Dale R. Romsos

Leptin alters metabolic rates before acquisition of its anorectic effect in developing neonatal mice

Leptin inhibits food intake and increases metabolic rates in adult mice. Neonatal mice need to maximize food intake and also maintain high thermoregulatory metabolic rates to optimize survival, suggesting that leptin may function differentially in neonatal versus adult animals. The efficacy of exogenous leptin to alter these two physiological functions during development was thus examined in C57BL/6J lean (+/+ or ob/+) and ob/ ob(leptin-deficient) mice. Intraperitoneal leptin administration (1 mg/kg body wt) to lean and ob/ obpups from 7 to 10 days of age did not affect milk intake, oxygen consumption, body weight, or epididymal fat pad weights. Intracerebroventricular injection of 1 μg leptin to 9-day-old pups also failed to influence milk intake or oxygen consumption. Because neither lean nor ob/ obpups responded to exogenous leptin, high endogenous plasma leptin concentrations per se in these lean mice do not explain their resistance to leptin. Leptin administered intracerebroventricularly also failed to alter milk/food intakes of 17-day-old pups but markedly increased oxygen consumption of these older mice. By 28 days of age, intracerebroventricular leptin inhibited food intake. The well-defined actions of leptin to reduce food intake and enhance metabolic rates do not develop synchronously. The ability of leptin to accelerate metabolic rates is acquired early in life and independent of its anorectic action, which may promote survival of neonates.Leptin inhibits food intake and increases metabolic rates in adult mice. Neonatal mice need to maximize food intake and also maintain high thermoregulatory metabolic rates to optimize survival, suggesting that leptin may function differentially in neonatal versus adult animals. The efficacy of exogenous leptin to alter these two physiological functions during development was thus examined in C57BL/6J lean (+/+ or ob/+) and ob/ob (leptin-deficient) mice. Intraperitoneal leptin administration (1 mg/kg body wt) to lean and ob/ob pups from 7 to 10 days of age did not affect milk intake, oxygen consumption, body weight, or epididymal fat pad weights. Intracerebroventricular injection of 1 microg leptin to 9-day-old pups also failed to influence milk intake or oxygen consumption. Because neither lean nor ob/ob pups responded to exogenous leptin, high endogenous plasma leptin concentrations per se in these lean mice do not explain their resistance to leptin. Leptin administered intracerebroventricularly also failed to alter milk/food intakes of 17-day-old pups but markedly increased oxygen consumption of these older mice. By 28 days of age, intracerebroventricular leptin inhibited food intake. The well-defined actions of leptin to reduce food intake and enhance metabolic rates do not develop synchronously. The ability of leptin to accelerate metabolic rates is acquired early in life and independent of its anorectic action, which may promote survival of neonates.

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.

Na+,K+-ATPase enzyme units in skeletal muscle from lean and obese mice

Abstract [ 3 H]-Ouabain binding to muscle preparations was utilized to estimate the number of Na + ,K + -ATPase enzyme units in hindlimbs from 8 week old lean and obese mice. Specific [ 3 H]-ouabain binding per mg particulate protein was 36% lower in obese mice; whereas, the affinity of the binding sites for ouabain was similar in obese and lean mice. Since obese mice had less muscle than lean mice, the number of Na + ,K + -ATPase enzyme units in hindlimbs from obese mice was less than half the number observed in lean mice.

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.

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.

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<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<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<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.

Influence of Dietary Safflower Oil and Tallow on Growth, Plasma Lipids and Lipogenesis in Rats, Pigs and Chicks

Summary Rats, chicks and pigs were fed diets containing safflower oil or tallow. Plasma triglyceride levels were elevated when tallow, rather than safflower oil was added to the diet of rats, unchanged in chicks and lowered when tallow, rather than safflower oil was fed to pigs. The rate of fatty acid synthesis in rat and chick liver was higher, whereas the rate of lipogenesis in adipose tissue preparations from rats and pigs was lower when tallow, rather than safflower oil was fed. These results indicate that there are species-specific, as well as organ-specific, metabolic responses to various dietary fats.

Effect of dietary fructose on in vitro and in vivo fatty acid synthesis in the rat

Abstract Diets containing either glucose or fructose as the only source of carbohydrate were fed to rats. [U- 14 C] Glucose and [U- 14 C] fructose incorporation into fatty acids by liver slices from these animals indicated that the results obtained were markedly affected by substrate concentration. At a level of 10 mM fructose incorporation into hepatic fatty acids was greater than that observed with glucose; however, the reverse was true when the substrate concentration was increased to 100 mM. High levels of fructose in the incubation medium markedly depressed ATP levels in liver slices and also inhibited [ 14 C]-acetate incorporation into fatty acids. Rats fed fructose had higher hepatic ATP levels than did rats fed glucose. In vivo estimates of fatty acid synthesis were obtained using [ 14 C] acetate as well as 3 H 2 . These determinations indicated that while the total capacity of the animal to synthesize fatty acids was unchanged, the relative importance of the liver was increased and that of the extra-hepatic tissues decreased when fructose, rather than glucose, was fed to rats.

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.

Oxygen Consumption and Body Fat Content of Young Lean and Obese (OB/OB) Mice

Summary Rates of oxygen consumption were determined daily from birth to 19 days of age and weekly thereafter until 16 weeks of age in lean and obese mice. As early as 5 days after birth obese mice consumed less oxygen than lean mice. Obese mice weighed more than lean mice by 6 days of age and contained 38% more fat than lean mice at 7 days of age. At 14 days of age obese mice contained 53% more fat than lean mice. Beyond 3 weeks of age oxygen consumption of obese mice was less than observed in lean mice when the results were expressed per g body weight, but the values for obese and lean mice were similar when expressed per animal. These results demonstrate that alterations in energy metabolism occur very early in the life of obese mice.