Different effects of lipolytic hormones and phosphodiesterase inhibitors on cyclic 3′,5′-AMP levels in isolated fat cells

Summary Cyclic AMP levels of isolated fat cells of rats were increased about 50-fold by noradrenaline (1 M) and isoprenaline (1 M) within 4 min of incubation and had declined markedly after 10 min. The effect of a variety of lipolytic hormones on cyclic AMP levels was dose-dependent over a wide range of concentrations, ACTH and glucagon being most potent on a molar basis. Among the adrenergic compounds tested isoprenaline elicited the grea test response. Glucagon, in maximally effective concentrations, caused only half the increase in cyclic AMP values produced by other hormones. This pattern was largely paralleled by the lipolytic effects of the hormones as measured by glycerol production. Various inhibitors of cyclic AMP phosphodiesterase had only small effects on cyclic AMP accumulation. Methylxanthines caused only a 3- to 5-fold elevation of cyclic AMP levels at concentrations which induced maximal lipolytic effects. Papaverine (0.5 mM) and phentolamine (0.1 mM) had virtually no effect and did not potentiate hormone effects on cyclic AMP production. However, theophylline (1 mM) caused a nearly tenfold increase in the effects of isoprenaline when added to 100 000 cells per ml in the medium. At a concentration of 20 000 fat cells per ml isoprenaline alone increased cyclic AMP levels 300-fold and the addition of theophylline had no further stimulatory effect. Our results suggest that lipolysis induced by hormones is mainly mediated by an accumulation of cyclic AMP, whereas methylxanthines must have additional effects. The potentiation of lipolytic hormones by methylxanthines cannot be attributed to an inhibition of phosphodiesterase alone, but seems to be due mainly to their antagonism with an inhibitory factor, which is produced by fat cells and released into the incubation medium.