Interaction between Na+/K+-pump and Na+/Ca2+-exchanger modulates intercellular communication

Ouabain, a specific inhibitor of the Na(+)/K(+)-pump, has previously been shown to interfere with intercellular communication. Here we test the hypothesis that the communication between vascular smooth muscle cells is regulated through an interaction between the Na(+)/K(+)-pump and the Na(+)/Ca(2+)-exchanger leading to an increase in the intracellular calcium concentration ([Ca(2+)](i)) in discrete areas near the plasma membrane. [Ca(2+)](i) in smooth muscle cells was imaged in cultured rat aortic smooth muscle cell pairs (A7r5) and in rat mesenteric small artery segments simultaneously with force. In A7r5 coupling between cells was estimated by measuring membrane capacitance. Smooth muscle cells were uncoupled when the Na(+)/K(+)-pump was inhibited either by a low concentration of ouabain, which also caused a localized increase of [Ca(2+)](i) near the membrane, or by ATP depletion. Reduction of Na(+)/K(+)-pump activity by removal of extracellular potassium ([K(+)](o)) also uncoupled cells, but only after inhibition of K(ATP) channels. Inhibition of the Na(+)/Ca(2+)-exchange activity by SEA0400 or by a reduction of the equilibrium potential (making it more negative) also uncoupled the cells. Depletion of intracellular Na(+) and clamping of [Ca(2+)](i) at low concentrations prevented the uncoupling. The experiments suggest that the Na(+)/K(+)-pump may affect gap junction conductivity via localized changes in [Ca(2+)](i) through modulation of Na(+)/Ca(2+)-exchanger activity.     

Comparison of the effects of DHEA and food restriction on serum calcitonin

In the first of two studies, female Wistar rats were fed ad libitum or 60% of the ad libitum intake. In the second study, female Sprague Dawley rats were given subcutaneous injections of DHEA (2-4 mg/day) five times per week or received similar volumes of the solvent vehicle. Animals in both studies were maintained on their respective regimens for six months. At the termination of the study, the food restricted animals weighed significantly less than the animals fed ad libitum; in addition, their serum calcitonin concentration was markedly lower and was over 60% less than that of the ad libitum fed animals. In contrast, DHEA treatment had no significant effect on the body weight or on the plasma calcitonin of the Sprague Dawley rats. Since food restriction maintains calcitonin concentrations toward youthful levels, it is clear that at least one of the anti-aging effects of food restriction is not mediated by DHEA.     

Lifelong dietary modulation of calcitonin levels in rats

Studies were carried out on specific pathogen-free rats to evaluate the effects of aging and dietary manipulation on serum and thyroid calcitonin (CT) levels. Male Fischer 344 rats were randomized at 6 weeks of age to six dietary groups and subsequently maintained on the following dietary regimens. Group 1 rats were fed ad libitum throughout life; group 2 rats were fed 60% of the ad libitum food uptake, but received the same amounts of calcium, phosphorus, and vitamin D; group 3 rats were fed as the group 2 animals until 6 months of age and from then on were fed ad libitum; group 4 rats were fed ad libitum until 6 months of age and then switched to 60% food restriction; group 5 rats were fed ad libitum on food isocaloric with that of group 1 rats, but containing only 60% of the protein. Group 6 rats were killed at 6 weeks of age to serve as baseline controls. Ten rats were killed in each of the remaining five groups 15 h postprandial at 6-month intervals. The following observations were made. Serum CT increased with age similarly in the ad libitum fed group 1 and 5 rats. Food restriction markedly inhibited the increase in serum CT, and the effect was more profound in animals whose food intake was restricted after 6 months of age (group 4) than in animals on lifelong food restriction (group 2). In rats switched from food restriction to ad libitum feeding (group 3) at 6 months of age, serum CT increased with age to levels identical with those of lifelong ad libitum fed group 1 animals. Thyroid CT showed a similar pattern of age-dependent and dietary modulated changes. In contrast, aging and dietary modulation had no appreciable effect on serum calcium levels, except at 27 months of age when the serum calcium level of group 1 animals increased dramatically from the level for 24-month-old animals. There was a weak positive correlation between serum calcium and serum CT (r = 0.627; P = 0.02) and a highly significant positive correlation between serum CT and thyroid CT (r = 0.917; P = 0.001). These findings indicate that elective and therapeutic restriction of food intake might also attenulate CT levels in humans, with potentially adverse implications for skeletal homeostasis.     

Modulation of age-related hyperparathyroidism and senile bone loss in Fischer rats by soy protein and food restriction

Studies were carried out to explore the influence of soy protein and food restriction on age-related changes in serum PTH and bone. Three groups of male Fischer 344 rats were studied from 6 weeks of age. Group A rats were fed ad libitum diet A, which has casein as the protein source. Group B rats were fed diet B (with casein as protein source) at 60% of the mean ad libitum food intake. Group C rats were fed ad libitum diet C, which has soy protein as the protein source. The animals were killed at periodic intervals beginning at 6 months of age after an overnight fast. Serum PTH, measured with an intact N-terminal-specific RIA, and immunoreactive calcitonin increased progressively with aging. The increase was markedly suppressed by food restriction, and in the case of PTH by the soy protein diet as well. Serum creatinine started to increase after 18 months of age, and both dietary regimens of groups 2 and 3 retarded the increase. Aging was associated with a fall in serum 25-hydroxyvitamin D, and loss of bone occurred during the terminal part of life in the ad libitum-fed animals. These were prevented by food restriction, while the soy protein diet delayed the onset of bone loss. We conclude from these findings and other data from this study that in the male F344 rats 1) an age-related increase in serum PTH precedes an age-related increase in serum creatinine concentration; 2) an age-related decline in renal function probably contributes to age-related hyperparathyroidism, which, in turn, contributes to senile bone loss; 3) food restriction inhibits age-related hyperparathyroidism and senile bone loss; 4) on the basis of the data from rats fed a soy protein-containing diet, a decline in renal function and progressive hyperparathyroidism are not inevitable consequences of aging in the ad libitum fed rats.     

Aging and dietary modulation of rat skeleton and parathyroid hormone

Studies were carried out on SPF F344 male rats to evaluate the effects of aging and life-prolonging food restriction, without malnutrition, on rat skeleton and circulating PTH. Six-week-old F344 rats were divided into five groups. Group 1 rats were fed ad libitum a diet that contained 21% protein. Group 2 rats were fed 60% of the mean food intake of group 1 rats from 6 weeks of age for the rest of their lives. Group 3 rats were fed 60% of the ad libitum food intake until 6 months of age and then switched to ad libitum feeding. Group 4 rats were fed ad libitum until 6 months of age, and then switched to 60% of the ad libitum food intake. Group 5 rats were fed ad libitum a diet that contained only 12.6% protein so that these animals ingested the same amount of protein per day as the group 2 rats. In group 1 animals, bone length, weight, density, and calcium content increased rapidly with age and plateaued at about 12 months of age. There was no evidence of bone loss in these animals until about 24 months of age, but by 27 months, the animals had lost appreciable amounts of bone. The circulating immunoreactive PTH levels of the animals increased with advancing age, with a marked rise at 27 months. The age-related changes in bone and serum PTH levels of rats in groups 3 and 5 were similar to those of group 1 animals, except that a terminal increase in serum PTH did not occur in group 5 rats. In the groups 2 and 4 animals which were food restricted for the longest period, bone growth and maturation were slowed down, but the animals did not experience senile bone loss or marked terminal increase in circulating PTH. The salutary effects of food restriction were, therefore, not due specifically to the restriction of protein intake or to restricting food intake only during the period of rapid growth.     

Prolactin increases Na+ transport across adult bullfrog skin via stimulation of both ENaC and Na+/K+-pump

PRL is involved in osmoregulation in lower vertebrates. Its serum concentration starts to increase during the metamorphosis of bullfrog tadpoles. Adult bullfrog skin transports Na(+) from the apical to the basolateral side across the skin. PRL is involved in the regulation of this transport. We investigated the effect of ovine PRL on the epithelial Na(+) channel (ENaC), Na(+)/K(+)-pump, and basolateral K(+) channels, which regulate Na(+) transport across adult bullfrog skin, by measuring the short-circuit current (SCC). At 0.1 microg/ml, PRL had no effect on the SCC. PRL (1 microg/ml) was sufficient to stimulate the SCC since 1 and 10 microg/ml of PRL each increased SCC 1.8-fold. Current-fluctuation analysis revealed that PRL (10 microg/ml) increased the density of active ENaC almost 1.8-fold. The effect of PRL on the Na(+)/K(+)-pump was investigated using apically nystatin-permeabilized skin with Ca-free Na-Ringers' solution on each side. PRL (10 microg/ml) increased SCC in this condition around 1.1-fold, suggesting that PRL stimulates the Na(+)/K(+)-pump [although PRL (1 microg/ml) had no effect on this SCC]. The effect of PRL on basolateral K(+) channels was investigated using apically nystatin-permeabilized skin with high-K Ringer's solution on the apical side. PRL (10 microg/ml) had no effect on the SCC, suggesting that PRL does not affect basolateral K(+) channels. Thus, although PRL stimulates the Na(+)/K(+)-pump, this effect probably contributes less than that on ENaC to the regulation of Na(+) transport across adult bullfrog skin.     

The source of leptin, but not leptin depletion in response to food restriction, changes during early pregnancy in mice

Maternal food restriction during pregnancy results in adverse consequences for offspring, including obesity and cardiovascular disease. Early pregnancy is a critical period for this programming effect. Leptin is a regulator of energy homeostasis that also affects placental and fetal development. As food restriction results in decreased serum leptin levels, at least in non-pregnant animals, leptin depletion may be one mechanism by which food restriction affects development. The objective of this study was to test whether moderate food restriction affects serum leptin concentrations during the first half of pregnancy. We found that restriction to 50% of ad libitum consumption levels resulted in a significant decrease in serum leptin concentrations in both pregnant and non-pregnant female mice. There was no significant difference in serum leptin concentrations between non-pregnant females and at pregnancy day 11.5 when fed ad libitum. However, there was a difference in the source of leptin during pregnancy, with greater production in visceral fat in pregnant mice, and greater production in subcutaneous fat in non-pregnant mice. Leptin concentrations were dependent on time of day and time of sampling relative to feeding, particularly in restricted mice. There was a significant difference in serum leptin concentrations between fed and restricted mice when they were fed and sampled in afternoon, but not when they were fed and sampled in morning. We conclude that food restriction results in a significant decrease in leptin concentration during the first half of pregnancy in mice, but that detection of this relationship is subject to experimental design considerations.     

Assessment of the role of the glucocorticoid system in aging processes and in the action of food restriction

The Glucocorticoid Cascade Hypothesis of Aging and the hypothesis that food restriction retards the aging processes by preventing the development with age of hyperadrenocorticism were investigated. A longitudinal life span study of the daily concentration pattern of plasma corticosterone was conducted in male Fischer 344 rats fed ad libitum or restricted to 60% of the mean food intake of ad libitum fed rats. In another group of ad libitum fed and food-restricted rats, the influence of age on the response of plasma corticosterone levels to restraint stress was measured as was the time course of the return of plasma corticosterone to basal levels following the stress. The findings do not support the hypothesis that food restriction retards the aging processes by preventing the development of hyperadrenocorticism with advancing age. They also indicate that the Glucocorticoid Cascade Hypothesis does not describe a major aspect of the aging processes. Rather, the results suggest the possibility that a lifetime of daily periods of mild hyperadrenocorticism may, if anything, retard the aging processes.     

Insulin- and glucose-induced phosphorylation of the Na(+),K(+)-adenosine triphosphatase alpha-subunits in rat skeletal muscle.

Phosphorylation of the alpha-subunits of Na(+),K(+)-adenosine triphosphatase in response to insulin, high extracellular glucose concentration, and phorbol 12-myristate 13-acetate was investigated in isolated rat soleus muscle. All three stimuli increased alpha-subunit phosphorylation approximately 3-fold. Phorbol 12-myristate 13-acetate- and high glucose-induced phosphorylation of the alpha-subunit was completely abolished by the PKC inhibitor GF109203X, whereas insulin-stimulated phosphorylation was only partially reduced. Notably, insulin stimulation resulted in phosphorylation of the alpha-subunit on serine, threonine, and tyrosine residues, whereas high extracellular glucose or phorbol 12-myristate 13-acetate stimulation mediated phosphorylation only on serine and threonine residues. Insulin stimulation resulted in translocation of Na(+),K(+)-adenosine triphosphatase alpha(2)-subunit to the plasma membrane and increased Na(+),K(+)-adenosine triphosphatase activity in the same membrane fraction. High glucose had no effect on alpha-subunits distribution. Immunoprecipitation with antiphosphotyrosine antibody and subsequent Western blot analysis with anti-alpha(1)- and -alpha(2)-subunit antibodies revealed that both alpha(1)- and alpha(2)-subunit isoforms underwent phosphorylation on tyrosine residues in response to insulin, although with different time course and magnitude. Thus, we show that insulin-stimulated phosphorylation of Na(+),K(+)-adenosine triphosphatase alpha-subunit occurs via a PKC- and tyrosine kinase-dependent mechanism, whereas high glucose-induced phosphorylation is only PKC-dependent. Phosphorylation of Na(+),K(+)-adenosine triphosphatase alpha-subunits may be involved in regulation of Na(+),K(+)-adenosine triphosphatase activity by insulin or high extracellular glucose in skeletal muscle.     

Inhibitory effect of hypophysectomy and food restriction on glomerular basement membrane thickening, proteinuria and renal enlargement in aging male Wistar rats

Age-related thickening of the glomerular basement membrane (GBM) was studied in three groups of male Wistar rats: (a) ad libitum fed, (b) hypophysectomized and (c) food-restricted eating the same amount of food as hypophysectomized rats, but about 45% of the ad libitum fed group. Studies were begun at 50 days (2 months) and continued throughout life. Multiple regression was used to statistically assess the effects of age and treatments. In ad libitum fed male rats GBM thickness increased from 114 nm at 50 days (2 months) to 632 nm at 1,000 days (33 months). GBM thickness at 1,000 days was 296 nm in hypophysectomized rats and 392 nm in food restricted rats. Hypophysectomy had a significantly greater inhibitory action on GBM thickening than food restriction, in rats eating the same quantity of food per day. However, a major part of the effect of hypophysectomy may be due to the permanent fall in food intake (from 16.3 to 7.9 g/day) resulting from the operation. Accompanying the age-related thickening of the GBM in ad libitum fed rats were proteinuria and renal enlargement, both of which were inhibited by hypophysectomy and food restriction.     

Effects of isolation and food restriction begun at 50 days on the development of age-associated renal disease in the male Wistar rat

The development of proteinuria with increasing age was studied in three groups of male Wistar rats: ad libitum fed and isolated, ad libitum fed and group housed 6 to 8 rats per cage, and food restricted (one-third of the isolated ad libitum food intake) and isolated. Studies were begun at age 50 days and continued throughout life. Ad libitum fed rats when isolated ate more food, grew faster, had larger maximum body weights and developed proteinuria at a faster rate than those that were group housed. There was a small increase in the severity of glomerular pathology in old age. However, systolic blood pressure was not affected significantly by isolation, nor was life duration. Food restriction of isolated rats inhibited body growth, prevented the development of proteinuria, reduced the incidence of glomerular and tubular pathology in old age and prolonged life. Electron microscopic examination of the kidneys of old food-restricted rats revealed a much lower incidence of foot process retraction and spreading on the basement membrane of the glomerulus than in ad libitum fed rats. Cardiac enlargement was also prevented by long-term food restriction.     

Mechanism of Thyroid Calorigenesis: Role of Active Sodium Transport

The hypothesis that thyroid calorigenesis is mediated by stimulation of active Na(+) transport was tested by measuring the Q(o2) of liver slices and skeletal muscle (diaphragm) from thyroxine- and triiodothyronine-injected thyroidectomized and normal rats in media fortified with ouabain (10(-3) M) and/or free of Na(+) or K(+). In both tissues, more than 90% of the increase in Q(o2) produced by injections of thyroid hormone in euthyroid rats was derived from increased energy utilization by the Na(+) pump. In triiodothyronine-treated thyroidectomized rats, activation of Na(+) transport accounted for 90% or more of the increment in Q(o2) in liver and 40% or more of the increment in diaphragm. Intracellular Na(+), K(+), and Cl(-) concentrations were measured in euthyroid and hyperthyroid liver and diaphragm. The transmembrane Na(+) and K(+) concentration differences were significantly increased in both tissues by the administration of triiodothyronine. These results indicate that thyroid hormone activates Na(+) extrusion and K(+) accumulation either by increasing the local concentration of ATP or by direct stimulation of the Na(+) pump.     

The effects of acute and life-long food restriction on basal and stress-induced serum corticosterone levels in young and aged rats

The effects of short term (1-month) and life-long 60% ad libitum food restriction on the adrenocortical response to restraint stress were compared in young and aged Fischer 344 rats. In rats restricted for 1 month (study 1), the adrenocortical response differed as a function of age. In 8-month-old animals, the initial steep rise in corticosterone in response to stress was of similar magnitude in ad libitum and restricted animals. In 23-month-old animals the corticosterone response was severely blunted in restricted animals. In life-long restricted animals (study 2), the corticosterone response to restraint stress was tested at 8, 16, and 24 months of age. The general pattern of response to stress in these animals was similar to that in study 1. The 16- and 24-month-old animals showed the same blunted response to stress found in the 23-month-old animals restricted for only 1 month, suggesting that the severe restriction per se and not life-long food restriction blunted the response to stress in aged animals. The similarity between the response to stress in study 1 and study 2 was evident even though animals were tested in one case before feeding when corticosterone levels were high, and in the other 4-5 h after feeding when corticosterone levels were lower. In study 3 it was found that in food-restricted young rats, the mean corticosterone level over a 24-h period was significantly elevated above that in ad libitum fed young rats. In aged rats, however, except before daily feeding, corticosterone levels of food-restricted rats remained significantly below those of ad libitum fed animals, whose levels were, in turn, significantly elevated compared to those of ad libitum fed young rats. These findings suggest that in aged animals severe food restriction reduces basal corticosterone levels, adrenal responsiveness to stress, and adrenal size and has the potential to protect against the consequences of high corticosterone levels in aging.     

Albumin-synthesizing capacity of hepatocytes isolated from rats fed diets differing in protein and energy content

The rate of albumin synthesis has been estimated in hepatocytes prepared from groups of rats maintained on diets of different protein content. These diets were fed either ad libitum or at 50% restriction of ad libitum consumption. The data show that the physiological capacity of hepatocytes to synthesize albumin varies with dietary intake. Albumin production by cells prepared from animals fed ad libitum was directly related to the protein energy:total energy ratio of the food. Restricting consumption of the control diet to 50% of ad libitum intake did not reduce albumin synthesis rates, and similar restriction of the low protein diets ameliorated some of the depression in albumin production observed in hepatocytes isolated from animals fed the same diets ad libitum. The results are discussed with reference to the occurrence of hypoalbuminaemia in children with protein-energy malnutrition.     

A Ca++-Dependent and -Selective Ionophore as Part of the Ca++ + Mg++-Dependent Adenosinetriphosphatase of Sarcoplasmic Reticulum

Solubilized Ca(++) + Mg(++)-dependent adenosinetriphosphatase (EC 3.6.1.3; ATP diphosphohydrolase) from sarcoplasmic reticulum increased bimolecular lipid membrane (oxidized cholesterol) conductance several hundred-fold. The relative conductance change and the relative permeability elicited by this material has the following sequence: Ba(++) > Ca(++) > Sr(++) > Mg(++) > Mn(++) > Zn(++), Na(+), K(+), Cs(+), Li(+), and Rb(+). Zn(++) and Na(+) strongly inhibit the increase in Ca(++) conductance obtained with solubilized Ca(++) + Mg(++)-dependent adenosinetriphosphatase. The Ca(++)-ionophore is an integral part of the Ca(++) + Mg(++)-dependent adenosinetriphosphatase enzyme and may function as a Ca(++)-carrier in the overall Ca(++)-pump of sarcoplasmic reticulum.     

C-Peptide stimulates Na<Superscript>+</Superscript>,K<Superscript>+</Superscript>-ATPase activity via PKC alpha in rat medullary thick ascending limb

AIMS/HYPOTHESIS: C-peptide, the cleavage product of proinsulin processing exerts physiological effects including stimulation of Na(+),K(+)-ATPase in erythrocytes and renal proximal tubules. This study was undertaken to assess the physiological effects of connecting peptide on Na(+),K(+)-ATPase activity in the medullary thick ascending limb of Henle's loop. METHODS: Na(+),K(+)-ATPase activity was measured as the ouabain-sensitive generation of (32)Pi from gamma[(32)P]-ATP and (86)Rb uptake on isolated rat medullary thick ascending limbs. The cell-surface expression of Na(+),K(+)-ATPase was evaluated by Western blotting of biotinylated proteins, and its phosphorylation amount was measured by autoradiography. The membrane-associated fraction of protein kinase C isoforms was evaluated by Western blotting. RESULTS: Rat connecting peptide concentration-dependently stimulated Na(+),K(+)-ATPase activity with a threshold at 10(-9) mol/l and a maximal effect at 10(-7) mol/l. C-peptide (10(-7) mol/l) already stimulates Na(+),K(+)-ATPase activity after 5 min with a plateau from 15 to 60 min. C-peptide (10(-7) mol/l) stimulated Na(+),K(+)-ATPase activity and (86)Rb uptake to the same extent, but did not alter Na(+),K(+)-ATPase cell surface expression. The stimulation of Na(+),K(+)-ATPase activity was associated with an increase in Na(+),K(+)-ATPase alpha-subunit phosphorylation and both effects were abolished by a specific protein kinase C inhibitor. Furthermore, connecting peptide induced selective membrane translocation of PKC-alpha. CONCLUSION/INTERPRETATION: This study provides evidence that in rat medullary thick ascending limb, C-peptide stimulates Na(+),K(+)-ATPase activity within a physiological concentration range. This effect is due to an increase in Na(+),K(+)-ATPase turnover rate that is most likely mediated by protein kinase C-alpha phosphorylation of the Na(+),K(+)-ATPase alpha-subunit, suggesting that C-peptide could control Na(+) reabsorption during non-fasting periods.     

Food restriction enhances the proteolytic capacity of the aging rat liver

The effects of age and food restriction upon the proteolytic capacity of the liver of SPF Fischer 344 rats have been investigated. The proteolytic capacity of the perfused liver of ad libitum fed animals increased from 3 to 6 months of age and then declined linearly through 24 months of age. Food restriction had no effect at 3 months of age (6 weeks of food restriction), but by 6 months of age the proteolytic capacity was approximately 25% greater than that of the liver of ad libitum fed animals; this increment was increased through 24 months of age. Therefore, food restriction, while not preventing the age-related decline in protein degradation, did produce a greater proteolytic capacity at any age from 6 months on.     

The effect of exercise training on insulin resistance in sedentary year old rats

The purpose of this study was to determine if exercise training of 12-month-old Sprague-Dawley rats could reverse the resistance to insulin-induced glucose uptake that has been shown to occur in these animals. Twelve-month-old rats were trained to run 1 1/2 miles/day in motorized exercise wheel cages, and the ability of insulin to stimulate glucose uptake in these rats was compared with values observed in two groups of similar aged sedentary rats--one fed rat chow ad libitum and the other a calorie-restricted diet for 4 months. Body weight increased and insulin-stimulated glucose uptake decreased as rats fed chow ad libitum grew from 12 to 16 months of age. In contrast, 4 months of either exercise training or calorie restriction prevented weight gain and loss of insulin-stimulated glucose uptake. Thus, the intensity of exercise training attained in this study did not result in an improvement in insulin action in older rats above and beyond that related to the reduction in rate of body weight gain.     

Maternal nutrition affects the ability of treatment with IGF-I and IGF-II to increase growth of the placenta and fetus, in guinea pigs

The aim of this study was to investigate how administration of IGF-I and IGF-II, during early to mid pregnancy, affects maternal growth and body composition as well as fetal and placental growth, in ad libitum fed, and in moderately, chronically food restricted guinea pigs. From day 20 of gestation, mothers (3-4 months old) were infused with IGF-I, IGF-II (565 microg/day) or vehicle for 17 days and then killed on day 40 of gestation. Maternal organ weights, fetal and placental weights were assessed. Treatment with IGFs did not alter body weight gain and had small effects on body composition in the mothers. Both IGF-I and IGF-II increased fetal and placental weights in ad libitum fed dams and IGF-I increased placental weight in food restricted dams. In conclusion, treatment with IGF-I during the first half of pregnancy stimulates placental growth in both ad libitum fed and food restricted guinea pigs without affecting maternal growth while fetal growth is stimulated by IGF treatment only in ad libitum fed animals.     

Effects of starvation in rats on serum levels of follicle stimulating hormone, luteinizing hormone, thyrotropin, growth hormone and prolactin; response to LH-releasing hormone and thyrotropin-releasing hormone.

Adult male Sprague-Dawley rats averaging 300 g each were subjected to complete food removal for 7 days (acutely starved), 7 days complete food removal followed by 2 weeks of 1/4 ad libitum food intake (chronically strved), 7 days complete food removal and 2 weeks of 1/4 ad libitum intake followed by ad libitum feeding for 7 days (refed), or fed ad libitum throughout (controls). Serum LH, FSH, TSH, PRL, and GH levels were measured by radioimmunoassays for each group of rats. The in vivo response to the combination of synthetic LHRH and TRH also was tested in each group of rats. Circulating LH, TSH, GH, and PRL were significantly depressed in acutely and chronically starved rats, and FSH was lowered only in acutely starved rats. After 7 days of refeeding, serum levels of LH and FSH were significantly greater than in ad libitum fed controls, PRL returned to control levels, and TSH and GH increased but were still below control levels. After LHRH + TRH injection serum LH and TSH were increased significantly in all groups of rats, FSH and PRL rose in acutely but not in chronically starved rats, and GH was not elevated in any group. The increases in serum LH, FSH, TSH and prolactin in response to LHRH + TRH injection in acutely or chronically starved rats were equal to or greater than in the ad libitum fed controls. These data indicate that severe reductions in food intake result in decreased release of at least 5 anterior pituitary hormones, and this is due primarily to reduced hypothalamic stimulation rather than to inability of the pituitary to secrete hormones.