Differences between male and female rats in the regulation of hepatic glycogenolysis. The relative role of calcium and cAMP in phosphorylase activation by catecholamines

Abstract

The relative importance of alpha- and beta-adrenergic pathways and of their respective intracellular mediators, calcium and cAMP, in the stimulation of phosphorylase alpha induced by catecholamines was studied in hepatocytes isolated from mature male and female rats. The fractional efflux of 45Ca was used as an index of intracellular calcium activity. Our results show that, in females: 1) the activation of phosphorylase alpha induced by 10(-8) to 10(-5) M epinephrine correlates with a rise in cellular cAMP as well as with an increase in 45Ca fractional efflux, 2) both alpha- and beta-agonists stimulate phosphorylase, 3) neither alpha- nor beta-antagonists effectively block the rise in phosphorylase caused by epinephrine, and 4) propranolol suppresses the rise in cAMP while phenoxybenzamine blocks the rise in calcium efflux. On the other hand, we found that in the male: 1) phosphorylase alpha activity is exclusively correlated with a rise in fractional calcium efflux, 2) epinephrine (10(-8) to 10(-7) M) does not increase cAMP and it causes a greater rise in calcium efflux than in the female at all concentrations, 3) phenylephrine increases calcium efflux and phosphorylase activity without affecting cAMP, 4) phenoxybenzamine totally blocks epinephrine action, and 5) beta-agonists and beta-antagonists are without effects. We conclude that, in females, epinephrine utilizes both alpha- and beta-adrenergic pathways which activate phosphorylase by calcium or cAMP, respectively, while, in adult male rats, epinephrine increases phosphorylase alpha activity by an alpha-mediated, calcium-dependent and cAMP-independent pathway.