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.

Leptin rapidly inhibits hypothalamic neuropeptide Y secretion and stimulates corticotropin-releasing hormone secretion in adrenalectomized mice.

Leptin may rapidly inhibit food intake by altering the secretion of hypothalamic neuropeptides such as neuropeptide Y (NPY), a stimulator of food intake, and/or corticotropin-releasing hormone (CRH), an inhibitor of food intake. We measured concentrations of NPY and CRH in specific hypothalamic regions [i.e., arcuate nucleus (ARC), paraventricular nucleus (PVN), ventromedial nucleus and dorsomedial nucleus] of 7- to 8-wk-old lean and ob/ob mice at 1 or 3 h after intracerebroventricular leptin administration. No rapid-onset effects of leptin on hypothalamic NPY or CRH concentrations were observed in intact mice. The addition of leptin to hypothalamic preparations from intact mice also did not alter NPY or CRH secretion. Glucocorticoids may oppose leptin actions. Consistent with this, leptin administration to adrenalectomized mice markedly reduced CRH concentrations in the ARC within 3 h after injection. This rapid reduction in CRH concentration in the ARC after leptin administration is more likely due to stimulated CRH release from this region than to decreased synthesis/transport from the PVN because leptin stimulates CRH synthesis in the PVN. Within 20 min after exposure to leptin, NPY secretion from hypothalamic preparations obtained from adrenalectomized mice was lowered by 27% and CRH secretion was elevated by 51%. The current study demonstrates that leptin rapidly influences the secretion of hypothalamic NPY and CRH and that these actions of leptin within the hypothalamus are restrained by the presence of endogenous corticosterone.

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/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.

Neuropeptide Y and corticotropin-releasing hormone concentrations within specific hypothalamic regions of lean but not ob/ob mice respond to food-deprivation and refeeding.

Leptin is proposed to control food intake at least in part by regulating hypothalamic neuropeptide Y (NPY), a stimulator of food intake, and corticotropin-releasing hormone (CRH), an inhibitor of food intake. Ob/ob mice are leptin-deficient and would thus be expected to exhibit alterations in hypothalamic NPY and CRH. We therefore measured concentrations of NPY and CRH in discrete regions of the hypothalamus (i.e., ARC, arcuate nucleus; PVN, paraventricular nucleus; VMH, ventromedial nucleus; DMH, dorsomedial nucleus; and SCN, suprachiasmatic nucleus) of 6.5-7-wk-old ob/ob and lean mice with free access to stock diet, 24 h after food deprivation, and 1 h after refeeding. Fed ob/ob mice had 55-75% higher concentrations of NPY in the ARC, VMH and SCN than lean mice. Food deprivation increased NPY concentrations approximately 70% in the ARC, PVN and VMH of lean mice, and refeeding lowered NPY concentrations approximately 70% in the PVN of these mice. NPY in these hypothalamic regions of ob/ob mice was unresponsive to food deprivation or refeeding. The most pronounced change in CRH concentrations within the regions examined (i.e., ARC, PVN and VMH) occurred in the ARC of lean mice where refeeding lowered CRH concentrations by 75% without influencing ARC CRH concentrations in ob/ob mice. The hypothalamic concentrations of two neuropeptides involved in body weight regulation (i.e., NPY and CRH) in leptin-deficient ob/ob mice respond abnormally to abrupt changes in nutritional status.

Leptin rapidly lowers food intake and elevates metabolic rates in lean and ob/ob mice.

Leptin, the ob gene product, is released from adipose tissue and likely acts in the central nervous system, particularly within the hypothalamus, to exert many of its effects. Obesity in C57BL/6J ob/ob mice is caused by a mutation in the ob gene resulting in a lack of functional leptin. In this study, we first compared effects of a single intracerebroventricular (ICV) injection of 3 pmol (50 ng) or 60 pmol (1 microg) leptin on food intake and oxygen consumption of lean and ob/ob mice deprived of food for 4 h during the 48-h period postinjection. Injection of 3 pmol leptin minimally lowered food intake in these mice without influencing oxygen consumption. Injection of 60 pmol of leptin rapidly lowered food intake within 30 min in both lean and ob/ob mice, with effects persisting for 24 h. Lean and ob/ob mice treated with leptin consumed 40 and 60% less food, respectively, in 24 h than vehicle-treated controls. Injection of leptin (60 pmol ICV) suppressed food intake of adrenalectomized mice as well (by 25 and 40% in lean mice and by 20 and 68% in ob/ob mice at 3 and 24 h, respectively), indicating that glucocorticoids are not essential for leptin to suppress food intake. Leptin increased oxygen consumption in conditions in which diet-induced thermogenesis was low, i.e., in fed ob/ob mice and in food-deprived lean mice, but not in fed adrenalectomized ob/ob mice or in fed lean mice. ICV injection of 60 pmol leptin along with 230 pmol (2 microg) of neuropeptide Y (NPY) attenuated NPY-induced feeding in ob/ob, but not in lean mice, suggesting an enhanced potential for crosstalk between the leptin and NPY signaling systems in ob/ob mice lacking endogenous leptin. Leptin exerts rapid-onset actions within the central nervous system to coordinate control of food intake and metabolic rate.

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.

Persistently enhanced sensitivity of pancreatic islets from ob/ob mice to PKC-stimulated insulin secretion.

Islets from 2-wk-old ob/ob and lean littermate mice were cultured for 4-12 days and then perifused or statically incubated to identify early-onset differences in their regulation of insulin secretion. Islets from these young ob/ob and lean mice increased insulin secretion similarly in response to glucose (10 or 20 mM), whereas responsiveness to glucose plus acetylcholine (10 microM) was greater in islets from ob/ob mice than lean mice. This phenotype-specific effect of acetylcholine was mimicked by phorbol 12-myristate 13-acetate (PMA, 100 nM), a protein kinase C (PKC) agonist, whereas prior downregulation of PKC abolished this phenotype-specific effect of acetylcholine. A high concentration of PMA (1 microM) equally and substantially increased insulin secretion from islets of ob/ob and lean mice, suggesting an enhanced regulatory sensitivity rather than altered responsiveness of the PKC system in islets of ob/ob mice. Addition of BAY K 8644, a Ca2+ channel agonist, to the perifusate enhanced acetylcholine-induced insulin secretion from islets of lean mice to attain the high rates observed in islets from ob/ob mice exposed to acetylcholine alone. We propose that acetylcholine-induced PKC regulation of insulin secretion is altered in islets from ob/ob mice, that this alteration may directly or indirectly involve Ca2+ channels, and that it persists even when islets are cultured for up to 12 days.

Dexamethasone rapidly increases hypothalamic neuropeptide Y secretion in adrenalectomized ob/ob mice.

A single intracerebroventricular injection of dexamethasone (DEX) rapidly (within 30 min) suppresses brown adipose tissue thermogenesis and increases plasma insulin concentrations in adrenal-ectomized (ADX) ob/ob mice but not in ADX lean mice. Intracerebroventricular neuropeptide Y (NPY) administered intracerebroventricularly causes these same metabolic changes within 30 min in both ob/ob and lean ADX mice. We therefore hypothesized that DEX exerts these rapid-onset metabolic actions in ob/ob mice via a phenotype-specific enhancement of NPY secretion within the central nervous system. In support of this hypothesis, DEX (a type II glucocorticoid receptor agonist) administered intracerebroventricularly selectively lowered NPY concentrations in the whole hypothalamus of ADX ob/ob mice by 35% and in the arcuate nucleus region by approximately 70% within 30 min but not in the brain stem or hippocampus or in any of these regions of lean mice. DEX also functioned in vitro to enhance depolarization-dependent release of NPY from hypothalamic blocks of ADX ob/ob mice but not of ADX lean mice. Thus DEX acts in the hypothalamus of ob/ob mice in a phenotype-specific manner to evoke rapid transport of NPY from cell bodies within the arcuate nucleus to terminal regions including the dorsomedial and ventromedial hypothalamic regions for release.

Adrenalectomy reverses pre-existing obesity in adult genetically obese (ob/ob) mice.

OBJECTIVE: To determine if adrenalectomy would reverse the pre-existing gross obesity, characteristic of adult genetically obese (ob/ob) mice. DESIGN: Adult (12 week old) female ob/ob mice were adrenalectomized and fed a stock diet for 6 or 14 weeks. They were housed at 23-25 degrees C or 33 degrees C. Food intake and total body energy were determined. RESULTS: Adrenalectomy abolished the hyperphagia characteristic of ob/ob mice. Adrenalectomized ob/ob mice consumed 8-17% less food than intact lean mice. Adrenalectomized ob/ob mice housed at 23-25 degrees C lost 55% of their pre-existing body energy within 6 weeks after surgery and 75% of their body energy within 14 weeks after surgery. At 14 weeks after surgery, body weights and body energy content of the adrenalectomized ob/ob mice were comparable with values for intact lean mice. Intact ob/ob mice pair-fed to adrenalectomized ob/ob mice lost only half as much body energy as the adrenalectomized ob/ob mice did, indicating that adrenalectomy not only diminished food intake in ob/ob mice but also increased their energy expenditure per unit food consumed. Adrenalectomized ob/ob mice housed at 33 degrees C lost only half as much body energy in 6 weeks as did mice housed at 23-25 degrees C. CONCLUSION: Adrenalectomy reverses the gross obesity characteristic of adult ob/ob mice by abolishing their hyperphagia and increasing their energy expenditure per unit food consumed.

Consumption of a glucose diet enhances the sensitivity of pancreatic islets from adrenalectomized genetically obese (ob/ob) mice to glucose-induced insulin secretion.

Consumption of a glucose diet for 4 d markedly elevates plasma insulin concentrations in adrenalectomized ob/ob mice. The present study examined regulation of insulin secretion from perifused pancreatic islets of female adrenalectomized genetically obese (ob/ob) and lean mice fed a glucose diet for 4 d. These mice were fed a high carbohydrate commercial diet for 21 d, or the high carbohydrate commercial diet for 17 d and a purified high glucose diet for the last 4 d of the 21-d feeding period. Adrenalectomy equalized plasma insulin concentrations, pancreatic islet size, rates of insulin secretion in response to 20 mmol/L glucose and insulin mRNA relative abundance in ob/ob and lean mice fed the commercial diet, but the threshold for glucose-induced insulin secretion determined by a linear glucose gradient remained lower in islets from adrenalectomized ob/ob mice than in those from lean mice (3.8 +/- 0.1 vs. 4.9 +/- 0.2 mmol/L glucose), and addition of acetylcholine to the perifusate lowered the threshold to only 2.0 +/- 0.1 mmol/L glucose in islets from ob/ob mice vs. 3.3 +/- 0.1 mmol/L glucose in lean mice. Switching from the commercial diet to the glucose diet for 4 d increased plasma insulin concentrations -10-fold in islets from adrenalectomized ob/ob mice without affecting islet size, 20 mmol/L glucose-induced insulin secretion or insulin mRNA abundance. Consumption of the glucose diet did, however, markedly lower the threshold for glucose-induced insulin secretion in islets from adrenalectomized ob/ob mice to approximate the abnormally low glucose thresholds in intact ob/ob mice.(ABSTRACT TRUNCATED AT 250 WORDS)

A single intracerebroventricular injection of dexamethasone elevates food intake and plasma insulin and depresses metabolic rates in adrenalectomized obese (ob/ob) mice.

A single intracerebroventricular (ICV) injection of dexamethasone rapidly (within 30 min to 3 h) increases plasma insulin and suppresses oxygen consumption in adrenalectomized ob/ob mice with minimal effects in lean mice. Food intake of these adrenalectomized ob/ob mice was unaffected by ICV dexamethasone during these short-term studies. However, in a longer-term study with adrenalectomized gold thioglucose-lesioned obese mice, food intake increased fourfold 6-8 d after a single ICV injection of dexamethasone. We have now further examined the time course of dexamethasone actions in adrenalectomized ob/ob and lean mice in the 96-h post-injection period. A single ICV injection of dexamethasone increased food intake 32% and plasma insulin 81%, and depressed oxygen consumption 11%, in adrenalectomized ob/ob mice during the 24-h period after injection, without increasing food intake or plasma insulin in lean mice. Oxygen consumption was 14% lower in lean mice 24 and 48 h after dexamethasone injection relative to saline-injected lean mice. Food intake and oxygen consumption in ob/ob mice returned to levels in saline-injected controls at 48 and 72 h after injection, respectively. Oxygen consumption of lean mice also returned to control levels at 72 h post-injection. Plasma insulin concentrations were similar in dexamethasone- and saline-treated ob/ob and lean mice at 96 h post-injection (the only time point examined other than 24 h). A single ICV injection of dexamethasone exerts both rapid (within 30 min to 3 h) and sustained (days) metabolic actions in ob/ob mice.

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.

Adrenalectomy suppresses insulin secretion from pancreatic islets of ob/ob mice.

The aim of this work was to examine effects of adrenalectomy and intracerebroventricular injection of dexamethasone on insulin secretion from pancreatic islets of ob/ob mice. Female ob/ob and lean mice fed a stock diet were adrenalectomized at 5 wk of age and studied at 8-9 wks of age. Removal of the adrenal glands from ob/ob mice fed a stock diet normalized their plasma insulin concentration, islet size, and essentially normalized insulin secretion from islets incubated in 10 mM glucose. Insulin secretion from islets was, however, not fully normalized when the incubation buffer contained higher concentrations of glucose (20 mM), or acetylcholine, a potentiator of glucose-induced insulin secretion. Within 90 min after intracerebroventricular administration of dexamethasone plasma insulin concentrations were elevated and glucose-induced (10 mM) insulin secretion from isolated islets of adrenalectomized ob/ob mice increased, but dexamethasone was ineffective in lean mice. Regulation of insulin secretion from pancreatic islets of adrenalectomized ob/ob mice remains abnormal even though adrenalectomy normalized plasma insulin concentrations in these mice. Alterations in glucocorticoid-mediated, central nervous system regulation of insulin secretion likely contribute to hyperinsulinemia in ob/ob mice.

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)

Corticotropin-releasing hormone decreases feeding, oxygen consumption and activity of genetically obese (ob/ob) and lean mice.

Acute effects of intracerebroventricularly administered corticotropin-releasing hormone (CRH) on deprivation-induced food intake, whole-body oxygen consumption, brown adipose tissue metabolism, and several locomotive behaviors were examined in 6- to 7-wk-old female genetically obese (ob/ob) and lean mice. Corticotropin-releasing hormone depressed food intake in a dose-dependent manner, with a tendency for greater suppression of intake in intact ob/ob mice than in lean mice. Adrenalectomy abolished this tendency for CRH to be more potent in ob/ob mice than in lean mice. Corticotropin-releasing hormone also lowered the oxygen consumption of ob/ob and lean mice, without affecting brown adipose tissue metabolism as assessed by measurement of GDP binding to brown adipose tissue mitochondria. Grooming activity was lowered in CRH-injected mice. The CRH-induced lowering of oxygen consumption and grooming activity in mice contrasts with CRH-induced elevations of oxygen consumption and grooming in rats, suggesting species-specific responses to this peptide. Because effects of CRH were similar in adrenalectomized ob/ob and lean mice, it is unlikely that obesity-producing abnormalities in ob/ob mice are related to abnormal CRH action mechanisms. However, potential abnormalities in CRH synthesis and/or release cannot be excluded.

Type II glucocorticoid receptors in the CNS regulate metabolism in ob/ob mice independent of protein synthesis.

A single intracerebroventricular injection of dexamethasone rapidly (within 30 min) decreases brown adipose tissue thermogenesis by 25% as assessed by GDP binding and increases plasma insulin twofold in adrenalectomized ob/ob mice. The present study investigated the type of corticoid receptor(s) that mediate these effects and determined whether protein synthesis was necessary for expression of these glucocorticoid actions in ob/ob mice. Intracerebroventricular injection of aldosterone (a type I-corticoid receptor agonist) was ineffective in altering peripheral metabolism in adrenalectomized ob/ob mice, whereas RU-486 (a type II-corticoid receptor antagonist) abolished the effects of dexamethasone. Thus type II-like corticoid receptors, not type I receptors, mediated the rapid effects of dexamethasone in adrenalectomized ob/ob mice. Anisomycin (0.5 mg) administered subcutaneously almost completely suppressed (-92%) cerebral protein synthesis, but anisomycin did not abolish the rapid effects of dexamethasone in adrenalectomized ob/ob mice. Thus protein synthesis is not a prerequisite for rapid effects of dexamethasone in adrenalectomized ob/ob mice.

Threshold for glucose-stimulated insulin secretion in pancreatic islets of genetically obese (ob/ob) mice is abnormally low.

Pancreatic islets were isolated from 8-9-wk-old female genetically obese (ob/ob) and lean mice to determine the glucose threshold for insulin secretion, and to examine effects of acetylcholine on insulin secretion. Only equal-sized islets from ob/ob and lean mice were incubated to eliminate confounding effects of phenotypic differences in islet size. Even after this adjustment, islets from ob/ob mice still hypersecreted insulin in response to 20 mmol/L glucose. The threshold for glucose-induced insulin secretion determined by perifusing islets with a linear glucose gradient averaged 1.9 +/- 0.1 mmol/L glucose in fed ob/ob mice and 3.1 +/- 0.1 mmol/L glucose in ob/ob mice after 24 h of food deprivation. These low thresholds indicate that islets from ob/ob mice are constantly stimulated by glucose. Islets from lean mice exhibited considerably higher thresholds (4.8 +/- 0.1 and 7.1 +/- 0.1 mmol/L glucose in fed and 24-h food-deprived lean mice, respectively). Rates of insulin secretion per each unit (mmol/L) increase in glucose above threshold concentrations were unaffected by phenotype or feeding state. Addition of acetylcholine to the perifusing buffer further lowered the threshold for insulin secretion to 0.5 mmol/L glucose in pancreatic islets from ob/ob mice and also doubled the rate of increase in insulin secretion at glucose concentrations above the threshold. The combination of the very low threshold for glucose-induced insulin secretion and the exaggerated insulin secretory response to acetylcholine in pancreatic islets of ob/ob mice are likely critical factors in the hyperinsulinemia of these mice.

Alpha-2 adrenergic receptor activity in porcine adipocytes is androgen and age/cell size dependent.

1. Using epinephrine plus propranolol we demonstrated alpha-2 adrenergic receptor (alpha 2AR) activity (49% inhibition of theophylline stimulated lipolysis) in adipocytes of adult intact male pigs (61 kg body wt and 97 microns adipocyte diameter). 2. Dose titration with an alpha 2AR antagonist (yohimbine) confirmed alpha 2AR associated activity. 3. No alpha 2AR activity was observed in younger male or castrated male pigs. 4. The inhibitory action on lipolysis via the alpha 2AR in pigs is dependent on androgen status and adipocyte size or age.

Similar effects of NPY on energy metabolism and on plasma insulin in adrenalectomized ob/ob and lean mice.

A single intracerebroventricular (icv) injection of dexamethasone (250 ng) lowers brown adipose tissue (BAT) thermogenesis and whole body metabolic rates and raises plasma insulin concentrations within 30 min in adrenalectomized ob/ob mice with minimal effects in adrenalectomized lean mice. The present study was conducted to determine if intracerebroventricular neuropeptide Y (NPY), a neuropeptide regulated in part by glucocorticoids, would mimic effects of dexamethasone in these mice. NPY lowered BAT metabolism and whole body oxygen consumption and raised plasma insulin concentrations within 30 min in adrenalectomized ob/ob mice similarly to dexamethasone; but, unlike dexamethasone, NPY was as effective in modulating these metabolic responses in adrenalectomized lean mice as in ob/ob mice. Further, intracerebroventricular NPY increased food intake equally in both ob/ob and lean mice, whereas dexamethasone did not alter food intake during the 30 min postinjection period. These data are consistent with the hypothesis that NPY mediates some of the effects of intracerebroventricular dexamethasone action in ob/ob mice and that the divergence between ob/ob and lean mice lies in glucocorticoid control of NPY release/synthesis rather than in NPY action mechanisms.

Corticosterone and tri-iodothyronine transport into brain of obese (ob/ob) mice.

Unidirectional influx of [3H]-corticosterone and [125I] 3,5,3'-tri-iodothyronine (T3) into brain, and of [3H]-corticosterone into livers, of 8-10 week old lean and obese (ob/ob) mice was measured with a bolus injection, tissue-sampling technique. Uptake of corticosterone by brain and liver of ob/ob mice was comparable to that observed in lean mice, and unaffected by corticosterone concentration in the bolus (17-32 nM or 25 microM) or by addition of lean or ob/ob mouse blood serum to the bolus injectate. Uptake of T3 by ob/ob mouse brain was lower than observed in lean mice when the bolus contained physiological concentrations of T3 (2.6-19 nM). An increase in T3 concentration in the injectate to 5 microM depressed T3 uptake by brain of lean mice, but not of ob/ob mice. Addition of blood serum from ob/ob mice to the bolus further lowered influx of T3 into the brain of lean and ob/ob mice. Even though unidirectional influx of T3 into the brain of ob/ob mice was impaired, steady-state T3 concentrations in ob/ob mice brain were similar to lean values. The enhanced sensitivity of ob/ob mice to the obesity-producing effects of glucocorticoids is not caused by an increased influx of corticosterone into brain or liver.