[Insulin action on adenylate cyclase. The effect of insulin on adenylate cyclase of rat fat cells in the presence of theophylline].
Insulin-effects on adenylate cyclase activity, cyclic AMP (cAMP) content and free fatty acid (FFA) accumulation of rat epididymal fat cells were examined under conditions of maximal inhibition of phosphodiesterase by sufficient amounts of theophylline for the purpose of localizing the site of the antilipolytic action of insulin. After brief preincubation of the cells with physiological amounts of insulin in the presence or absence of 0.1 mM adrenalineee, fat cells were homogenized following the addition of theophylline and then further incubated. Under these conditions, small but significant enhancement of adenylate cyclase activity, cAMP content and FFA accumulation by insulin alone was observed, while insulin remarkably inhibited adrenalin-stimulated FFA accumulation, reducing adenylate cyclase activity and cAMP levels. This increase of cAMP content by insulin was only seen 5 or 15 minutes after the addition of theophylline. Furthermore, this insulin effect was also observed in the experiments which were performed in a medium containing high concentrations of albumin (2%). The concomitant accumulation of FFA might have resulted from the stimulation of lipolysis, rather than from the synthesis of FFA, since there was no added glucose in the medium. And finally, the hydrolysis of 14C-tripalmitate by a fraction of the cell homogenate under the presence of theophylline was more extensive after preincubation of the cells with insulin than without insulin. In summary, insulin, which is recognized as a typical antilipolytic hormone, activated adenylate cyclase and increased lipolysis at its physiological concentrations when it alone exerted its effect upon fat cells under the conditions where phosphodiesterase was completely inhibited by theophylline. Accordingly, the present results indicate the bimodal effect of insulin on adenylate cyclase and lipolysis under the presence of theophylline; enhancement when applied alone, and depression with adrenalin. So it is most likely that the "negative synergism" occurs as a net effect when a mild activator acts together competitively with a strong activator toward the same target. These data suggest the fundamental roles of adenylate cyclase systems in the mechanism of lipolysis regulation by insulin.
D000262:Adenylyl Cyclases / Q000378:metabolism*; D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D000242:Cyclic AMP / Q000096:biosynthesis; D005230:Fatty Acids, Nonesterified / Q000096:biosynthesis; D066298:In Vitro Techniques; D007328:Insulin / Q000494:pharmacology*; D008049:Lipase / Q000378:metabolism; D008297:Male; D051381:Rats; D013806:Theophylline / Q000494:pharmacology*
D005230:Fatty Acids, Nonesterified; D007328:Insulin; D013806:Theophylline; D000242:Cyclic AMP; D008049:Lipase; D000262:Adenylyl Cyclases