Differential Effects of Protein Kinase C Activators and Inhibitors on α- and β-Adrenoceptor-mediated Positive Inotropic Effect in Isolated Rabbit Papillary Muscle

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Abstract
Background: A number of novel agents that activate or inhibit protein kinase C (PKC) in vitro have been developed to evaluate the physiologic role of PKC in regulation of cellular function. However, most of the PKC inhibitors also affect the protein kinase A, and the effects of these agents in intact myocardium remain still controversial. The present study was carried out to examine the effects of these agents on the positive inotropic effect (PIE) medi ated by α- and β-adrenoceptors in isolated rabbit papillary muscle. Methods and Results: A potent PKC activator, phorbol 12, 13-dibutyrate (PDBu) at 10 and 30 nM, induced a significant PIE. PDBu at 3 nM and higher inhibited the α-mediated PIE and abolished it at 100 nM without affecting the β-mediated PIE. Phorbol 12-myristate 13- acetate (PMA) and 1-oleyl-2-acetyl-sn-glycerol (OAG) elicited a similar selective inhibitory action on the α-mediated PIE. The PIE of PDBu was abolished by chelerythrine and partially inhibited by staurosporine, but H-7 or calphostin-C did not affect the PIE. These PKC inhibitors consistently inhibited the α-mediated PIE by 20-30% at concentrations that they did not affect the β-mediated PIE. None of the PKC inhibitors influence the PDBu-induced inhibitory action on the α-mediated PIE, an indication that they failed to reach the site of the inhibitory action of PDBu. Conclusion: Selective modulation by the PKC activators and inhibitors of the α-mediated PIE with little effect on the β-mediated PIE implies that the activation of PKC has a physiological relevance to the α-mediated PIE. However, the externally administered PKC activators do not mimic the effect of diacylglycerol that is generated endogenously by α-stimulation. By con trast, externally applied PKC inhibitors selectively antagonize the α-adrenoceptor-mediated PIE in rabbit ventricular myocardium.