Characterization of the liver P2-purinoceptor involved in the activation of glycogen phosphorylase

Abstract

Evidence has been presented for the existence in rat liver of P2-purinoceptors which are involved in the control of glycogenolysis. Isolated rat hepatocytes and purified liver plasma membranes have been used to study the binding of the ATP analogue adenosine 5''-[.alpha.-[35S]thio]triphosphate (ATP.alpha.[35S]) to these postulated P2-purinoceptors. The nucleoide analogue behaves as a full agonist for the activation of glycogen phosphorylase in isolated hepatocytes, 0.3 .mu.M being required for half-maximal activation. Specific binding of ATP.alpha.[35S] to hepatocytes and plasma membranes occurs within 1 min and is essentially reversible. The analysis of the dose-dependency at equilibrium indicates the presence of binding sites with Kd of 0.23 .mu.M with hepatocytes and Kd of 0.11 .mu.M with plasma membranes. The relative affinities of 10 nucleotide analogues were deduced from competition experiments for ATP.alpha.[35S] binding to hepatocytes, and these correlated highly with their biological activity (activation of glycogen phosphorylase in hepatocytes). For all the agonists, binding occurs in the same concentration range as the biological effect. These data clearly suggest that the detected binding sites correspond to the physiological P2-purinoceptors involved in the regulation of liver glycogenolysis. The rank order of potency of some ATP analogues suggests that liver possesses the P2V-subclass of P2-purinoceptors.