The α-adrenergic mediated effect in rat liver: Correlation between [ 3H]-dihydroergocryptine binding to plasma membranes and glycogen phosphorylase activation in isolated hepatocytes

Abstract

Alpha-adrenergic receptors of purified rat liver plasma membranes were characterized by the direct binding of [ 3H]-dihydroergocryptine. Since glycogen phosphorylase (EC 2.4.1.1; 1,4-alpha- d-glucan: orthophosphate alpha-glucosyl transferase) activation in isolated rat hepatocytes is known to be mediated mainly through an alpha-adrenergic mechanism, we compared the abilities of various adrenergic agonists and antagonists in competing with [ 3H]-dihydroergocryptine for the binding to alphasites in purified membranes and in activating phosphorylase or in inhibiting its activation in isolated hepatocytes. The activation of phosphorylase by agonists displayed an order of potencies typical of alpha-adrenergic mediators: (−)norepinephrine ( K a = 0.31 μM) ⩾ (−)epinephrine ( K a = 0.76 μM) ⪢ (−)isoproterenol ( K a = 24 μM), and the stereospecificity expected: (−)epinephrine ( K a = 0.76 μM) > (+)epinephrine ( K a = 5.2 μM), (−)norepinephrine ( K a = 0.31 μM) > (+)norepinephrine ( K a = 3.3 μM). Alpha-adrenergic antagonists (phentolamine K iapp =1.71 μM) were much more potent than betaadrenergic antagonists [(−)propranolol K iapp = 113 μM] in preventing the activation of glycogen phosphorylase by epinephrine. In our system imidazoline derivatives such as clonidine appeared to be mainly antagonists for the alpha-adrenergic mediated events. Close correlations ( r = 0.92, P < 0.01 and r = 0.91, P < 0.001) were observed, respectively, for adrenergic agonists and antagonists between either activation or prevention of the activation of the enzyme and competition with dihydroergocryptine for alpha-adrenoreceptors. These results validate the use of [ 3H]-dihydroergocryptine as a marker of the physiological alpha-adrenergic receptor in rat liver.