Low increase in cGMP induced by organic nitrates and nitrovasodilators improves contractile response of rat ventricular myocytes

Whether organic nitrates are bioactivated to NO in cardiac muscle cells and may thus directly affect cardiac contractile function has remained an open question. Therefore, we determined the effects of the organic nitrates glyceryl trinitrate (100 mu mol/L), pentaerythritol tetranitrate (10 mu mol/L), and isosorbide-5-mononitrate on electrically stimulated contractile response (CR) and cAMP and cGMP content of isolated adult rat ventricular cardiomyocytes compared with different concentrations of the spontaneous NO donors S-nitroso-N-acetyl-d,1-penicillamine (SNAP) and 2,2-diethy-1-hydroxy-1-nitroso-hydrazine (DEA/NO). A high concentration of spontaneous NO donors (100 mu mol/L) caused a large increase in cGMP content that was accompanied by a decrease in CR to 73.8+/-6.7% (SNAP) and 80.9+/-6.1% (DEA/NO) of the control values. Inhibition of cGMP-dependent protein kinase by 10 mu mol/L KT 5822 converted this effect into a pronounced improvement of CR (163.5+/-14.0%). By contrast, the organic nitrates caused a small but significant increase in cGMP, which was accompanied by an increase in cAMP and CR identical to that induced by 10 nmol/L isoprenaline (141.6+/-6.4%). A similar effect was observed with a low concentration (1 mu mol/L) of SNAP and DEA/NO. All increases in CR induced by nitrates were abolished after inhibition of cAMP-dependent protein kinase by Rp-cAMPS (10 mu mol/L). The positive contractile effect of isoprenaline was enhanced by 1 mu mol/L SNAP. This effect was also demonstrated in isolated rat papillary muscles. These results indicate that in cardiac muscle (1) organic nitrates are bioactivated to NO; (2) this results in a moderate increase in cGMP, which causes an improved CR by increasing cAMP and activating cAMP-dependent protein kinase; and (3) a large increase in cGMP, produced by high doses of NO donors, reduces CR because of the activation of cGMP-dependent protein kinase.