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Partially purified, non-suppressible, insulin-like material (NSILA-S) was studied with respect to its effect on the levels of 3',5'-cyclic adenosine monophosphate (cAMP) and its mechanism of action in the control of this nucleotide in rat fat cells. NSILA-S prevents the rise of cAMP in fat cells under the influence of isoproterenol with similar kinetics to insulin. A maximal effect is observed at about 70 ng/ml with a biological activity equivalent to 200 muU/ml of insulin. NSILA-S inhibits norepinephrine-stimulated adenylate cyclase activity in fat cell ghosts and partially purified plasma membrane preparations. At 10 mM Mg2+, the inhibition is characterized by an effect of Vmax without change in affinity towards ATP (apparent KM 30 muM). Similarly there is no observed change in affinity towards Mg2+. With respect to inhibition of norepinephrine-stimulated adenylate cyclase, the dose-response curve of NSILA-S is similar to that already found with intact cells. The effect of norepinephrine is inhibited throughout the dose-response range between 5 X 10(-7) and 5 X 10(-4) M. In contrast to previous observations with insulin in ghosts, NSILA-S inhibits the basal adenylate cyclase activity. Cyclic nucleotide phosphodiesterase activity in homogenates as measured at 1.0 muM substrate is increased by 90% after previous incubation of fat cells with NSILA-S. The study suggests that the anti-lipolytic effect of NSILA-S is mediated by a lowering of cAMP through inhibition of the adenylate cyclase and/or stimulation of the phosphodiesterase system.

In view of controversial findings regarding the mechanism for the increased intracellular hepatic cyclic 3':5' adenosine monophosphate levels in diabetic rats, we studied the dose-response relationship of the adenylate cyclase to glucagon stimulation in severely diabetic and in diabetic, insulin-treated rats. An enhanced response to glucagon and an additional augmenting effect of guanosine triphosphate on hormonal stimulation of the adenylate cyclase activity were found in diabetes which were reversible with insulin treatment. The results suggest a role of the regulatory guanyl nucleotide-binding protein in diabetes leading to an increased dose response relationship of the hepatic adenylate cyclase system to glucagon.