Abstract
The influence of protein kinase C (PKC) inhibitors, staurosporine, NA 0345 and H-7, on the α1- and β-adrenoceptor-mediated positive inotropic effect (PIE) was studied in rabbit ventricular myocardium. Staurosporine (1-10 nM), NA 0345 (10-100 nM) and H-7 (1-10 μM) selectively attenuated the PIE mediated by α1-adrenoceptors at concentrations that did not affect the β-mediated PIE and basal force of contraction. Staurosporine at higher concentrations (> 10 nM) decreased the basal force, while NA 0345 and H-7 did not. In membrane fractions derived from rabbit ventricular muscle, neither staurosporine, NA 0345 nor H-7 modified the specific [3H]prazosin binding at the concentrations that elicited the functional modulation. Accumulation of [3H]inositol monophosphate (IP1) induced by α1-adrenoceptor stimulation was not affected by the PKC inhibitors. Phorbol 12, 13-dibutyrate (PDBu), a PKC activator, also selectively attenuated the α1-mediated PIE, but in association with the inhibition of the α1-mediated IP, accumulation. Staurosporine (1 nM), but not H-7, antagonized the PDBu-induced inhibitory action on the a1-mediated PIE. These findings indicate that staurosporine, NA 0345 and H-7 produce a selective inhibition of the α1-mediated PIE, probably through inhibition of the α1-adrenoceptor-mediated activation of PKC. On the contrary, externally administered phorbol ester may act by uncoupling of α1-adrenoceptors to activation of phospholipase C through a pathway different from endogenous diacylglycerol to lead to a selective inhibition of the α1-mediated PIE.
