CHRONIC EXPOSURE OF RAT FAT-CELLS TO INSULIN ENHANCES LIPOLYSIS AND ACTIVATION OF PARTIALLY PURIFIED HORMONE-SENSITIVE LIPASE

The activity of adipose tissue hormone-sensitive lipase in animals wit h hyperinsulinemia has been reported to be increased compared with tha t in control animals. We examined whether this results from a direct e ffect of insulin on the tissue and whether ft is accompanied by altera tion in the regulation of lipolysis. When rat epididymal fat pads are incubated in culture medium with bovine serum albumin for 2-4 h with 2 ng/ml or 50 muU/ml of insulin, hormone-sensitive lipase activity in t he postmicrosomal supernatant fraction after acid precipitation and ac tivation with ATP-Mg2+ increases significantly compared with preparati ons from tissues incubated with the vehicle. The specific activities o f hormone-sensitive lipase in sonicates of adipocytes after primary cu lture with insulin at concentrations from 10 to 4000 ng/ml (250 muU to 100 mU/ml) increase in an insulin-dose-related manner. Lipolysis in r esponse to 10(-7) M isoproterenol also increases in an insulin-dose-de pendent manner. Enhancement of isoproterenol-mediated lipolysis is not attributable to a difference in the triglyceride content of the cells . Lipolysis caused by the beta-agonist could be completely blocked by the simultaneous presence of insulin in both control and insulin-treat ed cells reflecting normal responsiveness of both types of cells to th e acute effect of insulin. Although an increase in lipolysis is seen w ith norepinephrine and growth hormone after insulin treatment, other l ipolytic agents such as ACTH, thyrotropin, and glucagon evoke similar responses in insulin-treated and control cells. The simultaneous prese nce of growth hormone and insulin during the 16-h culture results in a dditive effects on the subsequent response of the cells to 10(-7) M is oproterenol compared with the responses of the cells cultured with eac h hormone alone. Beta-agonist-mediated cAMP accumulation in the presen ce of Ro-20.1724, a specific phosphodiesterase inhibitor, is significa ntly higher in cells cultured in the presence of insulin than in contr ol cells. Forskolin (1-25 muM) increases the lipolytic responses of in sulin-treated cells compared with control cells, but the maximal respo nse of the insulin-treated cells to forskolin is lower than that to is oproterenol. We conclude that changes produced by chronic insulin trea tment involve more than one site along the lipolytic cascade.