Leptin is secreted from adipose tissue, and is thought to act as a 'lipostat', signalling the body fat levels to the hypothalamus resulting in adjustments to food intake and energy expenditure to maintain body weight homeostasis. In addition, plasma leptin concentrations have been shown to be related to insulin sensitivity independent of body fat content, suggesting that the hyperleptinemia found in obesity could contribute to the insulin resistance. We investigated the effects of leptin on insulin binding by isolated adipocytes. Adipocytes isolated from Sprague-Dawley rats exhibited a dose-dependent reduction in the uptake of 125I-labelled insulin when incubated with various concentrations of exogenous leptin. For example, addition of 50 nM leptin reduced total insulin binding in isolated adipocytes by 19% (P < 0.05). Analysis of displacement curve binding data suggested that leptin reduced maximal insulin binding in a dose-dependent manner, but had no significant effect on the affinity of insulin for its binding site. We conclude that leptin directly inhibited insulin binding by adipocytes, and the role of leptin in the development of insulin resistance in obese individuals requires further investigation.

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The hexosamine biosynthesis pathway and protein kinase C (PKC) activation mediate hyperglycaemia-induced impaired glucose transport, but the relative role of each pathway is unknown. Following a 2 h preincubation of rat adipocytes in the presence of either high glucose (30 mM) plus insulin (0.7 nM) or glucosamine (3 mM), both high glucose and glucosamine inhibited subsequent basal and insulin-stimulated glucose transport, measured at 5.0 mM glucose. Azaserine, an inhibitor of the enzyme glutamine:fructose-6-phosphate aminotransferase, abolished the effect of high glucose, but not that of glucosamine. Ro-31-8220, an inhibitor of PKC, reversed the effects of both high glucose and glucosamine, suggesting that flux through the hexosamine biosynthesis pathway impaired glucose transport acutely by activating PKC. Both high glucose and glucosamine caused a 3-fold increase in PKC activity; this effect of high glucose, but not that of glucosamine, was partially decreased by azaserine. Neither high glucose nor glucosamine altered basal or insulin-stimulated plasma membrane GLUT1 levels, whereas both treatments decreased basal, but not insulin-stimulated, GLUT4 levels. Azaserine abolished the effect of high glucose, but not that of glucosamine, on basal plasma membrane GLUT4 levels. Ro-31-8220, which returned glucose transport to control values, caused a further decrease in plasma membrane GLUT4 levels. It is concluded that, in rat adipocytes, an acute increase in flux through the hexosamine biosynthesis pathway inhibits glucose transport by activation of PKC.

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