Mechanisms involved in catecholamine effect on glycogenolysis in catfish isolated hepatocytes

Isolated catfish hepatocytes were treated with epinephrine, norepinephrine, isoproterenol, and phenylephrine in the presence or in the absence of propranolol or phentolamine as beta and alpha inhibitors, respectively. Glycogen phosphorylase a activity and glycogen content, as well as glucose released from cells, were tested. Phosphorylase activity was stimulated by all the catecholamines and was accompanied by a decrease of glycogen content in cells and by an increase in glucose output into the medium. Whereas phentolamine did not affect the catecholamine action on any parameter considered, propranolol inhibited the effect of epinephrine, norepinephrine, and phenylephrine, but hardly altered that of isoproterenol. The effect of epinephrine and norepinephrine, as modified by propranolol and not by phentolamine, is consistent with a beta action of these catecholamines. The fact that propranolol and not phentolamine inhibited the phenylephrine effect indicates that in catfish hepatocytes phenylephrine behaves as a beta agonist and/or that propranolol may also bind to alpha receptors. Results also indicate that in catfish liver cells isoproterenol, whose effect is scarcely influenced by propranolol, is not a pure beta agonist.