Leptin constrains acetylcholine-induced insulin secretion from pancreatic islets of ob/ob mice.

Hypersecretion of insulin from the pancreas is among the earliest detectable metabolic alterations in some genetically obese animals including the ob/ob mouse and in some obesity-prone humans. Since the primary cause of obesity in the ob/ob mouse is a lack of leptin due to a mutation in the ob gene, we tested the hypothesis that leptin targets a regulatory pathway in pancreatic islets to prevent hypersecretion of insulin. Insulin secretion is regulated by changes in blood glucose, as well as by peptides from the gastrointestinal tract and neurotransmitters that activate the pancreatic islet adenylyl cyclase (e.g., glucagon-like peptide-1) and phospholipase C (PLC) (e.g., acetylcholine) signaling pathways to further potentiate glucose-induced insulin secretion. Effects of leptin on each of these regulatory pathways were thus examined. Leptin did not influence glucose or glucagon-like peptide-1-induced insulin secretion from islets of either ob/ob or lean mice, consistent with earlier findings that these regulatory pathways do not contribute to the early-onset hypersecretion of insulin from islets of ob/ob mice. However, leptin did constrain the enhanced PLC- mediated insulin secretion characteristic of islets from ob/ob mice, without influencing release from islets of lean mice. A specific enhancement in PLC-mediated insulin secretion is the earliest reported developmental alteration in insulin secretion from islets of ob/ob mice, and thus a logical target for leptin action. This action of leptin on PLC-mediated insulin secretion was dose-dependent, rapid-onset (i.e., within 3 min), and reversible. Leptin was equally effective in constraining the enhanced insulin release from islets of ob/ob mice caused by protein kinase C (PKC) activation, a downstream mediator of the PLC signal pathway. One function of leptin in control of body composition is thus to target a PKC-regulated component of the PLC-PKC signaling system within islets to prevent hypersecretion of insulin.

Enhanced sensitivity of pancreatic islets from preobese 2-week-old ob/ob mice to neurohormonal stimulation of insulin secretion.

Insulin secretion from perifused islets of preobese, 2-week-old, genetically obese (ob/ob) mice and their lean littermates was examined to identify early-onset abnormalities in regulation of insulin secretion by ob/ob mice. The ob/ob mice were slightly hyperinsulinemic (+20%) and hypoglycemic (-12%) at 2 weeks of age. Pancreatic islet size, DNA content, and insulin content were similar in ob/ob and lean mice. The responsiveness of islets to glucose, as determined by 20 mM glucose-induced insulin secretion, and the sensitivity of islets to glucose, as determined by the glucose threshold for insulin secretion, were unaffected by phenotype, but two insulin secretagogues that potentiate glucose-induced insulin secretion via activation of the phospholipase-C signal transduction pathway (i.e. acetylcholine, and cholecystokinin) were more effective in stimulating insulin secretion from islets of ob/ob mice than from islets of lean mice. Both responsiveness and sensitivity to acetylcholine and cholecystokinin potentiation of glucose-induced insulin secretion were enhanced in islets from ob/ob mice. Further, glucose-dependent insulinotropic polypeptide, which stimulates glucose-induced insulin secretion via activation of adenylate cyclase, interacted with acetylcholine to further augment differences in insulin secretion between islets from ob/ob and lean mice. The signal transduction pathway common to acetylcholine and cholecystokinin, and cross-talk between this pathway and the glucose-dependent insulinotropic polypeptide signal transduction pathway are loci for early-onset defects in control of insulin secretion from islets of ob/ob mice.

Opiates and medial hypothalamic knife cuts cause hyperphagia through different mechanisms.

Several experiments were performed to determine whether the hyperphagia caused by medial hypothalamic knife cuts and that induced by opiate agonists are mediated by a common mechanism. In the first set of experiments, male Sprague-Dawley rats were given bilateral parasagittal medial hypothalamic knife cuts or a sham procedure and fed a high-fat Crisco-chow diet. Knife-cut and sham-operated rats were approximately equally sensitive to the suppressive effects of naloxone on food intake. The kappa opiate receptor agonist ketocyclazocine generally increased daytime food intake in sham-operated rats; in contrast, the normal hyperphagia of knife-cut rats was in most cases either unchanged or decreased by ketocyclazocine. In a second set of experiments, neither diet composition nor hypothalamic knife cuts significantly affected the feeding responses to naloxone or the stimulatory effects of the kappa agonist butorphanol tartrate. It was hypothesized that the differential effects of ketocyclazocine in knife-cut and sham-operated rats are a consequence of the sedative effects of the drug combined with the elevated baseline of the knife-cut subjects. This hypothesis was supported by a third experiment, in which ketocyclazocine also reduced nocturnal intake in unoperated rats and butorphanol increased intake. That feeding responses to naloxone and butorphanol were essentially unchanged by hypothalamic knife cuts suggests that the opioid feeding system is independent of the longitudinal feeding inhibitory pathway believed to be involved in knife-cut-induced hyperphagia.

Cold acclimation of obese (ob/ob) mice: effects of energy balance.

Obese (ob/ob) and lean mice at 4 weeks of age were housed at 23 degrees C or 14 degrees C for 4 to 8 weeks to examine effects of acclimation to mild cold on energy balance. Energy intake of young lean mice increased by about 50% when housed at 14 degrees C, but energy intake of cold-acclimated obese mice increased by only 8%. Efficiency of energy retention (ratio of energy gain to energy intake) in obese mice declined from 22% +/- 1.2% at 23 degrees C to 10% +/- 1.8% after 4 weeks at 14 degrees C. Lean mice exhibited a less pronounced response to temperature; their efficiency of energy retention declined from 7% +/- 1.3% at 23 degrees C to 4% +/- 2.2% after 4 weeks at 14 degrees C. After 8 weeks of cold exposure, body weights and efficiency of energy retention became equal in obese and lean mice. Calculated heat production of cold-acclimated obese and lean mice was 40% higher than that of respective controls. Obese mice reacclimated to 23 degrees C after being kept for 4 weeks at 14 degrees C consumed the same amount of energy and were 16% more efficient than obese maintained at 23 degrees C; reacclimated lean mice consumed 12% more energy but were 53% less efficient than lean mice maintained at 23 degrees C. The results indicate that obese mice are able to increase heat production and markedly reduce their efficiency of energy retention when acclimated to mild cold but that they, unlike lean mice, rapidly revert to a high efficiency of energy retention after 4 weeks of reacclimation to 23 degrees C.

Cold acclimation of obese (ob/ob) mice: effects on skeletal muscle and bone.

Obese (ob/ob) and lean mice at 4 weeks of age were housed at 23 degrees C or 14 degrees C for 4 or 8 weeks to determine effects of acclimation to mild cold on the growth of skeletal muscle, bone, and fat. Body weights at 12 weeks of age averaged 48 +/- 0.6 g and 27 +/- 1.9 g for obese mice housed at 23 degrees C and 14 degrees C and 29 +/- 0.5 g and 26 +/- 0.6 g and 26 +/- 0.6 g for lean mice housed at 23 degrees C and 14 degrees C, respectively. At 23 degrees C, muscle weights of obese mice were approximately 60% of those in lean mice. Muscles of obese mice did not grow during the first 4 weeks at 14 degrees C (4 to 8 weeks of age) but did show a small gain during the second 4 weeks (9 to 12 weeks of age) at 14 degrees C. As a result, by the end of 8 weeks at 14 degrees C, muscles of obese mice weighed only 35% to 45% as much as muscles of lean mice. Growth of the tibia and femur followed the same pattern as the muscles. Obese mice housed at 23 degrees C from 4 to 12 weeks of age contained about six times as much fat as lean mice at this age. Although exposure to 14 degrees C for 8 weeks depressed the accumulation of fat in obese mice, they still contained approximately three times the percentage body fat as lean counterparts.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of thyroxine on Na+,K+-ATPase and norepinephrine turnover in brown adipose tissue of obese (ob/ob) mice.

Thyroxine treatment improves some of the defective thermogenic properties of obese (ob/ob) mice. Because brown adipose tissue (BAT) is an important thermogenic organ in mice, effects of thyroxine treatment on Na+, K+-ATPase, a thyroid-hormone responsive enzyme, and on rates of norepinephrine (NE) turnover, an indicator of sympathetic nervous system activity, in BAT of lean and obese mice were evaluated. Female mice, six weeks old, were injected with approximately 4 micrograms thyroxine daily for 2 weeks. Numbers of Na+, K+-ATPase enzyme units in BAT were similar in control lean and obese mice. Thyroxine treatment increased numbers of Na+, K+-ATPase enzyme units by 60% and 100% in lean and obese mice, respectively, indicating that the BAT of obese mice was responsive to thyroxine treatment. Fractional rates of NE turnover were 75% faster in BAT of control lean mice than in obese mice. Thyroxine treatment decreased functional rates of NE turnover in BAT of lean mice by 35%, but had no effect on NE turnover in BAT of obese mice. Rates of NE turnover in heart and pancreas of control lean and obese mice were unaffected by phenotype. Although the decreases in fractional rates of NE turnover in heart (-23%) and pancreas (-11%) of lean mice in response to thyroxine injections were not statistically significant, the calculated rates of NE turnover (fractional rate of NE turnover times the NE content of the organ) in these organs of lean mice were decreased 25% to 30% (P less than 0.05) in response to thyroxine. Thyroxine injections did not affect NE turnover in either heart or pancreas of obese mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of dietary carbohydrate, fat, and protein on norepinephrine turnover in rats.

To investigate effects of diet composition on rates of norepinephrine (NE) turnover in sympathetically innervated organs, weaning rats were fed for 2 to 21/2 weeks diets varying in carbohydrate (74.2% to 7.4% of total metabolizable energy) and fat (5.2% to 72.0%), or diets varying in protein (9.9% to 39.6%) and carbohydrate (77.8% to 48.1%). Changing the proportions of carbohydrate and fat in the diet, while maintaining similar intakes of energy and all other essential nutrients did not affect rates of NE turnover in heart, white adipose tissue (WAT), liver or pancreas and only minimally affected NE turnover in interscapular brown adipose tissue (IBAT). Decreasing the proportion of protein in the diet from 39.6% to 9.9% accelerated rats of NE turnover in heart (52%), IBAT (20%), WAT (42%), and liver (37%). When rats fed a diet containing 19.8% protein were also given a 10%(wt/vol) sucrose solution to drink for three days, their rates of NE turnover increased in heart (45%), IBAT (17%), liver (71%), and pancreas (55%). This response to sucrose depended on the protein content of the diet, since rats fed a 9.9% protein diet in which rates of NE turnover was already accelerated had no further increase in NE turnover when given the sucrose solution to drink. These data demonstrate that diet composition can affect activity of the sympathetic nervous system, as indicated by changes in rates of NE turnover. Changing the proportion of protein in the diet was more effective in altering NE turnover than changing the proportion of carbohydrate or fat.

Effects of a warm environment on energy balance in obese (ob/ob) mice.

Low rates of thermoregulatory heat production associated with low metabolic activity of brown adipose tissue, the primary site of thermoregulatory heat production, contribute substantially to the high efficiency of energy retention in obese (ob/ob) mice housed at 20 degrees C to 28 degrees C. To eliminate the need for thermoregulatory heat production lean and ob/ob mice were housed at 34.5 degrees C. At this temperature ob/ob mice still retained energy with a greater efficiency than lean littermates. Next, we investigated the possibility that the high efficiency of energy retention in ob/ob mice housed at 34.5 degrees C was related to depressed dietary-induced thermogenesis associated with low metabolic activity of brown adipose tissue. The sympathetic nervous system is a primary regulator of brown adipose tissue metabolism. Therefore, rates of norepinephrine (NE) turnover in brown adipose tissue, as an indicator of sympathetic nervous system stimulation of the tissue, were measured. Lean and ob/ob mice housed at 34.5 degrees C had equally low rates of NE turnover in their brown adipose tissue. Thus, the high efficiency of energy retention in ob/ob mice maintained at 34.5 degrees C is caused by factors other than low sympathetic stimulation of brown adipose tissue.

Functional correlates of Na+,K+-ATPase in lean and obese (ob/ob) mice.

Concentrations of Na+,K+-ATPase enzyme units are lower in skeletal muscle and liver of adult obese (ob/ob) mice than in their lean counterparts. The present studies were designed to provide information on functional correlates of Na+,K+-ATPase in ob/ob mice. Obese mice had lower potassium (K+) content in muscle and liver and higher sodium (Na+) content in muscle and liver and higher sodium (Na+) content in muscle than lean counterparts. The calculated intracellular Na+/K+ ratio in muscle of obese mice was approximately twice as high as in muscle of lean mice. Oxygen consumption was measured in mice maintained at 14 degrees, 25 degrees, or 33 degrees for 40 min and injected with 0.3 or 0.9 microgram ouabain per g body weight. Ouabain, a specific inhibitor of Na+,K+-ATPase, decreased oxygen consumption less in obese mice (12%--25%) than in lean mice (19%--38%). These results suggest that Na+ pump activity may be reduced in obese mice.

Effects of cimaterol, a beta-adrenergic agonist, on lipid metabolism in rats.

Rats fed a high carbohydrate diet containing 10 or 100 ppm cimaterol for 4 weeks gained 41% to 59% less fat and 70% to 76% more protein than controls, with no major changes in either energy gain or efficiency of energy retention. Effects of cimaterol on lipid metabolism in these rats were assessed. Cimaterol stimulated lipolysis in vivo and in vitro, but failed to influence rates of de novo fatty acid synthesis in either liver or white adipose tissue. Activities of fatty acid synthetase and malic enzyme in these tissues were also unaffected by cimaterol. Cimaterol administered in vivo failed to affect lipoprotein lipase activity in white adipose tissue, but elevated enzyme activity 67% to 75% in the extensor digitorium longus muscle. Lipoprotein lipase activity in the extensor digitorum longus muscle was also elevated by 66% during a 2 hour incubation with 1 mmol/L cimaterol. We conclude that cimaterol selectively stimulates both lipolysis in white adipose tissue and lipoprotein lipase activity in skeletal muscle, to direct energy away from adipose tissue deposition toward skeletal muscle accretion.

Effects of cimaterol, a beta-adrenergic agonist, on protein metabolism in rats.

Fractional accretion rates of total body 3-methylhistidine containing proteins (actin and myosin) were elevated 40% to 120% in rats fed a high-carbohydrate diet containing 10 or 100 ppm cimaterol for 1 week. Fractional degradation and fractional synthesis rates of these proteins were examined by measuring total body 3-methylhistidine content and urinary excretion of 3-methylhistidine. Consumption of the diet containing 100 ppm cimaterol for 1 week caused a 25% reduction in fractional degradation rates and a concomitant 32% increase in fractional synthesis rates of 3-methylhistidine containing proteins. Effects of cimaterol on fractional accretion, degradation, and synthesis rates of 3-methylhistidine containing proteins diminished after 1 week. Cimaterol failed to influence plasma insulin, triiodothyronine, or corticosterone concentrations. The dramatic increase in accretion of 3-methylhistidine containing proteins observed during the first week rats are fed diets containing cimaterol is caused by reciprocal action on protein degradation and synthesis.

Impaired thermoregulation in cold-exposed rats with hypothalamic obesity.

Rats with obesity-producing, hypothalamic knife cuts were fed a high fat diet and placed in the cold (2 degrees C) for six days starting 3, 11, or 24 days after surgery. Between surgery and cold exposure, knife-cut rats consumed 90% to 122% more energy and gained more weight (32 +/- 4, 112 +/- 5, and 241 +/- 9 g) than sham-operated rats (15 +/- 2, 34 +/- 2, and 58 +/- 3 g). When exposed to cold, sham-operated rats increased (22% to 30%) energy intake whereas knife-cut rats decreased (5% to 51%) intake. After 24 hours at 2 degrees C body temperatures of knife-cut rats were 1.2, 0.7, and 0.7 degrees less than those of control rats; body temperatures continued to decrease to 2.9, 3.0 and 2.5 degrees less than control rats after six days at 2 degrees C. Fasting for 12 hours at 2 degrees C caused a further reduction in body temperature to 4.9, 4.8, and 5.9 degrees less than in control rats. Cold exposure increased urinary excretion of norepinephrine and epinephrine (indicators of sympathoadrenal activity) in all rats. Guanosine diphosphate (GDP) binding to brown adipose tissue (BAT) mitochondria (an indicator of the thermogenic capacity of the tissue) was similar in cold-exposed, knife-cut, and sham-operated rats. Cold acclimation before hypothalamic knife-cut surgery prevented the cold-induced decrease in body temperatures of knife-cut rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevated neuronal c-Fos-like immunoreactivity and messenger ribonucleic acid (mRNA) in genetically obese (ob/ob) mice.

Adult genetically obese (ob/ob) mice display a number of metabolic alterations, the primary cause of which may be a defect in their central nervous system (CNS). The protein encoded by the protooncogene c-fos, c-Fos, functions as a nuclear transcription factor, and also serves as a marker of neuronal activity. The specific objectives of this study were (1) to use c-Fos immunohistochemistry to identify regions with altered neuronal activity in 6-7 week old male lean and ob/ob mice; (2) to examine c-fos relative mRNA abundance by northern blot analysis in brains of these mice and compare it with that of neuropeptide Y (NPY), a peptide well known to alter feeding and (3) determine changes in c-Fos immunoreactivity and mRNA caused by food deprivation. Fos-like immunoreactivity (FLI) tended to be higher in ad libitum fed ob/ob mice than in lean controls in most brain regions examined. The most prominent and consistent differences were in the paraventricular nuclei (PVN) where the numbers of Fos-positive nuclei were approximately 3 fold higher in ob/ob mice. Food deprivation for 24 h increased FLI in the PVN in lean mice but did not further augment FLI in the PVN of ob/ob mice. Arcuate nuclei of lean and ob/ob mice showed minimal FLI staining under ad libitum fed conditions. Food deprivation however, induced FLI in arcuate nuclei of both lean and ob/ob mice. The abundance of c-fos mRNA in whole brain of ob/ob mice averaged several fold higher than in leans under both fed and fasted conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Leptin-deficient mice commence hypersecreting insulin in response to acetylcholine between 1 and 2 weeks of age.

Leptin-deficient Lep(ob)/Lep(ob)mice develop hyperinsulinemia early in life, before they begin to overeat or develop insulin resistance. Pancreatic islets from these young mice do not yet hypersecrete insulin in response to glucose, but they hyperrespond to acetylcholine. Islets from 4-day, and 1-, 2-, and 4-week-old mice were used in the present study to determine when leptin-deficient mice first hypersecrete insulin in response to acetylcholine. This relative hypersecretion of insulin from islets of leptin-deficient mice occurred between 1 and 2 weeks of age. The divergence in insulin secretion occurred at this time because islets from lean, leptin-sufficient mice became relatively less responsive to acetylcholine between 1 and 2 weeks of age, whereas islets from leptin-deficient mice maintained a high responsiveness to acetylcholine during development. Leptin addition to islets isolated from 4-day, and 2-, and 4-week-old leptin-deficient mice rapidly (i.e., within 30 min) suppressed acetylcholine-induced insulin secretion at each stage of development. In contrast, islets from 4-day, and 2- and 4-week-old leptin-sufficient mice became progressively less responsive to leptin with development. Leptin targets pancreatic islets early in development to specifically constrain the overall capacity for acetylcholine-induced insulin secretion, and to acutely modulate this secretion.

Reduced sympathetic nervous system activity in rats with ventromedial hypothalamic lesions.

To determine if alterations in sympathetic nervous system (SNS) activity occur in rats with ventromedial hypothalamic (VMH) lesions, norepinephrine (NE) turnover rates were examined in various tissues of lesioned and control, weanling rats. VMH-lesioned rats fed a high-carbohydrate diet ad libitum for 4 weeks following surgery were not hyperphagic, but they gained 50% more body energy than control rats. VMH lesions extended the half-life of 3H-NE in interscapular brown adipose tissue (BAT) by 42%, in abdominal white adipose tissue (WAT) by 201%, in heart by 61% and in pancreas by 85%, and reduced total NE turnover (ng/organ/hr) in BAT (38%), WAT (57%), heart (30%) and pancreas (53%). Reduced SNS activity in BAT is consistent with the decreased energy expenditure (heat production) and increased energy efficiency observed in VMH-lesioned rats. In WAT, decreased SNS activity coupled with hyperinsulinemia would facilitate energy storage as fat by reducing lipid mobilization. In the pancreas, reduced SNS activity would contribute to hyperinsulinemia. These results support the hypothesis that VMH lesions decrease SNS activity in several organs. This change in autonomic tone is very likely a major factor in the development of obesity in VMH-lesioned animals.

Adrenalectomy increases norepinephrine turnover in brown adipose tissue of obese (ob/ob) mice.

The hyperphagia and rapid body weight gain normally observed in young obese (ob/ob) mice were abolished by removal of their adrenal glands, whereas food intake and weight gain of lean mice were not significantly affected by adrenalectomy. Adrenalectomy lowered body energy density (kcal/g carcass) in obese mice more than could be attributed to reduced food intake per se, suggesting that their energy expenditure was also increased. In control obese mice, low stimulation of brown adipose tissue by the sympathetic nervous system, as indicated by the low fractional rates of norepinephrine (NE) turnover in their brown adipose tissue may have contributed to the reduced energy expenditure in these animals. Adrenalectomy increased the rates of NE turnover in brown adipose tissue of obese mice to rates nearly equal to those observed in lean mice without affecting NE turnover in this tissue of lean mice. Likewise, removal of the adrenals normalized the low rates of NE turnover in hearts of obese mice without affecting lean mice. Rates of NE turnover in two other organs, white adipose tissue and pancreas, of control and adrenalectomized obese mice were similar to rates observed in lean counterparts. The adrenal may thus contribute to both the hyperphagia and the low energy expenditure by brown adipose tissue that together cause gross obesity in ob/ob mice.

Self-selected intake of carbohydrate, fat, and protein by obese (ob/ob) and lean mice.

Obese mice, self-selecting from three food cups containing carbohydrate, fat and protein supplemented with minerals and vitamins, selected the same proportion of energy from protein (approximately 20%) as did lean mice. Young obese mice also selected the same proportion of carbohydrate and fat as selected by lean mice, but adult obese mice self-selected a higher proportion of energy from fat and a lower proportion from carbohydrate than did lean mice. The high preference by adult obese mice for fat was confirmed in additional studies employing two food cups, one containing a high-carbohydrate diet and the other a high-fat diet. We speculate that the preference by adult obese mice for fat is related to their insulin resistance.

Specific inhibition of hepatic fatty acid synthesis exerted by dietary linoleate and linolenate in essential fatty acid adequate rats.

Dietary linoleate and linolenate were investigated for their ability to specifically inhibit liver and adipose tissue lipogenesis in meal-fed (access to food 900-1,200 hr), essential fatty acid (EFA) adequate rats. Supplementing a high carbohydrate diet containing 2.5% safflower oil with 3% palmitate 16:0, oleate 18:1, or linoleate 18:2 did not affect in vivo liver or adipose tissue fatty acid synthesis. However, 18:2 addition to the basal diet did result in a significant (P less than 0.05) decline of liver fatty acid synthetase (FAS) and glucose-6-phosphate dehydrogenase (G6PD) activities. When the safflower oil content of the basal diet was reduced to 1%, the addition of 3% 18:2 or linolenate 18:3 significantly (P less than 0.05) depressed hepatic FAS, G6PD, and in vivo fatty acid synthesis by 50%. Addition of 18:1 caused no depression in hepatic FAS activity but did result in a significant (P less than 0.05) decline in liver G6PD activity and fatty acid synthesis which was intermediate between basal and basal +18:2- or +18:3-fed animals. Adipose tissue rates of lipogenesis were completely unaffected by dietary fatty acid supplementation. Similarly, the addition of 3 or 5% 18:3 to a basal diet for only one meal resulted in no change in lipogenesis relative to that in animals fed the basal diet. The data indicate that, like rats fed EFA-deficient diets, dietary 18:2 and 18:3 exert a specific capacity to depress rat liver FAS and G6PD activities and rate of fatty acid synthesis.

Muscle growth and satellite cell proliferative activity in obese (OB/OB) mice.

Muscle growth of male obese (ob/ob) and lean mice at 2, 3, 5 and 8 wk were analyzed on the basis of weights of gastrocnemius, plantaris and soleus muscles from each hind leg. The carcasses (prepared by removing skin, viscera, head, feet and tail) were analyzed for fat content so that the effect of phenotype on the relationship between muscle weight and fat-free carcass weight could be assessed. For each age group the obese mice had less muscle relative to fat-free carcass weight than lean mice, with the difference being significant at 3 wk (P less than .05) and 8 wk (P less than .025). The proliferative activity of muscle satellite cells in 2- and 3-wk-old obese and lean mice was measured on isolated muscle fibers by autoradiography. Muscle fiber diameter and number of nuclei/unit length were unaffected by phenotype, but the proportion of muscle nuclei showing proliferative activity was lower (P less than .01) in obese than in lean mice at 2 wk (1.05 vs 1.93%, respectively) and 3 wk of age (.23 vs .59%, respectively). These results are consistent with the suggestion that muscle growth is limited by satellite-cell proliferative activity, although direct evidence for a cause and effect relationship is not provided.

Interrelationships of erythrocyte glutathione peroxidase system enzymes in the fasted Beagle.

Eight adult female Beagles were fasted for 21 days to investigate responses of the erythrocyte glutathione peroxidase system enzymes. Blood samples were collected once a week. Erythrocyte reduced glutathione was maintained within 10% of base-line value. Hexose monophosphate pathway dehydrogenases showed a significant inverse relationship to glutathione reductase (GR) activity. Further, active GR, as a percentage of total GR, increased as hexose monophosphate pathway dehydrogenase activity fell. A similar relationship between these enzymes has been reported in human glucose-6-phosphate dehydrogenase deficiency. Also, glutathione peroxidase showed an inverse linear response to erythrocytic reduced glutathione. Though the responses of this system are complex, they appear interrelated.