Evidence that glucose metabolism regulates leptin secretion from cultured rat adipocytes

Synergistic effects of feeding and dexamethasone on serum leptin levels

The objectives of this study were to determine the time course of the stimulatory effect of dexamethasone on serum leptin and whether it depends on food intake. Dexamethasone (4mg) was administered I.V. over 1 minute to healthy human volunteers (n=8) under fasting and feeding conditions (2000 kcal given at three meals over 7 hours). At 10 hours, serum leptin levels were increased only in the fed subjects (delta leptin 10.6+/-1.6 vs -2.4+/-1.9 ng/ml, p=0.01, n=8). To assess the interactive effect of food and dexamethasone on serum leptin, a subgroup (n=4) was studied under 4 conditions: 1) dexamethasone/fast; 2) dexamethasone/food; 3) saline/fast; 4) saline/food. Serum leptin declined from baseline under the fasting conditions, with or without dexamethasone. Feeding prevented the drop in serum leptin. In the dexamethasone/food condition, leptin levels rose from baseline after 7 hours and doubled after 10 hours (p<0.05). The rise in serum leptin was significantly greater in the food/dexamethasone condition compared to all other conditions (p<0.05). In summary, dexamethasone has no independent effect on serum leptin in the absence of food intake. Rather, dexamethasone appears to potentiate the food-induced increase in serum leptin. This synergism may be mediated by insulin and/or other factors associated with food ingestion.

Lactation suppresses diurnal rhythm of serum leptin

Rats consume most of their daily food intake at night; serum leptin levels and adipose tissue leptin mRNA content are elevated at night in non-lactating rats fed ad libitum. Lactation induces massive hyperphagia with most food still consumed at night, but the nocturnal increase in leptin secretion was not observed in lactating rats. Thus the link between nocturnal food intake and increased serum leptin is broken during lactation and the hypoleptinaemia may be an important factor promoting the hyperphagia of lactation.

Signals that regulate food intake and energy homeostasis

Feeding behavior is critical for survival. In addition to providing all of the body's macronutrients (carbohydrates, lipids, and proteins) and most micronutrients (minerals and vitamins), feeding behavior is a fundamental aspect of energy homeostasis, the process by which body fuel stored in the form of adipose tissue is held constant over long intervals. For this process to occur, the amount of energy consumed must match precisely the amount of energy expended. This review focuses on the molecular signals that modulate food intake while integrating the body's immediate and long-term energy needs.

Marked and rapid decreases of circulating leptin in streptozotocin diabetic rats: reversal by insulin

Evidence for regulation of circulating leptin by insulin is conflicting. Diabetes was induced in rats with streptozotocin (STZ; 40 mg.kg(-1).day(-1) x 2 days) to examine the effect of insulin-deficient diabetes and insulin treatment on circulating leptin. After 12 wk, plasma leptin concentrations in untreated rats were all < 0.4 ng/ml versus 4.9 +/- 0.9 ng/ml in control animals (P < 0.005). In rats treated with subcutaneous insulin implants for 12 wk, which reduced hyperglycemia by approximately 50%, plasma leptin was 2.1 +/- 0.6 ng/ml, whereas leptin concentrations were 6.0 +/- 1.6 ng/ml in insulin-implanted rats receiving supplemental injections of insulin for 4 days to normalize plasma glucose (P < 0.005 vs. STZ untreated). In a second experiment, plasma leptin was monitored at biweekly intervals during 12 wk of diabetes. In rats treated with insulin implants, plasma leptin concentrations were inversely proportional to glycemia (r = -0.64; P < 0.0001) and unrelated to body weight (P = 0.40). In a third experiment, plasma leptin concentrations were examined very early after the induction of diabetes. Within 24 h after STZ injection, plasma insulin decreased from 480 +/- 30 to 130 +/- 10 pM (P < 0.0001), plasma glucose increased from 7.0 +/- 0.2 to 24.8 +/- 0.5 mM, and plasma leptin decreased from 3.2 +/- 0.2 to 1.2 +/- 0.1 ng/ml (delta = -63 +/- 3%, P < 0.0001). In a subset of diabetic rats treated with insulin for 2 days, glucose decreased to 11.7 +/- 3.9 mM and leptin increased from 0.5 +/- 0.1 to 2.9 +/- 0.6 ng/ml (P < 0.01) without an effect on epididymal fat weight. The change of leptin was correlated with the degree of glucose lowering (r = 0.75, P < 0.05). Thus insulin-deficient diabetes produces rapid and sustained decreases of leptin that are not solely dependent on weight loss, whereas insulin treatment reverses the hypoleptinemia. We hypothesize that decreased glucose transport into adipose tissue may contribute to decreased leptin production in insulin-deficient diabetes.