Pharmacological differentiation of synergistic contribution of L-type Ca²⁺ channels and Na⁺/H⁺ exchange to the positive inotropic effect of phenylephrine, endothelin-3 and angiotensin II in rabbit ventricular myocardium

The present study was designed to delineate pharmacologically the role of sarcolemmal L-type Ca²⁺ channels and Na⁺/H⁺ exchange in the positive inotropic effect (PIE) of phenylephrine mediated by alpha-1 adrenoceptors, endothelin (ET) and angiotensin II (Ang II) that stimulate phosphoinositide (PI) hydrolysis in the rabbit ventricular muscle. The PIE of these receptor agonists was compared with the PIE of isoprenaline that accumulates cyclic AMP. For this purpose, we investigated the influence of a Ca²⁺ antagonist, verapamil, and of an inhibitor of Na⁺/H⁺ exchange, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), alone or in combination, on the cumulative concentration-response curve (CRC) for phenylephrine (with 0.3 μM bupranolol), ET-3 and Ang II in isolated right ventricular papillary muscles of the rabbit, which were electrically stimulated at 1 Hz in Krebs-Henseleit solution at 37°C. Verapamil at 0.3 and μM decreased the basal force of contraction to 37.0 ± 4.0% and 13.2 ± 1.1% of the control, respectively, while EIPA even at 10 μM affected the basal force to much less extent and decreased it to 87.0 ± 1.4%. Verapamil (0.3 and 1 μM) and EIPA (1 and 10 μM), when used alone, each significantly attenuated but did not abolish the PIEs induced by phenylephrine, ET-3 and Ang II, while the simultaneous administration of verapamil (1 μM) and EIPA (10 μM) consistently and almost completely inhibited the PIE induced by these receptor agonists. By contrast, the PIE of isoprenaline was retained even in the presence of verapamil and EIPA. These results indicate that both the influx of Ca²⁺ ions through L-type Ca²⁺ channels and activation of Na²⁺ H⁺ exchange contribute synergistically to the PIE that is mediated by alpha-1 adrenergic, ET and Ang II receptor agonists, while these mechanisms are not essential for the beta-adrenoceptor-mediated PIE.