Desensitization of alpha 1 adrenoceptor-stimulated glycogen phosphorylase activity in vascular smooth muscle

Desensitization of alpha 1-adrenoceptor-mediated activation of glycogen phosphorylase was investigated in rabbit aorta. Activation of glycogen phosphorylase by epinephrine was antagonized by the alpha 1-receptor selective antagonist prazosin but not by yohimbine (alpha 2-receptor selective) or by propranolol (beta-receptor antagonist). Preincubation of rabbit aortic ring segments for 5 h with norepinephrine (NE, 10(-5) M) led to a 30-fold loss in sensitivity and a 55% decrease in maximal activation of the enzyme by alpha agonists. Preincubation of aortic ring segments with phenylephrine (10(-5) M) in the presence of propranolol (10(-6) M) also caused desensitization of glycogen phosphorylase activation. The desensitization was heterologous since maximal activation of the enzyme by histamine or KCl was also markedly diminished in segments preincubated with NE. In contrast to these results, catecholamine-induced desensitization to alpha 1-adrenoceptor-mediated smooth muscle contraction in aortic ring segments resulted in loss in sensitivity but not maximal force of contraction on subsequent stimulation by alpha 1 agonists. These results suggest that the mechanism responsible for desensitization of glycogen phosphorylase is distal to receptor activation and may involve attenuation of responses to intracellular Ca2+-dependent enzymes which have limited reserve.