[Show abstract][Hide abstract] ABSTRACT: We hypothesized that individual differences in autonomic responses to psychological, physiological, or environmental stresses are inherited, and exaggerated autonomic responsiveness may represent an intermediate phenotype that can contribute to the development of essential hypertension in humans over time. alpha(2)-Adrenergic receptors (alpha(2)-ARs), encoded by a gene on chromosome 10, are found in the central nervous system and also mediate release of norepinephrine from the presynaptic nerve terminals of the peripheral sympathetic nervous system and the exocytosis of epinephrine from the adrenal medulla. We postulated that, because this receptor mediates central and peripheral autonomic responsiveness to stress, genetic mutations in the gene encoding this receptor may explain contrasting activity of the autonomic nervous system among individuals. The restriction enzyme Dra I identifies a polymorphic site in the 3'-transcribed, but not translated, portion of the gene encoding the chromosome 10 alpha(2)-AR. Southern blotting of genomic DNA with a cDNA probe after restriction enzyme digestion results in fragments that are either 6.7 kb or 6.3 kb in size. Transfection studies of these two genotypes resulted in contrasting expression of a reporter gene, and it is suggested from these findings that this is a functional polymorphism. In a study of 194 healthy subjects, we measured autonomic responses to provocative motion, a fall in blood pressure induced by decreasing venous return and cardiac output, or exercise. Specifically, we measured reactions to 1) Coriolis stress, a strong stimulus that induces motion sickness in man; 2) heart rate responses to the fall in blood pressure induced by the application of graded lower body negative pressure; and 3) exercise-induced sweat secretion. In all of these paradigms of stress, subjective and objective evidence of increased autonomic responsiveness was found in those individuals harboring the 6.3-kb allele. Specifically, volunteers with the 6.3-kb allele had greater signs and symptoms of motion sickness mediated by the autonomic nervous system after off-axis rotation at increasing velocity (number of head movements a subject could complete during rotation before emesis +/- SE: 295 +/- 18 vs. 365 +/- 11; P = 0.001). They also had greater increases in heart rate in responses to the lower body negative pressure-induced fall in blood pressure (increase in heart rate +/- SE: 3.0 +/- 0.4 vs. 1.8 +/- 0.3; P = 0.012), and the 6.3-kb group had higher sweat sodium concentrations during exercise (mean sweat sodium concentration in meq/l over 30 min of exercise +/- SE: 43.2 +/- 7.1 vs. 27.6 +/- 3.4; P < 0.05). This single-nucleotide polymorphism may contribute to contrasting individual differences in autonomic responsiveness among healthy individuals.
Journal of Applied Physiology 07/2004; 96(6):2231-9. · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: To test our hypothesis that differences in urinary calcium excretion among blacks and whites may be secondary to ethnic variations in acid (H(+)) metabolism and to prove that increases in titratable acid excretion would be found among individuals predisposed to the development of stress fractures.
We administered 8 g NH(4)Cl acutely to 11 black and 18 white healthy volunteers and measured urinary sodium, calcium, and acid excretions. We measured the Na(+)/H(+) antiporter activity using acid-loaded platelets as surrogate markers for this exchanger expressed in renal epithelial cells. We also compared differences in titratable acid excretion among a cohort of subjects with, and without, a history of stress fracture.
NH(4)Cl-induced increases in titratable urinary acid correlated with changes in the renal excretion of calcium and sodium, and stimulated acid excretion correlated with basal acid loss. Despite comparable changes in plasma pH, whites, when compared to blacks, had much greater basal acid excretion and NH(4)Cl-induced acid excretion. Whites also had much greater baseline calcium excretion rates when compared to blacks. Following acid loading, whites continued to exhibit greater calcium excretion rates than blacks. Acid loading significantly decreased sodium excretion in whites but not in blacks. Blacks also had significantly attenuated Na(+)/H(+) exchange activity. In a cohort of resting, athletic students, we found enhanced basal H(+) and phosphate excretion among subjects who experienced stress fractures during their rigorous physical training when compared to those individuals who did not.
Blacks may have a greater endogenous buffering capacity than whites, or the reported ethnic differences in sodium and calcium excretion rates between blacks and whites may be secondary to racial variations in renal H(+) excretion. We conclude that both ethnic differences in bone mineralization and bone integrity in athletes are mediated by heritable differences in titratable acid excretion.
Medicine & Science in Sports & Exercise 03/2002; 34(2):295-302. · 4.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We reported previously that genetic polymorphisms of the alpha 2-adrenergic receptor are associated with hyperinsulinemia, diabetes mellitus, and hypertension in blacks. The evolutionary driving force for maintaining such deleterious mutations in the black population is unknown. Recognizing that vascular alpha 2-adrenergic receptors mediate cold-induced vasoconstriction and that temperature maintenance is a primary thrust of cellular metabolism, we postulated that vascular alpha 2-adrenergic receptors contribute significantly to metabolic heat generation in homeotherms such as humans. Using aerobic lactate production as an indicator of thermogenesis, we measured metabolic heat production in HT29 cells that expressed the gene encoding human vascular alpha 2-adrenergic receptors. Epinephrine, an alpha 2-adrenergic receptor agonist, increased net lactate efflux from 226 +/- 20 to 280 +/- 20 nmol/min (mean +/- SE) (P = .06). Clonidine, a more specific alpha 2-adrenergic agonist, increased lactate efflux from 110 +/- 6 to 156 +/- 8 nmol/min (P < .01). Similarly, in the presence of physiological concentrations of glucose (5.5 mmol/L), insulin increased lactate production from 123 +/- 6 to 175 +/- 10 nmol/min (P < .01). Because differences in aerobic glycolysis may also explain the heat intolerance and abnormal fuel homeostasis found in genetically hypertensive rats, we also measured lactate production in cultured vascular smooth muscle cells isolated from stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive control Wistar-Kyoto rats (WKY). Vascular smooth muscle cells from SHRSP had significantly greater lactate efflux compared with cells from normotensive WKY (296 +/- 4 versus 172 +/- 2 nmol/min, P < .001). These differences were not due to abnormalities in glucose uptake, as lactate efflux was greater in SHRSP cells compared with WKY cells when dextrose was replaced with equimolar concentrations of fructose (230 +/- 6 versus 138 +/- 2 nmol/min, P < .001). alpha 2-Adrenergic agonists increase lactate efflux in HT29 cells, and abnormalities in vascular smooth muscle lactate metabolism in genetically hypertensive rats is independent of altered glucose uptake. These data provide support for our hypothesis that balanced polymorphisms of the alpha 2-adrenergic receptor could offer protection against cold stress by increasing the thermogenic response associated with aerobic lactate production.
[Show abstract][Hide abstract] ABSTRACT: An understanding of the reflex hormonal responses that occur with a redistribution of the blood volume upon exposure to microgravity or during water immersion is operationally relevant. Further, dopamine receptor antagonists, which are used in the treatment of the motion sickness induced by microgravity, or at sea, may affect fluid metabolism. Atrial natriuretic factor (ANF) is a potent diuretic and natriuretic hormone that is released following central blood volume expansion in man. ANF is also a potent inhibitor of angiotensin- and potassium-stimulated aldosterone release.
Since ANF-induced diuresis may be mediated by dopamine, we sought to determine whether inhibition of dopamine receptors blocks ANF-induced natriuresis and diuresis. Also, since ANF has been shown to inhibit aldosterone secretion induced by adrenocorticotropic hormone (ACTH) in vitro, we investigated whether ANF decreases aldosterone in man infused with exogenous ACTH.
In six healthy, sodium-replete men, infusion of synthetic ANF (0.01 microgram.kg.min, Anaritide, Wyeth Laboratories) significantly increased urine flow from 7.1 +/- 0.7 to 11.7 +/- 1.8 ml.min-1 (p < 0.05) and decreased aldosterone from 74.7 +/- 9.0 to 55.8 +/- 6.5 pg.ml-1 (p = N.S.). Metoclopramide (Met), a dopaminergic antagonist, increased plasma aldosterone from 104.5 +/- 8.9 to 163 +/- 12.5 pg.ml-1 (p < 0.05). ANF-induced diuresis was not inhibited by Met, but ANF significantly inhibited Met-stimulated increases in plasma aldosterone. ANF did not attenuate ACTH-stimulated increases in plasma aldosterone. Also, ANF-induced diuresis and natriuresis were not affected by concomitant infusions of ACTH, but the ANF-induced kaliuresis was significantly attenuated by ACTH.
These studies suggest: a) Synthetic ANF induces a diuresis, natriuresis and kaliuresis; b) the D2 receptor antagonist Met increases plasma aldosterone; c) ANF-induced diuresis is not subject to dopaminergic blockade with Met, but Met-induced increases in plasma aldosterone are inhibited by ANF; and d) ANF does not attenuate ACTH-stimulated increases in mineralocorticoid production. These studies have operational significance, as they demonstrate that D2 dopaminergic blockade by the antinausea agent metoclopramide does not prevent the effects of increased ANF in response to central volume expansion such as occurs during exposure to microgravity or following water immersion.
Aviation Space and Environmental Medicine 04/1996; 67(3):248-52. · 0.78 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It is likely that a number of independent heritable traits, each encoded by a singular gene, contribute to pathologic elevations in blood pressure in humans. Genetic polymorphisms of individual genes may result in intermediate phenotypes which, by themselves, do not raise blood pressure, but, coupled with environmental or epistatic forces, contribute to the prevalence of human hypertension. The gene for the alpha 2-adrenergic receptor encoded by chromosome 10 (C10 A2AR) is polymorphic, and Southern blotting with a cDNA probe following restriction enzyme digest of this gene results in fragments of either 6.3 kb or 6.7 kb in size. We reported an association between homozygosity for the 6.3 kb allele and hypertension in blacks. Blacks with hypertension also have an increased risk for thrombotic stroke, increased baroreceptor sensitivity, and decreased sodium excretion. We noted that the C10 A2AR, which modulates norepinephrine release in blood-pressure-regulating regions of the brain, is also expressed on platelets and in the kidney. We postulated that functional changes associated with the C10 A2AR gene polymorphism could be responsible for increased baroreceptor sensitivity, epinephrine-mediated platelet aggregation, and decreased sodium excretion in some individuals.(ABSTRACT TRUNCATED AT 250 WORDS)
American Journal of Hypertension 10/1995; 8(9):863-9. · 3.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: alpha 2-Adrenergic receptors are found on presynaptic neurons of the central and peripheral nervous systems, on blood vessels, on platelets, on adipocytes, and in the kidney and pancreas. Activation of these ubiquitous adrenoreceptors results in decreased neuronal norepinephrine release, vasodilation, a fall in blood pressure, platelet aggregation, increased sodium excretion, and decreased insulin release. We hypothesized that defects in alpha 2-adrenergic receptors, or postreceptor defects, could explain the increased prevalence of hypertension in blacks. To test our hypothesis, we first determined whether or not a polymorphism of the alpha 2-adrenergic receptor gene was associated with pathologic elevations in blood pressure in American blacks. Dra-I identified a restriction fragment-length polymorphism (RFLP) of 6.3 and 6.7 kb of the alpha 2-adrenergic receptor gene on chromosome 10 in humans. Of 227 patients studied, 13/107 hypertensive subjects were homozygous for the 6.3-kb allele, whereas only 3/120 normotensive volunteers were homozygotes (P = .008). When analyzed by race, 13/82 black hypertensive subjects were homozygous for the 6.3-kb allele, whereas only 2/59 normotensive blacks were homozygous for the 6.3-kb alleles (P = .02). However, only 1/61 white normotensive and 0/25 white hypertensive subjects were homozygous for the 6.3-kb allele (P = 1.00). Ethnic variation among blacks may explain our findings. Alternatively, a genetic polymorphism in, or near, the alpha 2-adrenergic receptor on chromosome 10 can contribute to the development of hypertension in blacks.
American Journal of Hypertension 05/1995; 8(4 Pt 1):390-4. · 3.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In white populations, a deletion polymorphism in the gene for angiotensin converting enzyme (ACE) appears to be associated with increased risk for myocardial infarction but not for hypertension. In a population-association study in African-Americans, we compared the frequency of the ACE deletion polymorphism in subjects with hypertension versus those with normal blood pressure. The frequency of the ACE deletion allele was greater in African-Americans with hypertension than in those with normal blood pressure (P < 0.05). These findings raise the possibility that in some patient subgroups, sequence variation in or near the ACE gene may contribute to the risk for hypertension.
American Journal of Hypertension 08/1994; 7(8):759-62. · 3.40 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We tested the hypothesis that 1-desamino-8-D-arginine vasopressin (DDAVP), a V2-receptor agonist, could inhibit the diuresis induced by water immersion in humans. Water and electrolyte excretion, plasma atrial natriuretic factor concentration, and plasma aldosterone concentration were measured initially and after 3 h of water immersion in 13 healthy sodium-replete men given either placebo or 20 micrograms of intranasal DDAVP. Guanosine 3',5'-cyclic monophosphate and urea excretion and urine osmolality were also determined. DDAVP inhibited the diuresis induced by water immersion in men: 758 +/- 168 (SE) ml/3 h in the placebo group vs. 159 +/- 28 ml/3 h in the DDAVP group (P less than 0.05). After 3 h of water immersion, plasma atrial natriuretic factor concentrations were increased from 11 +/- 2 to 20 +/- 4 pg/ml in the placebo group and from 14 +/- 2 to 33 +/- 4 pg/ml in the DDAVP group (P less than 0.05). Plasma aldosterone concentrations were decreased from 98 +/- 18 to 45 +/- 6 pg/ml in the placebo group (P less than 0.05) and from 54 +/- 17 to 25 +/- 5 pg/ml in the DDAVP group (P less than 0.05). Despite these changes in aldosterone and atrial natriuretic factor concentrations, which should increase sodium excretion, DDAVP decreased the natriuresis induced by water immersion in humans: 56 +/- 8 meq Na+/3 h in the placebo group vs. 36 +/- 6 meq Na+/3 h in the DDAVP group (P less than 0.05). DDAVP may be used to prevent the diuresis associated with central redistribution of blood volumes that occur during water immersion.(ABSTRACT TRUNCATED AT 250 WORDS)
Journal of Applied Physiology 10/1992; 73(3):932-6. · 3.43 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We studied the effect of yohimbine, a drug that inhibits presynaptic alpha 2-adrenergic receptors and increases the neuronal release of norepinephrine from the central and sympathetic nervous systems, on tolerance to cardiovascular stress in 10 untrained, healthy subjects. Using radioligand binding of tritiated yohimbine to platelets, these subjects were found to have a normal complement of alpha 2-adrenergic receptors (174 +/- 18 [+/- SEM] receptors/platelet) with normal Kd (1.93 +/- 0.17 nmol/l). Lower body negative pressure was used to test responses to cardiovascular stress in the subjects after they received either placebo or 20 mg yohimbine. Graded lower body negative pressure from 0 to -40 mm Hg significantly decreased systolic blood pressure from 116 +/- 3.7 to 106 +/- 5.8 mm Hg, increased heart rate from 54 +/- 3 to 68 +/- 7 beats/min, decreased forearm blood flow from 1.8 +/- 0.21 to 1.36 +/- 0.25 ml/100 ml/min, and increased forearm vascular resistance from 55.76 +/- 12.1 to 77.26 +/- 15.8 mm Hg/ml/min. Yohimbine increased the blood pressure at rest and during lower body negative pressure, but these changes were not significantly different from values recorded from the individuals when they were given placebo. Compared with placebo, however, yohimbine significantly increased forearm blood flow at rest (1.80 +/- 0.21 vs. 2.66 +/- 0.31 ml/100 ml/min, p less than 0.05) and during -40 mm Hg of lower body negative pressure (1.36 +/- 0.25 vs. 1.91 +/- 0.28 ml/100 ml/min, p less than 0.05). We also found that yohimbine significantly increased the plasma insulin concentration in these fasted subjects (9.4 +/- 2.4 vs. 14.5 +/- 1.4 ng/ml, p less than 0.05) without inducing hypoglycemia.(ABSTRACT TRUNCATED AT 250 WORDS)