Antagonism of insulin and lipolytic hormones in the control of adenylate-cyclase activity in fat cells,.

Abstract

The control of adenylate cyclase activity by lipolytic hormones was studied in fat cell “ghosts” in the presence and absence of physiological concentrations of insulin. Insulin was found to inhibit the effect of submaximal concentrations of norepinephrine, adrenocorticotrophin, and glucagon. Addition of the hormone to the assay system leads to an immediate reduction of the linear rate of formation of adenosine 3′:5′‐monophosphate (3′:5′‐AMP). For norepinephrine and adrenocorticotrophin a dose‐response relationship was established which was shifted by insulin in a “pseudocompetitive” fashion. Increasing the concentration of insulin beyond the physiological range resulted in a reversal of its inhibitory effect in the presence of lipolytic hormones. Insulin alone at concentrations between 5 and 5000 μU/ml had no effect, whereas a small stimulatory effect of high insulin concentrations (5–500 mU/ml) could be found in the presence of submaximal concentrations of norepinephrine. No effect of insulin was obtained against several concentrations of NaF. The effect of insulin upon the norepinephrine‐stimulated adenylate cyclase disappeared after trypsin treatment of intact cells prior to preparation of ghosts. Further incubation of intact fat cells after trypsin treatment lead to a recovery of the effect of insulin. Analogous to insulin, nonsuppressible insulin‐like material which is believed to be related to “somatomedin”, was found to inhibit adenylate cyclase activity in ghosts, but this effect was not abolished by trypsin treatment. Ghosts were found to contain 3′:5′‐AMP phosphodiesterase activity which was not influenced by direct addition of insulin as measured with low substrate concentrations or under the conditions of the adenylate cyclase reaction.It is suggested that insulin at low concentrations interferes with the activation of the adenylate cyclase system by lipolytic hormones, at a step between hormone receptor and catalytic unit. In view of similar characteristics of the antilipolytic effect of insulin in intact lipocytes and the observed modulation of the action of lipolytic hormones in ghosts, a fundamental physiological role of this effect upon the adenylate cyclase system in the mechanism of action of insulin is postulated.