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 protein kinase (PKG) is a necessary step in the chain of events leading to the 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 effect because they elevate cyclic GMP levels in a pool that does not activate the kinase. This hypothesis was tested in the present study by monitoring the effects of carbachol, SNP and atrial natriuretic peptide (ANP) on contractility, cyclic GMP content and PKG activity in rat intact ventricular preparations and freshly isolated ventricular cardiomyocytes. 2. The presence of PKG in both the intact vehicle and in isolated ventricular cardiomyocytes was confirmed by MonoQ anion exchange chromatography and Western blotting. The elution profile indicated that the conditions of the PKG assay were selective for measuring PKG activity. 3. Carbachol induced a marked negative inotropic effect in intact, perfused hearts and ventricular strips in the presence of isoproterenol. The negative inotropic effect of carbachol was not associated with significant changes in cyclic GMP content or PKG activity in intact ventricular tissue, or in PKG activity in isolated cardiomyocytes. 4. SNP and ANP significantly increased cyclic GMP levels and activated PKG in intact ventricular preparations. Both drugs also activated PKG in isolated cardiomyocytes. However, neither drug had any negative inotropic effect in isoprenaline-stimulated perfused hearts and ANP did not change the contractility of isoprenaline-stimulated isolated cardiomyocytes. 5. The results of this study demonstrate that the negative inotropic effects of muscarinic receptor agonists can occur in the absence of significant activation of PKG. Conversely, marked increases in ventricular cyclic GMP content and PKG activity caused by SNP or ANP were not 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.