Positive inotropic and negative lusitropic effect of angiotensin II: Intracellular mechanisms and second messengers

In the cat ventricle angiotensin H exerts a positive inotropic effect produced by an increase in intracellular calcium associated with a prolongation of relaxation. The signaling cascades involved in these effects as well as the subcellular mechanisms of the negative lusitropic effect are still not clearly defined. The present study was directed to investigate these issues in cat papillary muscles and isolated myocytes. The functional suppression of the sarcoplasmic reticulum (SR) with either 0.5 muM ryanodine or 0.5 muM ryanodine plus I um thapsigargin or the preincubation of the myocytes with the specific inhibitor of the inositol 1,4,5-triphosphate (IP3) receptors [diplienylborinic acid, ethanolamine ester (2-APB), 5-50 muM] did not prevent the positive inotropic effect and the increment in Ca2+ transient produced by 1 muM angiotensin II. In contrast, protein kinase C (PKC) inhibitors, chelerythrine (20 muM) and calphostin C (1 muM) completely inhibited both, the angiotensin II-induced increase in L-type calcium current and positive inotropic effect. The prolongation of half relaxation time produced by 0.5 muM angiotensin II [207 +/- 15.4 msec (control) to 235 +/- 19.98 msec (angiotensin H), P <0.05] was completely blunted by PKC inhibition. This antirelaxant effect, which was independent of intracellular pH changes, was associated with a prolongation of the action potential duration and was preserved after either the inhibition of the SR and the SR Ca(2+)ATPase (ryanodine plus thapsigargin) or of the reverse mode of the Na+/Ca2+ exchanger (KB-R7943, 5 muM). We conclude that in feline myocardium. the positive inotropic and negative lusitropic effects of angiotensin H are both entirely mediated by PKC without any significant participation of the IP3 limb of the phosphatidyltnositol/phospholipase C cascade. The results suggest that the antirelaxant effect of angiotensin H might be determined by the decrease in Ca2+ efflux through the Na+/Ca2+ exchanger produced by the angiotensin H-induced prolongation of the action potential duration. (C) 2001 Academic Press.