(3H)-Vasopressin Binding to Isolated Rat Hepatocytes and Liver Membranes: Regulation by GTP and Relation to Glycogen Phosphorylase Activation

Abstract

Specific vasopressin binding to rat hepatocytes and rat liver membranes was measured using biologically active (³H)-Tyr2-Lys8-vaso-pressin (8.5 Ci/mM). In both systems, vasopressin binding was found to be time-dependent, reversible, and saturable. The kinetic parameters for vasopressin binding were: apparent dissociation constants (Kd): 4.9 nM and 15 nM; maximal binding capacities: 0.83 pmoles/mg protein and 2.10⁵ sites/Cell for purified membranes and intact cells respectively. The relative affinities of 19 vasopressin structural analogues were deduced from competition experiments and compared to the previously determined glycogenolytic (or anti-glycogenolytic) potencies of these analogues. For both agonists and antagonists, a highly significant correlation was demonstrated between pKd and pKa (or pKi) values, suggesting that the detected binding sites are the physiological receptors involved in the glycogenolytic action of vasopressin on the rat liver. The affinity of antagonists for binding to these receptors is the same for both membranes and cells. In contrast, agonists which bind to vasopressin receptor sites have a higher affinity for purified membranes than for intact cells (Kd membranes/Kd cells = 8 + 1). GTP (0.1 mM) reduced the affinity of agonists but not of antagonists for binding to membranes and abolished the differences between Kd values for binding to hepatocytes and membranes.