Discriminative insulin antagonism of stimulatory effects of various cAMP analogs on adipocyte lipolysis and hepatocyte glycogenolysis

Although insulin effectively blocked hormone-stimulated glycerol output in adipocytes or phosphorylase activation in hepatocytes, the inhibitory effect of insulin on cAMP analog-stimulated cells depended on the cAMP analog used. Of the 20 analogs tested in adipocytes and 13 tested in hepatocytes, the effects of about half of them were effectively blocked by insulin, whereas the effects of many of them were not inhibited at all. In order to approach the explanation for this discriminative insulin action, the inhibitory effects of insulin on the responses to the analogs in the intact cells were correlated with the in vitro cAMP analog specificity for the hepatocyte cAMP-dependent protein kinase isozymes and the low K(m), hormone-sensitive phosphodiesterases from both cell types. No correlation was found between insulin resistance of analog-stimulated hepatocyte phosphorylase and the concentration of analog required in vitro for half-maximal activation of either type I or type II cAMP-dependent protein kinase from hepatocytes. However, a good correlation was found between insulin resistance of cAMP analog-stimulated responses and the analog I₅₀ values for the phosphodiesterase from both cell types. Using a new method capable of measuring hydrolysis at low analog concentrations, several of those analogs which had relatively low, but not high, phosphodiesterase I₅₀ values were shown to be directly hydrolyzed by the low K(m) adipocyte phosphodiesterase. The insulin inhibition of cell responses when stimulated by hydrolyzable analogs, but not by poorly hydrolyzable analogs, is best explained by insulin stimulation of the low K(m) phosphodiesterases from both cell types.