Comparison of the lipolytic effects of insulin and proinsulin on isolated fat cells

Proinsulin, like insulin, has a biphasic effect on the lipolysis of isolated fat cells. Low concentrations of the peptides inhibit the response to submaximal doses of epinephrine, while higher concentrations of the peptides enhance the effect of supramaximal doses of epinephrine. Half-maximal doses are: (a) inhibition of lipolysis: insulin, 2.3 times 10- minus 11M; proinsulin, 6.9 times 10- minus 10M; (b) enhancement of lipolysis: insulin, 3.1 times 10- minus 9M; proinsulin, 8.1 times 10- minus 8M. The ratios of the potencies of the peptides are the same for both effects. These results suggest that both the inhibitory and the lipolytic effects of insulin are not caused by contaminants, and are mediated by similar receptors.