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 [(3)H]angiotensin (sp. radioactivity 14Ci/mmol). The kinetic parameters for angiotensin binding to hepatocytes are: K(+1) (association rate constant). 100mum(-1).min(-1); K(-1) (dissociation rate constant), 2min(-1); K(d) (dissociation constant). 30nm; maximal binding capacity, 0.42pmol/10(6) 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 [(3)H]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.

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.…”

Effects of noradrenaline, vasopressin and angiotensin on the Na‐K pump in rat isolated liver cells

“…The action ofeach hormone was dose-dependent, blocked by preincubation with the corresponding antagonist and occured via activation of different types of receptors (see also Campanile et al, 1982;Keppens et al, 1982;Jard, 1983). Two different results seem to indicate that noradrenaline, angiotensin and vasopressin make use of a common mechanism to control Na-K pump activity in this tissue in keeping with their action on the Ca-dependent phosphorylase, the key enzyme for glycogen breakdown, and on Ca fluxes (see Williamson et al, 1981;Exton, 1981;.…”

Amiloride enhances the secretion but not the synthesis of renin in renal juxtaglomerular cells

“…Considering the relatively long times of incubation and the weak stability of native angiotensin II even in vitro (Keppens et al 1982), we used the more stable angiotensin II analogue [Sar t, Ala s] angiotensin II (1 gM) (Hall et al 1974) for our experiments. As shown in Fig.…”