α1-adrenoceptor-mediated phosphoinositide breakdown and inotropic response in rat left ventricular papillary muscles

Abstract

α1-Adrenoceptor stimulation of rat left ventricular papillary muscles by phenylephrine in the presence of propranolol resulted in rapid breakdown of phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2) and a triphasic inotropic response in a concentration-dependent manner. The release of inositol trisphosphate (IP3) was maximum within 30 seconds and remained high for at least 30 minutes. The IP3 formation was associated with a rapid, but small, increase in contractile force followed by a transient decline in the contractility prior to the development of a sustained and more pronounced positive inotropic response. Inhibition of PI-4,5-P2 hydrolysis by the α1-adrenergic antagonist prazosin or the PI-4,5-P2 phosphodiesterase inhibitor neomycin blocked all components of the inotropic responses. Combined addition of 2,3-diphosphoglyceric acid, a competitive inhibitor of IP3 phosphatase, with phenylephrine doubled the IP3 formation and potentiated the initial phases of inotropic responses but had no effect on the sustained positive inotropic response. Nifedipine and Mn2+ did not block the transient inotropic responses but inhibited the sustained positive inotropic response. α1-Adrenoceptor stimulation resulted in restoration of slow responses in the high K+depolarized muscles in the time course similar to that of the development in the sustained positive inotropic response. Addition of phorbol-12,13-dibutyrate alone or in combination with caffeine or A23187 failed to produce a sustained positive inotropic effect, but pretreatment with this phorbol ester (1-100 nM) for 30 minutes resulted in dose-dependent potentiation of α1-adrenoceptor-mediated sustained positive inotropic effect associated with enhanced slow responses. These results suggest that the inotropic effects mediated by cardiac α1-adrenoceptor stimulation occur through the phosphodiesteratic cleavage of PI-4,5-P2, such that IP3 may produce transient inotropic effects by mobilizing intracellular Ca2+, while diacylglycerol, along with cofactors that are also generated on α1-adrenoceptor stimulation, may provoke a sustained positive inotropic effect by potentiating slow Ca2+ channels through activation of protein kinase C.