Inhibition of lipolysis in hamster epididymal adipocytes by selective alpha-adrenergic agents. Evidence for cyclic AMP-dependent and independent mechanisms

Abstract

In this study the role of cyclic AMP in the antilpolytic effect of the alpha-adrenergic agents methoxamine and phenylephrine in hamster epididymal adipocytes was studied. Both methozamine and phenylephrine lowered the very high levels of cyclic AMP that were produced by high concentrations of isoproterenol (10 muM) or ACTH (100 MU/ml), and partially inhibited lipolysis. When lower concentrations of isoproterenol were used, the antilipolytic effect of phenylephrine and methoxamine was still evident. Under these conditions methoxamine produced a slight suppression of cyclic AMP levels while phenylephrine increased accumulation of cyclic AMP. It follows, therefore, that the inhibition of lipolysis by the alpha agents is most likely unrelated to changes in cyclic AMP levels; in contrast, phenylephrine promoted lipolysis and increased cyclic AMP levels. When the stimulus for lipolysis was provided by methylxanthines a different picture emerged. Methoxamine antagonized lipolysis and lowered cyclic AMP levels. In the presence of propranolol, phenylephrine lowered cyclic AMP levels and suppressed methylxanthine-accelerated lipolysis. It is suggested that when methy xanthines provide the stimulus for lipolysis the antilipolytic effect of methoxamine and phenylephrine (in the presence of propranolol) may be mediated by the suppression in cyclic AMP levels.