CYCLIC GMP-DEPENDENT PROTEIN-KINASE ACTIVATION IN THE ABSENCE OF NEGATIVE INOTROPIC EFFECTS IN THE RAT VENTRICLE

1 It has been suggested that activation of cyclic GMP-dependent protei n kinase (PKG) is a necessary step in the chain of events leading to t he production of negative inotropy by muscarinic receptor agonists in mammalian ventricles, and that some cyclic GMP-elevating agents, such as sodium nitroprusside (SNP), fail to exert a negative inotropic effe ct because they elevate cyclic GMP levels in a pool that does not acti vate the kinase. This hypothesis was tested in the present study by mo nitoring the effects of carbachol, SNP and atrial natriuretic peptide (ANP) on contractility, cyclic GMP content and PKG activity in rat int act ventricular preparations and freshly isolated ventricular cardiomy ocytes. 2 The presence of PKG in both the intact vehicle and in isolat ed ventricular cardiomyocytes was confirmed by MonoQ anion exchange ch romatography and Western blotting. The elution profile indicated that the conditions of the PKG assay were selective for measuring PKG activ ity. 3 Carbachol induced a marked negative inotropic effect in intact, perfused hearts and ventricular strips in the presence of isoproteren ol. The negative inotropic effect of carbachol was not associated with significant changes in cyclic GMP content or PKG activity in intact v entricular tissue, or in PKG activity in isolated cardiomyocytes. 4 SN P and ANP significantly increased cyclic GMP levels and activated PKG in intact ventricular preparations. Both drugs also activated PKG in i solated cardiomyocytes. However, neither drug had any negative inotrop ic effect in isoprenaline-stimulated perfused hearts and ANP did not c hange the contractility of isoprenaline-stimulated isolated cardiomyoc ytes. 5 The results of this study demonstrate that the negative inotro pic effects of muscarinic receptor agonists can occur in the absence o f significant activation of PKG. Conversely, marked increases in ventr icular cyclic GMP content and PKG activity caused by SNP or ANP were n ot accompanied by a negative inotropic effect. 6 These results suggest that increases in cyclic GMP levels and activation of PKG do not play important roles in the regulation of rat ventricular contractility by muscarinic receptor agonists.