Antilipolytic actions of insulin on basal and hormone-induced lipolysis in rat adipocytes

The levels of insulin, free fatty acids (FFA), and triglycerides in rat sera increase with age. The increase in serum FFA levels accompanied the stimulation of basal lipolysis (i.e., lipolysis in the absence of lipolytic agents) in fat cells and enlargement of the diameter of the cells. An overnight fast resulted in a significant increase in basal lipolysis in fat cells from 6- and 8 week-old rats. Although insulin inhibited lipolysis induced by norepinephrine and ACTH at a concentration of 10(-10) M, it failed to inhibit basal lipolysis even at a concentration of 10(-6) M. Propranolol, another antilipolytic agent like insulin, also did not affect basal lipolysis, Insulin did not inhibit the accelerated basal lipolysis in enlarged fat cells, fasted fat cells, and sonicated cells. These results indicate that insulin inhibits only the lipolysis induced by lipolytic agents such as norepinephrine and ACTH but not the basal lipolysis found in the absence of lipolytic agents, The possibility that free fatty acids produced by enlarged fat cells initiate insulin resistance and diabetes mellitus is discussed.