. A marked reduction in the calcium content of the tissues especially of the blood and brain, during tetany. 2. An increased output of calcium in the urine and faeces on the development of tetany. 3. An increased output of nitrogen in the urine. 4. An increased output of ammonia in the urine with 4a. an increased ammonia ratio in the urine. 5. An increased amount of ammonia in the blood. Much of this affords evidence of the existence of some type of acid intoxication. Its effects are, however, not neutralized by the introduction of alkaline sodium salts and may perhaps be regarded as especially important in producing a drainage of calcium salts from the tissues which can be remedied by the reintroduction of calcium salts. 14. Emphasis must be laid upon the remarkable difference which exists between the alterations inmetabolism following thyroidectomy and those following parathyroidectomy. In myxoedema there is lowered metabolism, decreased respiratory changes and lowered nitrogen output with depression of body temperature. In tetany there is increased metabolism, probably increased respiratory changes, certainly increase in nitrogen output and elevation of the temperature. 15. It is important, therefore, that in any experiments upon metabolism in relation to the thyroid and parathyroid gland, these glands should be clearly distinguished as structures exercising very different and in large part contrary effects upon metabolism. 16. In general the role of the calcium salts in connection with tetany may be conceived of as follows: These salts have a moderating influence upon the nerve cells. The parathyroid secretion in some way controls the calcium exchange in the body. It may possibly be that in the absence of the parathyroid secretion, substances arise which can combine with calcium, abstract it from the tissues and cause its excretion and that the parathyroid secretion prevents the appearance of such bodies. The mechanism of the parathyroid action is not determined, but the result, the impoverishment of the tissues with respect to calcium and the consequent development of hyperexcitability of the nerve cells, and tetany is proven. Only the restoration of calcium to the tissues can prevent this. 17. This explanation is readily applicable to spontaneous forms of tetany in which there is a drain of calcium for physiological purposes, or in which some other condition causes a drain of calcium. In such cases the parathyroid glands may be relatively insufficient.
Pathology, microbiology, and selected serum chemistries were evaluated in 144 male Fischer rats from 4 to 33 mo of age. The
rats were reared and maintained under barrier conditions, which successfully excluded the introduction of major infectious
disease agents throughout the entire study, including Mycoplasma pulmonis. A wide variety of pathology was found and tabulated, and many lesions were found to increase in severity and incidence with
age. There was a high correlation of renal disease severity with increasing age. Serum total protein and albumin decreased
with age, while alpha-1 globulin and cholesterol increased.
Kidney disease was studied patho-histologically, electron microscopically and immunologically in Wistar rats ranging in age from 2 weeks to 24 months. Glomerular lesions characterized by adhesion of podocyte foot processes were first detected at 3 months of age. The kidney lesions became more pronounced with age, as manifested by an increase of mesangial matrix, basement membrane thickening, crescent formation and hyalinization of glomeruli, and tubular degeneration. Evidence for the deposition of immune complexes in the kidney was obtained by immunofluorescence and electron microscopy, as well as by immuno-electrophoretic analysis of kidney elutions. Tests with antiserum reagent against Moloney leukemia virus antigen revealed its presence in some but not all glomeruli. The presence of other viruses in Moloney leukemia virus negative glomeruli was not ruled out. Serum autoantibody against an array of rat tissues could not be detected. Therefore, it would appear that autoimmune mechanism may not be the primary underlying cause of pathogenesis of the disease. Accordingly, the disease could be referred to as chronic immune complex glomerulonephropathy with nephrotic syndrome, but sources of the antigens in the complex were mostly unknown, althougn virus could be one portion of them. The possibility that diet antigens may also be present in the complex seems unlikely because attempts to demonstrate serum antibodies against diet pellet in old rats were unsuccessful.
The chapter illustrates recent developments that demonstrate quite conclusively that vitamin D is the precursor of a hormone of central importance in the control of serum calcium and phosphorus, which are in turn necessary for mineralization of bone, muscle contraction, and nerve function. In the absence of vitamin D, mineralization of bone collagen cannot occur thus leading to the disease rickets in the young and osteomalacia in the adult. Deprivation of vitamin D brings about the acute disease state of hypocalcemic tetany. Although parathyroid hormone is involved in the etiology of hypocalcemic tetany, vitamin D also plays a basic role in preventing this disturbance and that there is an interlocking function between the parathyroid hormone and vitamin D in this capacity. Because vitamin D is the precursor of a hormone that functions in the mobilization of calcium and phosphorus and because other hormonal factors interact with vitamin D in these functions, it will become evident that a disturbance of vitamin D conversion to its active hormone may result in a variety of pathological situations, which are the well recognized symptoms of rickets and osteomalacia. Vitamin D is a building block for one hormone, which has its endocrine system located in the kidney and the targets of hormone action in intestine, bone, and kidney. From the clinical point of view, methods for assessment of vitamin D metabolism and hormonal status are under development that will usher into the clinical chemistry area a new era of bone disease in treatment, diagnosis, and evaluation of treatment.
In this review evidence is presented, largely from the authors' laboratory, that indicates that the adaptive mechanisms in the rat involve elevated blood levels of calcitonin and 1,25-dihydroxycholecalciferol [1,25-(OH)2D3]. There is indirect evidence that parathyroid hormone also is increased. Briefly, the serum calcium level is maintained at a level 10% lower than that of nonlactating rats, in part by an action of calcitonin. Parathyroid hormone and 1,25-(OH)2D3, in concert, prevent further reduction in the serum calcium level. There is marked elevation of intestinal calcium absorption, which may involve 1,25-(OH)2D3, as well as other factors. Continued intake of vitamin D during lactation is required not only to prevent marked hypocalcemia in the mother, but also to prevent true vitamin D-deficiency rickets in the suckling pups. Calcium metabolism in lactating women is also reviewed and contrasted with that in the rat. Lactation in ruminants is discussed only briefly, mainly for purposes of species comparison. Several reviews contain a detailed discussion of special problems in ruminant lactation, such as parturient paresis (Simkiss, 1967; Mayer et al., 1969b; Braithwaite, 1976). Hormones other than the calcemic hormones, such as glucocorticoids, are not discussed further, because there is no information on their role in calcium metabolism in lactation. However, it is quite possible that future studies may uncover such a role because excess glucocorticoids may lead to alteration in bone and calcium metabolism (Nelson, 1979) and because serum levels of glucocorticoids are elevated in lactating rats (Kamoun and Haberey, 1969; Ota et al., 1974: Smotherman et al., 1976) and cows (Wagner and Oxenreider, 1972; Shayanfar et al., 1975).
Using a sensitive calcitonin (CT) immunoassay and a newly developed bioassay capable of detecting 0.025 MRC mU CT, we have studied acute and chronic plasma CT fluctuations in male and female rats. Immunoassay of serial plasma samples revealed progressive increases in plasma CT concentrations during aging; female rats have higher CT concentrations than age-matched males. Acute periodic CT fluctuations were discovered by immunoassay of plasmas obtained at 3-h intervals; the greatest values occurred just before and during feeding. Fed rats have higher CT than starved rats. We have used immunoadsorbent chromatography to concentrate specifically CT moieties from large volumes of plasma for concurrent immunoassay and bioassay measurements of circulating CT. These concurrent measurements of immunoextracted plasma CT demonstrate that for normal rats, our immunoassay measurements correspond to bioassay measurements. In 1-yr-old rats on a regulated feeding schedule, the biological (hypocalcemic) activity of CT recovered from 15 ml peripheral plasma ranged from less than 0.15 mU in starved males to 0.78 mU in feeding females. We conclude that biologically active CT circulates in normal rats and that the blood concentration of biologically active CT progressively increases during somatosexual maturation, being highest in old females, and increases acutely just before and during feeding.
Studies were carried out to investigate the role of 1,25(OH)2D3 in the skeletal resistance to the calcemic action of parathyroid hormone. The change in serum calcium after the intravenous infusion of 2 U of parathyroid extract (PTE)/kg body wt/hr for eight hours was evaluated in thyroparathyroidectomized (T-PTX) dogs before, and one, two and three days after, induction of uremia by bilateral ureteral ligation (11 dogs) or by bilateral nephrectomy (8 dogs). In another six nephrectomized and T-PTX dogs, 0.68 ug of 1, 25 (OH)2D3/day was given on the day of nephrectomy and for two days thereafter. Serum creatinine in each day of the study was not different among the three groups. The study also included the evaluation of the effect of sham operation (five dogs) and the administration of 1,25 (OH)2D3 to dogs with normal renal function (four dogs) on the calcemic response to PTE, as well as the reproducibility of such a response in the same animal. The results showed that 1) the calcemic response to PTE was markedly impaired after one day of bilateral ureteral ligation or nephrectomy, but the impairment was more severe after nephrectomy; 2) the calcemic response to PTE after two or three days of bilateral ureteral ligation was similar to that seen at one day after nephrectomy; 3) 1, 25 (OH)2D3 partially restored the calcemic response to PTE in the nephrectomized animals to levels similar to those seen after one day of bilateral ureteral ligation; 4) sham operation did not affect the response to PTE, and repeated infusion of PTE produced similar changes in the concentrations of serum calcium. The data indicate that (a) a deficiency of 1,25 (OH)2D3 is at least partly responsible for the skeletal resistance to the calcemic action of PTH in uremia; (b) uremia, per se, may also contribute to this phenomenon; and (c) the kidney after one day of complete bilateral ureteral ligation may still produce 1,25 (OH)2D3, but this ability is compromised after two days of ureteral obstruction.
To determine the influence of aging on the relative roles of parathyroid hormone (PTH) and calcitonin in the control of calcium homeostasis during fasting, we assessed changes in plasma calcium in fasted rats following simultaneous removal of the glands that secrete both hormones. Animals ranging in age from 3 to 34.7 wk were thyroparathyroidectomized or sham operated and bled by orbital puncture prior to and 1.5, 3, and 6 h after surgery. After thyroparathyroidectomy (TPTX), plasma calcium fell immediately and progressively in very young rats (3-6 wk old); in young animals (6.5-9.4 wk old), there was a delay of about 1.5 h pceded by a rise, which persisted for at least 3 h. Since the rise and fall in plasma calcium after TPTX are most likely due to calcitonin and PTH deficiencies, respectively, our observations are consistent with the following hypothesis; in the fasting state, the relative importance of calcitonin and PTH in the regulation of plasma calcium varies with age; PTH appears to play the dominant role in young rats; however, during maturation the importance of calcitonin appears to increase progressively.
Investigations were carried out in rats to determine the relative roles of kidney and bone in the regulation of plasma calcium by parathyroid hormone (PTH). The effects of endogenous PTH were studied by parathyroid ablation. Parathyroid glands were removed by thyroparathyroidectomy (TPTX) since parathyroidectomy appears to release calcitonin and thyroidectomy itself does not alter plasma calcium levels. Changes in plasma calcium, urinary calcium and urinary hydroxyproline were determined 3 hr or 24 hr after TPTX in rats of varying ages and in animals fed a calcium-deficient diet for 30 days to enhance PTH secretion and bone turnover. The following observations were made. 1) Plasma calcium but not hydroxyproline decreased 3 hr after TPTX. 2) The degree of hypocalcemia was inversely related to age. 3) By 24 hr, urinary hydroxyproline had decreased and plasma calcium fallen further. 4) Plasma calcium and urinary hydroxyproline decreased immediately in immature rats and rats fed a calcium-deficient diet. 5) TPTX acutely enhanced urinary calcium excretion. 6) The increase in urine calcium accounted for 8, 12, 45 and 67% of the fall in plasma calcium in 45, 134, 221 and 241 g rats, respectively. 7) In animals fed a low-calcium diet only 4% of the fall in plasma calcium was due to increased calciuria. From these findings we conclude that 1) PTH maintains plasma calcium by enhancing calcium transport from bone, resorption of calcified bone matrix and kidney reabsorption of calcium, 2) PTH-dependent bone resorption does not play an appreciable role in the acute control of plasma calcium except when the rate of bone turnover is high, 3) bone-mediated minute-to-minute regulation of plasma calcium by the hormone may be due instead to calcium transport from bone without osteolysis, and 4) PTHdependent contribution of the kidney to the maintenance of plasma calcium probably increases and surpasses that of bone with age.
A competitive protein binding assay with a sensitivity of 80 picograms has been developed for 1alpha,25-dihydroxyvitamin D(3), the hormonal form of vitamin D(3). lalpha,25-Dihydroxyvitamin D(3) displaced tritiated hormone from a cytosol-chromatin receptor preparation isolated from chick small intestine, providing a simple assay for the hormone. The concentration of lalpha, 25-dihydroxyvitamin D(3) in human plasma, as determined by this assay, is approximately 6 nanograms per 100 milliliters; in patients with renal disease the concentration of this kidney-produced hormone is significantly lower.
This review considers the problem of osteoporosis in the context of present knowledge (and its limitations) of the mechanisms of bone resorption and bone formation. Two particular points are emphasized: (1) osteoporosis is a disorder characterized by an imbalance between formation and resorption in favor of the latter; and (2) both bone formation and resorption are regulated by the local ionic environment as well as the activities of several hormones. A rational therapy for this condition will undoubtedly have to take account of both processes and of means of enhancing the one (formation) and suppressing the other (resorption). In the latter context, thyrocalcitonin has already been shown to have a profound effect in experimental animals, but one disturbing feature of its action is the very striking decrease in its effectiveness in older animals. Whether or not this will be a major limitation to its use in adult human subjects remains to be established.The final portion of this review considers the recent evidence that thyrocalcitonin and parathyroid hormone control metabolism by regulating the fluxes of calcium and magnesium across cellular and subcellular membranes as well as the activities of enzymes whose functions are controlled by these cations. Specific attention is directed to the effects of these hormones upon the activity of pyrophosphatase, an enzyme specifically implicated in bone metabolism.
Added thyrocalcitonin greatly diminished parathyroid-hormone-induced resorption of bone in tissue culture. The results indicate
that bone is primary site of action of thyrocalcitonin.
The role of the thyroid and parathyroid glands in counteracting the hypercalcemic action of exogenous parathyroid hormone was examined. Acutely parathyroidectomized, thyroparathyroidectomized, and sham-operated rats were injected either with commercial parathyroid extract or with partially purified parathyroid hormone preparation. The rats were bled 5 or 6 hr later. The serum calcium was slightly higher in the parathyroidectomized rats injected with the parathyroid hormone preparations than in the sham-operated rats injected with control solution, but never higher than in the shamoperated rats treated with hormone. In thyroparathyroidectomized rats injected with parathyroid hormone preparations the serum calcium rose sharply (3-5 mg of calcium/100 ml of serum) and the increase was significantly higher than in the other hormone-treated groups. These results provide additional evidence that the thyroid gland is important in the prevention of experimental hypercalcemia in rats, presumably by secreting thyrocalcitonin, and provide no evidence for the secretion of calcitonin or a thyrocalcitonin- releasing factor from the parathyroid glands.(Endocrinology 79: 655, 1996)
The role of the thyroid and parathyroid glands in counteracting the hypercalcemic action of exogenous parathyroid hormone was examined. Acutely parathyroidectomized, thyroparathyroidectomized, and sham-operated rats were injected either with commercial parathyroid extract or with partially purified parathyroid hormone preparation. The rats were bled 5 or 6 hr later. The serum calcium was slightly higher in the parathyroidectomized rats injected with the parathyroid hormone preparations than in the sham-operated rats injected with control solution, but never higher than in the shamoperated rats treated with hormone. In thyroparathyroidectomized rats injected with parathyroid hormone preparations the serum calcium rose sharply (3-5 mg of calcium/100 ml of serum) and the increase was significantly higher than in the other hormone-treated groups. These results provide additional evidence that the thyroid gland is important in the prevention of experimental hypercalcemia in rats, presumably by secreting thyrocalcitonin, and provide no evidence for the secretion of calcitonin or a thyrocalcitonin- releasing factor from the parathyroid glands.(Endocrinology 79: 655, 1996)
We have employed region-specific RIAs directed against amino-terminal (N), midregion (M), or carboxyl-terminal (C) regions of parathyroid hormone (PTH) to analyze glandular and circulating PTH species in normal subjects and in persons with primary hyperparathyroidism. Immunoaffinity methods have been developed for validation of RIA measurements and for extraction of circulating PTH spcies. Chromatographic analyses of normal and adenomatous parathyroid-gland extracts and electrophoretic analyses of parathyroid-gland immunoextracts revealed at least four PTH species, each with a distinct pattern of cross-reactivity in the sequence-specific RIAs. Intact PTH (mol wt, 9500) was detected by each type of RIA. The large carboxyl-fragment (mol wt, 5500), was detected only by M and C RIAs. A somewhat smaller fragment (mol wt, 4000) was identified that was detected by M RIA but not by C RIA. We also detected in parathyroid extracts a small N-fragment (mol wt, 3400), which had no reactivity in M or C RIAs. In normal and hyperparathyroid plasmas the predominant PTH species is a large C-fragment (mol wt, 5500) with cross-reactivity in both M and C RIAs. N RIA results indicated relatively low levels of intact PTH or small N-fragments in immunoextracts of peripheral plasmas. However, plasmas from persons with primary hyperparathyroidism did contain an additional prominent PTH species (mol wt, 4000) that was similar to M-fragment(s) found in parathyroid-gland extracts. Since small M-fragment(s) cross-reacts only with the M-antisera, we were able to develop, for isolation and measurement of the fragment(s), a sequential immunoextraction procedure that does not use chromatography. Using this procedure we found that normal basal sera have no detectable small M-fragment(s). However, circulating M-fragment(s) is detectable in normal persons during EDTA-induced hypocalcemial; the relative increase in M-fragment(s) is three times greater than that observed for total anti-C immunoreactivity, which includes large C-fragment(s) and intact PTH. The accumulation of circulating small M-fragment(s) in hypercalcemic hyperparathyroid persons and in normal persons during EDTA-induced hypocalcemia suggest that it may somehow indicate the level of parathyroid secretory activity. Since a small M-species in parathyroid tissues has size and immunochemical characteristics similar to circulating M-fragment(s), this species might be secreted by parathyroid tissue.
To evaluate the effects of age and sex on plasma calcitonin in human beings, the authors measured plasma levels of the hormone by radioimmunoassay in 137 normal subjects of both sexes. They found that irrespective of age, the increase in calcitonin in response to calcium infusion was greater in men than in women, but that basal levels of calcitonin and the response to calcium waned with age in both sexes. The study suggests that progressive calcitonin deficiency possibly contributes to the increased bone loss that occurs with age in both sexes, especially women. However, extensive evaluation of age-matched and sex-matched subjects and patients will be necessary to study the role of calcitonin secretion, along with other relevant factors, in the pathogenesis of age-related and sex-related bone diseases in human beings.
Serum calcitonin (CT) was measured in male and female Fischer fasted and fed rats from 1-24 months of age. Previous findings by ourselves and others that serum CT rises with advancing age and is higher in females than in males were confirmed and extended. The effects of feeding were examined with the use of three different protocols, overnight fasting then feeding for 1 h, feeding ad libitum, and feeding rats for 2 h after they had been on a fixed feeding schedule for 4 weeks. At 1 month of age serum CT was too low to measure in either sex, fed or fasted. From 2-24 months of age, feeding increased serum immunoreactive CT much more in females than in males. The increase in serum CT was accompanied by a slight decrease in serum calcium. We concluded that the release of CT in rats after eating is sex related.
This study has dealt with progressive changes which occur in renal phosphate and calcium excretion rates in rats during the first 27 hours after parathyroidectomy or the first injection of parathyroid extract. Following parathyroidectomy there is an immediate drop in urinary phosphate and a rise in urinary calcium. Within 24 hours the excretory rates for both these ions return to normal despite the continued high phosphate and low calcium levels of the serum. Following the initiation of parathyroid extract administration to normal rats the reverse is true. There is an immediate drop in urinary calcium and a rise in urinary phosphate. Twenty-four hours later the excretory rate for calcium returns to or rises above normal. The urinary phosphate, however, continues high throughout the entire period. These data are explained on the basis that immediate changes in renal excretory rates resulting from changes in parathyroid hormone titer are due to shifts in the renal threshold for these two ions. Excret...
Aging and parathyroid hormone, in Endocrinology of Calcium Metabolism
- T Fujita
- M Ohata
- K Ota
- K Tanimota
- Y Hanano
- M Funasako
- A Uezu
Parathyroid hormone, calcitonin and vitamin D, in CRC Handbook of Physiology in Aging
- D N Kalu
- D. N. Kalu
Further studies on the interrelationship between parathyroid hormone and vitamin D
- C Arnaud
- H Rasmussen
- C. Arnaud