beta-Adrenergic desensitization by chronic insulin exposure in 3T3-L1 cultured adipocytes

Cultured 3T3-L1 cells provide a model system for studies of the long-term regulation of lipolysis. Insulin acutely inhibits isoproterenol-stimulated lipolysis primarily by decreasing the apparent affinity apparent Km for isoproterenol. In contrast, chronic insulin exposure inhibits lipolysis by a reduction in the maximal effect of isoproterenol Vmax. The decrease in Vmax can be observed with insulin concentrations that are as low as 10(-9) mol/L at the time of addition. The effect is stable to washing, and the cells' responsiveness to isoproterenol returns partially with continued culture. Chronic insulin exposure also markedly reduced dibutyryl-cAMP-stimulated lipolysis indicating an insulin-induced change distal to cAMP concentration in the cascade of reactions controlling lipolysis in these cells. Time course and insulin dose-response experiments indicate an additional proximal alteration. These results indicate that: (1) 3T3-L1 cells are a useful model for studying the long-term regulation of lipolysis. (2) Chronic insulin exposure inhibits lipolysis by a mechanism that differs from the acute effect of insulin. (3) The chronic effects of insulin may be mediated through changes at multiple levels in the lipolytic cascade.