The effect of free fatty acids (FFAs) on insulin binding and action was investigated in isolated rat hepatocytes. Oleic acid (0.4 mM) added to the cells rapidly (within 45 min) reduced insulin binding and degradation (each by 45%; P < 0.001, n = 7) without changing the apparent receptor affinity. The effect was concentration dependent; a half-maximal inhibitory effect occurred at 0.150 ± 0.050 mM (mean ± SE). Oleic acid exerted no effect on insulin binding in energy-depleted (KCN-treated) cells. Oleic, palmitic, stearic, palmitoleic, and eicosapentaenoic acids were equally effective in reducing insulin binding. FFA did not change insulin binding to partially purified insulin receptors, thus excluding a direct effect on the insulin receptor. Furthermore, binding to partially purified receptors from solubilized cells pretreated with 0.2 mM oleic acid was not changed, indicating the effect of FFA in intact cells is on the rate of receptor internalization and/or recycling. Concomitant with the effect on insulin binding, oleic acid elicited a concentration-dependent reduction in nonstimulated cellular [14C]aminoisobutyric acid uptake (AIB; 29 ± 8%, P <0.05) and decreased the maximal effect of insulin (39 ± 7%, P < 0.05). Thus, in a concentration-dependent manner, different fatty acids can reduce the number of binding sites for insulin and the degradation of insulin by isolated liver cells. Basal and insulin-stimulated AIB transport was reduced, suggesting the presence of postbinding perturbations. These data suggest that FFA exerts an important modulating effect on insulin action in the liver.

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