ANGIOTENSIN-II STIMULATES THE SYNTHESIS OF ANGIOTENSINOGEN IN HEPATOCYTES BY INHIBITING ADENYLYLCYCLASE ACTIVITY AND STABILIZING ANGIOTENSINOGEN MESSENGER-RNA

Angiotensin II stimulates the hepatic synthesis and secretion of angio tensinogen, the substrate of renin. In the present study performed on freshly isolated rat hepatocytes we demonstrate that this effect of an giotensin II is mainly related to a transient inhibition of adenylylcy clase. Agents known to decrease intracellular cAMP (angiotensin II, va sopressin, guanfacine) or the cAMP-antagonist Rp-adenosine-3',5'-cycli c phosphothioate stimulated, whereas cAMP-stimulating agents (isoprote renol, forskolin, glucagon) or the cAMP-agonist Sp-adenosine-3',5'-cyc lic phosphothioate inhibited angiotensinogen synthesis. In contrast, a ll agents known to affect intracellular concentrations of calcium, as confirmed in Fura-2-loaded hepatocytes (Bay K 8644, calcimycin, calmid azolium, ionomycin, or methoxamine) failed to influence the synthesis of angiotensinogen. The inhibitory effect of angiotensin II as well as the stimulatory effect of glucagon on cAMP were inversely related to angiotensinogen mRNA and angiotensinogen secretion over a wide concent ration range of both peptides. Both the angiotensin II-dependent inhib ition of cAMP and the angiotensin II-induced increase in angiotensinog en mRNA were abolished by a pertussis toxin pretreatment. In hepatocyt e membranes, pertussis toxin ADP-ribosylated a single protein (approxi mately 41 kDa) probably representing the alpha-subunit of the G(i)-pro tein, coupling inhibitory receptors to adenylylcyclase. We further sho w that the increase of angiotensinogen mRNA and secretion mainly repre sents the result of mRNA stabilization, since in a nuclear run-on assa y, angiotensin II pretreatment of hepatocytes does not significantly a lter the rate of [P-32]UTP incorporation into angiotensinogen mRNA, wh ereas angiotensin II prolonged the half-life of angiotensinogen mRNA i n transcription-arrested as well as in [H-3]uridine pulse-labeled hepa tocytes about 2.5-fold from 80 to 190 min. It is concluded that angiot ensin II induces an increase in angiotensinogen synthesis in hepatocyt es by stabilizing of angiotensinogen mRNA and that this effect is medi ated through inhibition of adenylylcyclase.