α-adrenoceptors mediating the positive inotropic effect of phenylephrine in the right ventricular muscle of the monkey (Macaca fuscata)

Summary

The property of adrenoceptors mediating the positive inotropic effect (PIE) in the ventricular muscle of the Japanese monkey (Macaca fuscata) was investigated by the use of phenylephrine (PE) and adrenoceptor antagonists. The intrinsic activity (0.6) and the pD₂-value (5.41) for PE were comparable to those in other mammalian species. The β-adrenoceptor antagonist, pindolol (3×10⁻⁸ mol/l) shifted only the upper part of the concentration-response curve (CRC) for PE to the right; the pD₂-value for PE was not significantly affected by pindolol. On the other hand, phentolamine (10⁻⁶ mol/l) shifted the lower part of the CRC for PE more than the upper part. In the presence of both pindolol and phentolamine the curve was shifted to the right in a parallel manner. The time required for twitch relaxation was negatively correlated to the degree of PIE of PE in the presence of phentolamine but not pindolol. These results indicate that both β- and α-adrenoceptors mediate the positive inotropic action in the ventricular muscle of the Japanese monkey and that in contrast to the action via β-adrenoceptors the action via α-adrenoceptors is not accompanied by the “relaxant effect”.