(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-vasopressin (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 2.10(5) 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 antiglycogenolytic) 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 (0.1mM) 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.

Periportal and perivenous hepatocytes respond equally to glycogenolytic agonists

“…In both cell populations maximal activations are similar albeit somewhat lower with phenylephrine and ATP. KA values (agonist concentrations at which half maximal activation is obtained) have been computed as described previously [3,[13][14][15] These data do not however exclude an in vivo occurrence of zonation of hormonal glycogenolysis. Indeed, using a perfused system, H~iussinger et al [16] reported on liver heterogeneity in response to extracellular ATP, the periportal area being more responsive to ATP than the perivenous hepatocytes.…”

Size of Vasopressin Receptors from Rat Liver and Kidney

“…Using a series of vasopressin structural analogues, it was possible to demonstrate a close correlation between the relative abilities of these peptides to bind to hepatocytes and liver membranes and their rclative abilities to activate glycogen phosphorylase [1,2].These observations suggest that the detected vasopressin binding sites are the receptor involved in the glycogenolytic action of vasopressin.…”

“…Dissociation constants for vasopressin binding to kidney receptors [ 3 ] and to liver receptors [2] are similar. Guanyl nucleotides affect vasopressin binding to both kinds of receptor.…”

“…However, marked functional differences between these receptors have been demonstrated. Thus, vasopressin binding to kidney membranes triggers adenylate cyclase activation [4,5] but no such activation was found after vasopressin binding to liver membranes [ 2 ] . The relative potencies of a series of structural vasopressin analogues for binding to liver and kidney rcceptors are clearly _ _ _ _~ Ahhrevicition.…”

“…The relative potencies of a series of structural vasopressin analogues for binding to liver and kidney rcceptors are clearly _ _ _ _~ Ahhrevicition. p[NH]ppG, guanosine 5'-[B,i'-imido]triphospha te distinguishable which indicates that hormone binding sites on each type of receptor are different [2,3].…”