The liver angiotensin receptor involved in the activation of glycogen phosphorylase

Abstract

Specific angiotensin binding to rat hepatocytes and purified liver plasma membranes was measured by using biologically active [³H]angiotensin (sp. radioactivity 14Ci/mmol). The kinetic parameters for angiotensin binding to hepatocytes are: K₊₁ (association rate constant). 100μm⁻¹·min⁻¹; K–₁ (dissociation rate constant), 2min⁻¹; K_d (dissociation constant). 30nm; maximal binding capacity, 0.42pmol/10⁶ cells or 260000 sites/cell. Angiotensin binding to membranes is profoundly affected by GTP (0.1mm) and NaCl (100mm); these regulatory compounds greatly enhance both the rate of association and of dissociation and also the extent of dissociation. K_d amounts to 10nm in the presence of GTP+NaCl and to 1.5nm in their absence; maximal binding capacity is 0.70pmol/mg of protein, both with or without GTP+NaCl. The relative affinities of 11 angiotensin structural analogues were deduced from competition experiments for [³H]angiotensin binding to hepatocytes and to membranes (in the latter case, GTP + NaCl were not included, in order to study the higher affinity state of the receptor). These are highly correlated with their biological activity (activation of glycogen phosphorylase in hepatocytes). Binding to membranes occurs in the same concentration range as the biological effect. On the other hand, the existence of numerous spare receptors is suggested by the observation that binding of the agonists to hepatocytes requires 25-fold higher concentrations than those needed for their biological activity. These data clearly suggest that the detected binding sites correspond to the physiological receptors involved in the glycogenolytic action of angiotensin on rat liver.