D S MacKenzie

Texas A&M University, College Station, Texas, United States

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Publications (31)57.42 Total impact

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    ABSTRACT: The role of thyrotropin (thyroid-stimulating hormone, TSH) in driving peripheral thyroid function in non-mammalian species is still poorly understood. Thyroxine (T4), the principal hormone released from the thyroid gland in response to TSH stimulation, circulates with a robust daily rhythm in the teleost fish the red drum. Previous research suggests that the red drum T4 cycle is circadian in nature, driven by TSH secretion in the early photophase and inhibited by T4 feedback in the early scotophase. To determine whether TSH is produced in a pattern consistent with feedback inhibition by this T4 cycle, we used quantitative real time PCR (qPCR) to quantify the daily cycle of expression of the pituitary TSH subunits GSUα, and TSHβ. We found that TSH expression cycled inversely to, and 6-12 hours out of phase with, the T4 cycle, consistent with the hypothesis that TSH secretion drives the T4 cycle. To examine the potential role of deiodinases in negative feedback regulation of this TSH cycle, we also utilized qPCR to assess the pituitary expression patterns of the TH activating enzyme outer-ring deiodinase (Dio2) and the TH deactivating enzyme inner-ring deiodinase (Dio3). Dio2 was not expressed with an obvious daily cycle, whereas Dio3 expression mirrored the expression of TSH. These results are consistent with circulating T4 providing the negative feedback signal controlling both TSH production and Dio3 expression in the pituitary, and suggest that TH inactivation by inner ring deiodination is an important component of TSH negative feedback control.
    General and Comparative Endocrinology 10/2013; · 2.82 Impact Factor
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    ABSTRACT: Thyrotropin (TSH) is a pituitary glycoprotein hormone heterodimer that binds to its G-protein coupled receptor (TSH-R) at the thyroid to promote the synthesis and secretion of thyroid hormone. Very little is known about TSH-TSH-R interactions in teleost fish. Mammalian gonadotropins have been reported to have an intrinsic ability to activate teleost fish TSH-Rs, suggesting the TSH-R in teleost fish is more promiscuous than in other vertebrates. In this study we utilized the goldfish T(4)-release response and recombinant human TSH analogs as in vivo tools to evaluate the structural constraints on hormone-receptor interactions. We found that four positively charged lysines substituted for neutral or negatively charged amino acids within positions 11-20 of the glycoprotein hormone subunit α (GSUα) significantly increased biological activity of hTSH in fish, as it does in mammals. We further found that bovine follicle stimulating hormone but not luteinizing hormone, whose GSUα subunits also contain four lysine or arginine amino acid residues in the N-terminal portion of GSUα, was thyrotropic in goldfish, suggesting gonadotropin β subunit contributes to the heterothyrotropic activity. Though recombinant human FSH did not produce a dose-dependent increase in T(4), thyrotropic activity could be acquired with the addition of positively charged amino acids at the N-terminal portion of its GSUα, confirming the importance of the charge on those amino acids for activation of the goldfish TSH-R. These studies demonstrate that mammalian glycoprotein hormone analogs can be utilized to evaluate the conservation of receptor binding and activation mechanisms between fish and mammals.
    General and Comparative Endocrinology 02/2012; 177(1):70-5. · 2.82 Impact Factor
  • S K Scobell, D S Mackenzie
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    ABSTRACT: Few studies have examined the underlying hormonal mechanisms that mediate reproductive cyclicity, male pregnancy and reproductive behaviour in syngnathids. Progress in these areas has been hampered by the small size of most species in the family and a lack of validated techniques for assessing endocrine function. Research on a relatively small number of species has suggested that androgens are likely regulators of spermatogenesis and the development of the male brood pouch prior to pregnancy whereas prolactin and corticosteroids synergistically promote brood pouch function during pregnancy. No evidence supports a reversal of reproductive steroid hormone function in sex-role reversed behaviour, but neuropeptides such as arginine vasotocin or isotocin should be examined for their role in regulating parturition and mating behaviour. The diversity of reproductive patterns exhibited by syngnathids suggests that they will provide a unique opportunity to assess how hormonal regulation of integumentary function, gametogenesis and reproductive behaviour have evolved within a teleost lineage. Additionally, their coastal distribution and embryo retention make them potentially important subjects for studies on the effect of endocrine disruption on fitness.
    Journal of Fish Biology 06/2011; 78(6):1662-80. · 1.83 Impact Factor
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    ABSTRACT: Full-length cDNAs for thyrotropin β (TSHβ) and glycoprotein hormone α (GSUα) subunits were cloned and sequenced from the red drum (Sciaenops ocellatus). The cDNAs for TSHβ (877 bp) and GSUα (661 bp) yielded predicted coding regions of 126 and 94 amino acid proteins, respectively. Both sequences contain all invariant cysteine and putative glycosylated asparagines characteristic of each as deduced by comparison with other GSUα and TSHβ sequences from representative vertebrate species. Multiple protein sequence alignments show that each subunit shares highest identity (79% for the TSHβ and 86% for the GSUα) with perciform fish. Furthermore, in a single joint phylogenetic analysis, each subunit segregates most closely with corresponding GSUα and TSHβ subunit sequences from closely related fish. Tissue-specific expression assays using RT-PCR showed expression of the TSHβ subunit limited to the pituitary. GSUα mRNA was predominantly expressed in the pituitary but was also detected in the testis and ovary of adult animals. Northern hybridization revealed the presence of a single transcript for both TSHβ and GSUα, each close in size to mRNA transcripts from other species. Dot blot assays from total RNA isolated from S. ocellatus pituitaries showed that in vivo T3 administration significantly diminished mRNA expression of both the TSHβ and GSUα subunits and that goitrogen treatment caused a significant induction of TSHβ mRNA only. Both TSHβ and GSUα mRNA expression in the pituitary varied significantly in vivo over a 24-h period. Maximal expression for both TSHβ and GSUα occurred during the early scotophase in relation to a peak in T4 blood levels previously documented. These results suggest the production of TSH in this species which may serve to drive daily cycles of thyroid activity. Readily quantifiable, variable, and thyroid hormone-responsive pituitary TSH expression, coupled with previously described dynamic daily cycles of circulating T4 and extensive background on the growth, nutrition, and laboratory culture of red drum, suggests that this species will serve as a useful model for experimental studies of the physiological regulation of TSH production.
    Fish Physiology and Biochemistry 12/2010; 36(4):1277-90. · 1.55 Impact Factor
  • Dae-Yeon Moon, David W. Owens, Duncan S. MacKenzie
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    ABSTRACT: The effects of fasting and increased feeding on plasma thyroid hormones, glucose, and total protein levels were investigated in immature greens (Chelonia mydas) and Kemp's ridley (Lepidochelys kempi) sea turtles. Under constant temperature, nutritional status affected plasma thyroid hormones and glucose level, but not protein level in green and Kemp's ridley turtles. Plasma T4 in green turtles decreased with deprivation of food, but did not do so in Kemp's ridleys. Increased feeding did not affect circulating levels of plasma T4 in either species. Plasma T3, in contrast, tended to decrease during fasting and increase with refeeding or increased feeding. Blood glucose level fell with deprivation of food, but remained constant during increased feeding. Sea turtles may use glycogen or lipid during short-term fasting as in endothermic vertebrates.
    ZOOLOGICAL SCIENCE 01/2009; · 1.08 Impact Factor
  • Duncan S MacKenzie, Richard A Jones, Thomas C Miller
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    ABSTRACT: Thyrotropin (TSH), a pituitary glycoprotein hormone that stimulates the thyroid gland, has been cloned and sequenced from over a dozen teleost fish species. Although TSH is established as a primary driver of systemic thyroid status in mammals, its importance in the regulation of fish thyroid function is still uncertain. We review recent studies indicating that TSH structure is highly conserved across species representing six teleost families. These studies have found TSH messenger RNA consistently expressed in teleost pituitary tissue, although ectopic expression, particularly in gonads, has also been observed. They have also provided evidence for negative feedback inhibition of TSH expression by thyroid hormones, as well as stimulation by hypothalamic peptides. Descriptive studies have found increased TSHbeta expression associated with life history events thought to be promoted by thyroid hormones. These results, coupled with the discovery of a G-protein coupled TSH receptor in several teleost species, supports an active and conserved role for TSH in the regulation of teleost thyroid function. The relative importance of central pathways in regulating thyroid hormone provision to targets and the identity of a proposed thyrotropin-inhibiting factor in teleost fish are still unanswered questions whose resolution will be facilitated by development of methods to measure circulating TSH and its secretion from the pituitary gland.
    General and Comparative Endocrinology 01/2009; 161(1):83-9. · 2.82 Impact Factor
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    ABSTRACT: Channel catfish (Ictalurus punctatus) in pond culture, sampled once per day, have been reported to exhibit significant seasonal cycles in the thyroid hormones thyroxine (T4) and 3,5,3′-triiodothyronine (T3), rising from levels generally below 2 ng/ml in January to above 8 ng/ml in July. To determine if daily thyroid hormone cycles underlie these seasonal changes, we blood sampled groups of 20 catfish (10 males and 10 females) in the morning (approx. 1 h after sunrise), midday, and evening (approx. 1.5 h before sunset) on January 9, April 4, and July 29. From January to July, pond temperatures rose from 7 ° to 32 °, associated with significant (p < 0.05) increases in mean fish weight (from 477 to 1052 g) and in monthly mean food consumption (from 34 to 474.7 g/kg fish). On all three dates, significantly (p < 0.05) greater levels of both hormones (except T3 in April) were found in midday and evening compared to morning samples. In January, the daily change was small (from morning to midday, mean T3 rose from 2.2 to 3.6 ng/ml and mean T4 from 2.3 to 4.8 ng/ml), whereas in July it was considerably greater (from morning to evening, mean T3 rose from 7.2 to 17.8 ng/ml, and T4 from 9.0 to 22.4 ng/ml). No significant differences were found between midday and evening levels, or between males and females. Additionally, no seasonal phase-shifting of cycles was apparent. A subset of animals was examined to evaluate the potential contribution of peripheral mechanisms in generating these seasonal and daily cycles. Whereas we observed only minor changes in thyroid hormone binding to plasma proteins during any single day, a significant seasonal increase in the ratio of free T4:free T3 indices (from a mean of 1.3–1.5 in January to 2.0–2.1 in July) indicated enhanced T3 binding by plasma proteins in July. Furthermore, in vitro hepatic T4 and T3 deiodination activities showed across dates no significant change in T4 outer-ring deiodination to produce T3 (ranging from a mean of 53.1 to 70.1 pmol T4 deiodinated/h/mg microsomal protein), but a significant (p < 0.05) decrease in T4 inner-ring deiodination to degrade T4 to 3,3′5′-triiodothyronine (from a mean in January of 2.4 to 0.65 pmol T4 deiodinated /h/mg protein in April) and a significant (p < 0.05) decrease in T3 inner-ring deiodination to degrade T3 to 3,3′-diiodothyronine (from a mean in January of 115.5 to 3.1 pmol T4 deiodinated/h/mg protein in July). These results demonstrate that channel catfish under conditions of natural temperature and photoperiod exhibit robust daily cycles in total plasma T4 and T3 similar in magnitude to those reported for other fish species held under controlled laboratory conditions. These cycles maintain a similar phase throughout the year, indicating that apparent seasonal increases in thyroid hormones are not due to phase-shifting of daily cycles. However, seasonal studies sampling fish only in the morning would underestimate the magnitude of the annual changes in blood thyroid hormones. Thyroidal status, as judged from total plasma T4 and T3 levels in the afternoon, is greatest in July, coinciding with the postspawning peak in food consumption and growth. Enhanced T3 plasma protein binding and a shift from predominantly hepatic inner-ring deiodination in winter to outer-ring deiodination in summer suggest that peripheral mechanisms contribute to the generation of these seasonal changes.
    Aquaculture. 01/2007;
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    ABSTRACT: We collected adult cave swallows (Petrochelidon fulva) and cliff swallows (P. pyrrhonota) during the breeding seasons in 1999 and 2000 from eight locations along the Rio Grande from Brownsville to El Paso (unless otherwise specified, all locations are Texas, USA) and an out-of-basin reference location. Body mass, spleen mass, hepatosomatic index (HSI), gonadosomatic index (GSI), thyroxine (T4) in plasma, DNA damage measured as the half-peak coefficient of variation of DNA content (HPCV) in blood cells, as well as acetylcholinesterase and butyrylcholinesterase in brain were compared with concentrations of organochlorines, metals, and metalloids in carcasses to determine potential effects of contaminants on swallows during the breeding season. Concentrations of 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (p,p'-DDE) were significantly greater in swallows from El Paso than in those from most locations, except for Pharr and Llano Grande. All swallows from these three locations had p,p'-DDE concentrations of 3 microg/g wet weight or greater. Swallows from El Paso either had or shared the highest concentrations of p,p'-DDE, polychlorinated biphenyls, and 13 inorganic elements. Swallows from El Paso exhibited greater spleen mass and HPCV values as well as lower T4 values compared with those from other locations. Thyroxine was a potential biomarker of contaminant exposure in swallows of the Rio Grande, because it was negatively correlated with p,p'-DDE and Se. Spleen mass was positively correlated with selenium and HSI and negatively correlated with body mass, GSI, Mn, and Ni. Overall, the present study suggests that insectivorous birds living in areas of high agricultural and industrial activity along the Rio Grande bioaccumulate environmental contaminants. These contaminants, particularly p,p'-DDE, may be among multiple factors that impact endocrine and hematopoietic function in Rio Grande swallows.
    Environmental Toxicology and Chemistry 07/2006; 25(6):1574-84. · 2.62 Impact Factor
  • Kevin A Leiner, Duncan S Mackenzie
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    ABSTRACT: Several experiments were conducted to investigate the dynamics of central regulation of thyroid function in the red drum, Sciaenops ocellatus, by manipulating a well-characterized circadian rhythm of T(4) secretion. In the first experiment, red drum were reared under either a long (16L:8D) or short (8L:16D) photoperiod and fed at the same time relative to dawn. The same feeding time under different photoperiods maintained the same phase relationship between T(4) cycles under each photoperiod. This suggests that the circadian clock that determines when the hypothalamus-pituitary-thyroid (HPT) axis is activated is comprised of a feeding-entrained oscillator and a light-entrained oscillator that interact to determine the phase of the T(4) rhythm. Additionally, the amplitude of the main T(4) peak of the cycle was inversely related to the frequency of feeding, while the duration of the main T(4) peak was directly related to feeding frequency under a long photoperiod. Feeding time appears to modify the diurnal profile of circulating T(4) by stimulating post-prandial T(4) secretion that subsequently results in negative feedback on the HPT axis to regulate thyroidal status. In following experiments, red drum immersed in T(3), in lieu of a meal at a specific time that would diminish the main T(4) peak, exhibited a dose-dependent decline in amplitude of the T(4) cycle. This demonstrates that T(3) can exert negative feedback on the HPT axis of red drum to maintain appropriate thyroid hormone concentrations. These data are consistent with a dynamic and physiologically important central component of the regulation of thyroid function in fish.
    Journal of Experimental Zoology Part A Comparative Experimental Biology 08/2003; 298(1):32-43.
  • K A Leiner, D S MacKenzie
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    ABSTRACT: Previous studies with red drum and other species have indicated that diurnal rhythms of circulating thyroid hormones (thyroxine, T(4), and 3-5-3'-triiodo-L-thyronine, T(3)) are synchronized to the light cycle, and not to time of feeding. In this study we set out to address the effects of various lighting regimes on thyroid hormone levels in the red drum. The first experiment was undertaken to determine the effects of long and short photoperiods on diurnal thyroid hormone rhythms, growth rate, feed efficiency and food consumption. Red drum raised under a long photoperiod (16L:8D) grew significantly larger and exhibited greater feed efficiency than their short photoperiod (8L:16D) counterparts. There were no changes in food consumption or the diurnal profile of plasma thyroid hormones, e.g. increased peak amplitude or duration, that would explain this increase in growth rate and feed efficiency. The second experiment was undertaken to determine if diurnal thyroid hormone rhythms in the red drum originate from an endogenous circadian clock. To address this question, red drum were housed under a 12L:12D photoperiod and fed once daily at variable times before the lighting was switched to constant dim illumination for up to 3 days. The rhythm of circulating T(4) levels persisted for two complete cycles with constant amplitude in fish that continued to be fed during constant dim illumination, and did not appear to entrain to feeding. The T(4) rhythm also persisted for three complete cycles under constant conditions in feed-restricted fish, although with a diminished amplitude over time. To our knowledge, these data provide the first evidence for a free-running circadian rhythm of plasma T(4) levels in a fish. These findings implicate the involvement of an endogenous circadian clock that determines when the hypothalamo-pituitary-thyroid axis is activated.
    Comparative Biochemistry and Physiology - Part A Molecular & Integrative Physiology 09/2001; 130(1):141-9. · 2.17 Impact Factor
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    ABSTRACT: To characterize seasonal changes in thyroid function in a terrestrial reptile, thyroid hormones were measured over a period of 2 years in desert tortoises, Gopherus agassizii, maintained at the Desert Tortoise Conservation Center in Las Vegas, Nevada. In all samples, triiodothyronine was nondetectable (less than 0.1 ng/ml). Thyroxine (T(4)) exhibited distinct cycles in both sexes, being lowest during hibernation and rising toward the time of emergence. Females exhibited only one peak in T(4), during the early spring. In males, T(4) levels peaked in early spring and again in late summer. The desert tortoise has distinct activity patterns that include increased feeding, mating, and locomotor activity in the early spring and increased mating and combat in the late summer. In an experiment to determine whether food intake influences T(4), food was withheld for 2 weeks. Compared to continuously fed controls, T(4) declined significantly in unfed tortoises, but increased significantly within 36 h of refeeding, indicating that thyroid activity is responsive to nutrient intake. The second seasonal peak of T(4) only in males suggests that male reproductive activity in late summer is associated with thyroid activation. To evaluate this possibility, adult, subadult, and juvenile males were sampled during the months of the second seasonal peak in T(4). Although all three age groups showed similar foraging and thermoregulatory behaviors, T(4) peaked in July only in the reproductively active adults, which also exhibited significantly higher testosterone levels. Elevated T(4) in desert tortoises is thus associated with periods of increased feeding and reproductive activity, supporting a role for thyroid hormones in these energy-demanding activities.
    General and Comparative Endocrinology 03/2001; 121(2):214-22. · 2.82 Impact Factor
  • T.G. Gaylord, D.S. MacKenzie, D.M. Gatlin
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    ABSTRACT: A 6-week feeding trial was conducted to investigate the effects of short-term feed deprivation on inducing compensatory growth and changes in thyroid hormone levels of channel catfish. Feeding treatments consisted of the following four regimes of 2-week duration: satiate feeding (control), no feed for 3 days then feeding to apparent satiation for the next 11 days, no feed for 5 days then feeding to apparent satiation for 9 days, and no feed for 7 days then feeding to apparent satiation for 7 days. These regimes were repeated three times over the 6-week trial in which 25 channel catfish fingerlings, initially averaging 15g each, were stocked into each of 12, 38-l glass aquaria supplied with supplemental aeration and flow-through water. Depriving fish of feed had a pronounced effect in that fish lost weight in as little as 3 days. Returning the fish to a satiate feeding regime caused a resumption of growth, equal to control growth only in the case of the 3-day deprived treatment, but all periods of feed deprivation failed to induce a period of catch-up growth adequate to compensate for previously lost weight. Feed efficiency also was not improved by the periods of feed deprivation, and restricting feed in excess of 3 days lowered feed efficiency. Fish condition indices were not altered at the termination of the trial. Muscle lipid, muscle protein and liver protein also were not different among feeding regimes. Liver lipid was elevated in fish deprived of feed for more than 3 days every 2 weeks. Plasma thyroxine (T4) and triiodothyronine (T3) were equally depressed by 3 days from the onset of feed deprivation. Both hormones rose significantly within 24h of realimentation, with the greatest increase observed in animals subjected to the briefest feed deprivation. These results support a role for thyroid hormones in the promotion of growth in channel catfish. Whereas feed deprivation appears to rapidly reduce activity of the hypothalamo-pituitary-thyroid axis, the high correlation observed between T4 and T3 in all treatments suggests that peripheral deiodinating systems are capable of rapidly generating T3 from T4 upon realimentation. More rapid recovery of thyroid hormone production following realimentation may minimize the effects of feed deprivation on growth and feed efficiency of fish subjected to the 3-day deprivation treatment when compared to longer periods.
    Fish Physiology and Biochemistry 12/2000; 24(1):73-79. · 1.55 Impact Factor
  • K A Leiner, G S Han, D S MacKenzie
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    ABSTRACT: Available data in cyprinid and salmonid species indicate that nutrient intake sustains thyroidal rhythmicity and that time of feeding may influence the amplitude, but not the phase, of diurnal thyroid hormone cycles. Several experiments were conducted to characterize the nature of thyroidal rhythmicity in a more derived perciform teleost, the red drum. These studies were designed to test the following hypotheses: (1) that feeding time will alter the amplitude of the thyroid hormone rhythm without altering its phase and (2) that food deprivation will diminish the amplitude of the thyroid hormone rhythm. Circulating T(4) levels in this species exhibit high-amplitude diurnal rhythms, whereas circulating T(3) levels fluctuate within a more narrow range. Fish were reared under a 12L:12D photoperiod and fed 5% body weight once daily either at dawn or at dusk. Feeding time had no discernible effect on the phase of the T(4) cycle, but altered the amplitude of the cycle. Dawn-fed fish had significantly greater mean peak levels of T(4) than dusk-fed fish, although there was no difference in daily mean levels in both groups of fish. When red drum were deprived of food, significant declines in plasma glucose, HSI, and liver glycogen content occurred within 3 days. When red drum were sampled once per day after 3, 7, or 11 days of food deprivation there were no consistent changes in circulating T(4) and T(3) levels compared to those of fed controls. However, significant declines in circulating T(4) and T(3) levels in response to food deprivation were detected with a diurnal sampling protocol. Within 3 days of food deprivation, T(4) levels were significantly reduced compared to those in fed controls and not significantly different from T(4) levels after 10 days of food deprivation. T(3) levels exhibited a stepwise decline in circulating levels during food deprivation. These data indicate that both feeding time and nutrient status exert their effects on thyroid hormone rhythms by modifying the amplitude of these cycles. These data also underscore the importance of incorporating a consideration of endocrine rhythmicity into sampling protocols.
    General and Comparative Endocrinology 11/2000; 120(1):88-98. · 2.82 Impact Factor
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    ABSTRACT: Adult red drum (Sciaenops ocellatus) from a wild, autumn (fall) spawning population were studied over a 1-year period to evaluate seasonal changes in body composition in comparison with cultured red drum. Each month, female and male fish were captured and blood sampled. Standard length and weight were measured, and gonads, liver, intraperitoneal fat (IPF), and a sample of muscle tissue were collected from each fish. Gonadosomatic index (GSI), hepatosomatic index (HSI), IPF ratio, protein and lipid composition of muscle and liver tissues, and lipid class composition of liver samples were determined for each fish. All tissue indices exhibited a similar seasonal cycle in both sexes, with highest HSI in spring (March–April) and maximal IPF ratio in May, followed in September and October by minimal values for HSI and IPF ratio. Whereas GSI was low, gonadal histology demonstrated activation of spermatogenesis and oocyte development in July and August. Liver composition varied dramatically throughout the year. Liver lipid content ranged from 7.4% of wet weight in November to 30.2% in August, with triglycerides (TG) being the most abundant component at all times. Liver protein was more stable, ranging from 11.5% in August to 16.3% in September. Muscle composition was relatively constant, with muscle crude protein ranging from 20.5% to 25.6% of wet weight and muscle lipid ranging from 0.4% to 2.2% of wet weight. These data indicate that red drum utilize the liver as a major depot for lipid. Depletion of maximal lipid reserves from liver and IPF in late summer indicates that lipid stored during active spring and summer feeding supplies energy for reproduction which is mobilized rapidly (within 1 month) in this fall-spawning species. Body composition of wild fish is similar to that of laboratory cultured red drum.
    Aquaculture 01/2000; · 2.01 Impact Factor
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    R A Valverde, D W Owens, D S MacKenzie, M S Amoss
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    ABSTRACT: Adrenocortical responsiveness to turning stress was examined in wild, reproductively-active olive ridley sea turtles (Lepidochelys olivacea) in relation to their mass nesting (arribada) behavior. We hypothesized that the high sensitivity threshold (HST) observed in ovipositing sea turtles is associated with a diminished sensitivity of the hypothalamo-pituitary-adrenal (HPA) axis to stressful stimuli in arribada females. We tested this hypothesis by determining whether arribada females exhibited an increased activation threshold of the HPA axis to an imposed stressor (turning stress). Mean basal corticosterone (B) and glucose levels were below 1.0 ng/ml and 60 mg/dl, respectively. Basal B remained unchanged throughout a 24-hr period in basking females. Most animals responded to turning stress with elevated mean B levels (up to 6.5 ng/ml after 6 hr) and no increase in circulating glucose. Nearly 50% of females (and none of the males) were refractory to the stimulation. Males exhibited the most rapid response, with B levels significantly elevated by 20 min over basal levels. Among females, arribada and solitary nesters exhibited a slower rate of response than basking, non-nesting animals. These results demonstrate that olive ridleys exhibit stress-induced changes in circulating B which are slower than those observed in most reptilian and in mammalian, avian, and piscine species. Furthermore, the presence of refractory females and the relatively slower increase in B in arribada and solitary nesters indicate a hyporesponsiveness of the HPA axis to turning stress in nesting olive ridleys. The hyporesponsiveness may be part of a mechanism to facilitate arribada nesting. J. Exp. Zool. 284:652-662, 1999.
    Journal of Experimental Zoology 12/1999; 284(6):652-62.
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    ABSTRACT: The seasonal reproductive cycle of the Kemp's ridley sea turtle (Lepidochelys kempi) was studied under semi-natural conditions at the Cayman Turtle Farm, Grand Cayman, British West Indies, from June 1987 to July 1988. MaleL. kempidisplayed a prenuptial rise in serum testosterone 4 to 5 months prior to the mating period (March). Male testosterone then declined sharply during the mating period. FemaleL. kempialso displayed a prenuptial rise in serum testosterone, estradiol, and total calcium 4 to 6 months prior to the mating period (March). Female testosterone and estradiol declined during the nesting period (April to July) immediately following the mating period (March). Elevated levels in female estradiol and total calcium corresponded with the period of vitellogenesis as determined from gel electrophoresis and ultrasonography. Serum thyroxine also fluctuated seasonally with elevated levels observed in females associated with the period of vitellogenesis.L. kempidisplayed a distinct seasonal reproductive cycle in captivity. Nesting in the captive study group corresponded with nesting in the wild population at Rancho Nuevo, Mexico (April to July). Female endocrine cycles during the nesting period were similar to those observed in the wild population.
    General and Comparative Endocrinology 03/1998; · 2.82 Impact Factor
  • D.S. MacKenzie, C.M. vanPutte, K.A. Leiner
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    ABSTRACT: Hormones play a central role in the regulation of growth and nutrient utilization in fish. Consequently, fish endocrine systems are sensitive to alterations in nutrient intake. Procedures routinely employed in the development of diets and feeding protocols for cultured fish have pronounced effects on endocrine systems. We review the evidence that alterations in ration level (including food restriction and food deprivation), diet composition, photoperiod, and feeding time influence the most intensively-studied fish metabolic hormones: thyroid hormones, pancreatic hormones, and hormones of the growth hormone–insulin-like growth factor axis. Whereas effects of these dietary manipulations on total circulating hormone levels are commonly examined, nutrient intake may also influence hormone transport in blood, activation in peripheral tissues, receptor binding, and neuroendocrine pathways regulating hormone secretion. Information on the cellular and molecular mechanisms through which nutrients influence endocrine systems is still needed. Significant new information about the regulation of endocrine function can be derived from nutritional studies currently employed in aquaculture for the development of diets. Additional information on the influence of nutrients on endocrine function is essential for the design and interpretation of hormone supplementation studies, and should eventually allow development of feeding strategies which promote anabolic hormone production.
    Aquaculture 01/1998; 161(1). · 2.01 Impact Factor
  • D Y Moon, D S MacKenzie, D W Owens
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    ABSTRACT: Captive immature green (Chelonia mydas) and Kemp's ridley (Lepidochelys kempi) sea turtles were examined to determine if a hibernation-like state could be induced under controlled conditions. Both species demonstrated that they are able to acclimate to cold temperatures behaviorally. However, the two species appeared to respond differently to decreasing temperature. Whereas the green turtles tolerated the onset of cold water temperatures by reducing swimming activity, the ridleys became very agitated and active as they were exposed to temperatures below 20 degrees C. Nevertheless, both species displayed semi-dormant behavior at temperatures below 15 degrees C, coming to the surface to breathe periodically at intervals of up to three hours. At low temperatures, venous blood pO2 and pCO2 decreased, whereas venous blood pH increased. Feeding also decreased as either species was exposed to cold temperature: greens (at 15 degrees C) and ridleys (at 20 degrees C) decreased food consumption to 50% of control levels, and ceased feeding below 15 degrees C. Thus, these species tolerated temperature drops and the associated hypophagia. They did not exhibit cold-stunning behavior, as has been observed in wild sea turtles exposed to rapid temperature drops, or prolonged periods of hibernation-like dormancy, as has been proposed for wild sea turtles during cold winter months.
    Journal of Experimental Zoology 09/1997; 278(6):372-80.
  • J Heck, D S MacKenzie, D Rostal, K Medler, D Owens
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    ABSTRACT: Estrogen stimulation of the production of teh yolk protein precursor vitellogenin was demonstrated in immature Kemp's ridley sea turtles. 17beta-Estradiol injection elicited an increase in serum estrogen, protein, protein phosphorus, and total calcium within 7 days. Associated with these changes was the appearance of a single, dimethylformamide-precipitable, 205-kDa estradiol-induced serum protein, which became the predominant serum protein identified by polyacrylamide gel electrophoresis. The effects of estradiol injection were maintained for 3 months following termination of estradiol administration, despite a decline in serum estrogen levels. Although studies in other species have indicated that changes in circulating vitellogenin levels can influence thyroid hormone transport, no changes were observed throughout the study in total blood levels of thyroxine or triiodothyronine or protein binding of these hormones. We conclude that in the immature Kemp's ridley estrogen induces the synthesis and secretion of a vitellogenin which resembles that of other reptiles, but which does not influence thyroid hormone transport. The prolonged presence of vitellogenin in the blood may be due to a lack of an active ovarian uptake mechanism in these immature animals.
    General and Comparative Endocrinology 09/1997; 107(2):280-8. · 2.82 Impact Factor
  • H L Moon, D S Mackenzie, D M Gatlin
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    ABSTRACT: Four separate 8-week feeding trials were conducted to assess the effects of supplementing semipurified diets with either triiodothyronine (T3) or thyroxine (T4) at 0, 2, 10, and 50 mg/kg on growth and body composition of juvenile red drum (Sciaenops ocellatus) held in artificial brackish water (6‰) and artificial seawater (32‰). At both levels of salinity, increasing doses of T3 resulted in fish with reduced weight gain, feed efficiency, condition factor (weight × 100/length(3)), and muscle ratio (muscle weight × 100/body weight), as well as a lighter body color. Significant (p < 0.05) effects of T3 on the proximate composition of whole body, liver, and muscle were variable, generally reflecting decreased lipid and protein storage in liver and muscle, respectively. The two highest doses of T3 given to seawater adapted fish increased survival. Dietary T4 supplementation had no distinctive effects on appearance, growth or proximate body composition. These results indicate that whereas T3 may function to regulate protein and lipid metabolism in red drum, dietary supplementation with T3 leads to a hyperthyroidism-induced catabolic state. The elevated endogenous thyroid hormone levels found in fish fed optimal diets may thus adequately supply tissue needs during juvenile growth.
    Fish Physiology and Biochemistry 01/1994; 12(5):369-80. · 1.55 Impact Factor

Publication Stats

381 Citations
57.42 Total Impact Points

Institutions

  • 1993–2013
    • Texas A&M University
      • Department of Biology
      College Station, Texas, United States
  • 1981–1984
    • University of California, Berkeley
      • Department of Integrative Biology
      Berkeley, California, United States
  • 1979
    • CSU Mentor
      Long Beach, California, United States