Angiotensin II–Induced Growth Responses in Isolated Adult Rat Hearts: Evidence for Load-Independent Induction of Cardiac Protein Synthesis by Angiotensin II

Abstract

Abstract Cardiac myocyte hypertrophy often occurs in response to both hemodynamic and neurohumoral factors. To study whether activation of the renin-angiotensin system by itself may induce a cardiac growth response, the acute effects of angiotensin II on cardiac protein synthesis were studied in isolated rat hearts. New protein synthesis in isolated buffer-perfused adult rat hearts was measured by incorporation of [3 H]phenylalanine into cardiac proteins during a 3-hour perfusion protocol. Angiotensin II (1×10−8 mol/L), administered alone or in combination with the α 1-blocker prazosin (1×10−7 mol/L), stimulated protein synthesis in both ventricles. The rate of [3 H]phenylalanine incorporation into cardiac proteins was 3.9-fold (P<.005) and 2.6-fold (P<.01) higher in angiotensin II–perfused (n=6) than in vehicle-perfused (n=6) left and right ventricles, respectively. The induction of new protein synthesis by angiotensin II was blocked by the angiotensin II type 1 (AT 1) receptor antagonist losartan (1×10−7 mol/L, n=5). To study the pathways of angiotensin signal transduction, protein kinase C (PKC)-ε as well as cardiac c-fos and c-jun mRNA levels were analyzed. Angiotensin II (1×10−8 mol/L, n=20) resulted in a transient translocation of PKC-ε from the cytosol to the cellular membrane. However, compared with phorbol ester stimulation (phorbol 12-myristate 13-acetate [PMA], 1×10−7 mol/L; n=20), angiotensin II effects on PKC translocation were significantly less pronounced and required a more prolonged stimulation. There was no effect of angiotensin II in concentrations from 10−9 to 10−6 mol/L (n=22) on c-fos and c-jun mRNA levels in intact adult rat hearts studied over a time course from 15 to 120 minutes of perfusion. In contrast, norepinephrine (10−6 mol/L, n=6), phorbol ester (PMA, 10−7 mol/L; n=5), and calcium ionophore (A23187, 2.5×10−6 mol/L; n=5) infusion as well as elevated left ventricular systolic wall stress (n=6) were all followed by a threefold to fourfold induction of cardiac c-fos and c-jun mRNA levels (P<.005) compared with respective angiotensin II–infused or vehicle-infused rat hearts (n=12). In contrast, administration of angiotensin II in concentrations >10−9 mol/L caused a significant induction of c-fos in adult and neonatal cardiac myocytes. In conclusion, angiotensin II acutely stimulates protein synthesis in cultured adult isolated perfused rat hearts. Angiotensin II–activated signal transduction appears to involve AT 1 receptors and activation of PKC. However, further downstream signaling mechanisms remain elusive, since angiotensin II may stimulate protein synthesis in the adult intact heart without preceding c-fos and c-jun proto-oncogene induction.