We used authentic NO or NO from NO donors to show that the physiological levels of NO (<1 microM) induce a positive inotropic effect and demonstrated that the effect is evoked through a cGMP-dependent pathway. In isolated rat ventricular myocytes, authentic NO at 588 nM increased both cell shortening and the intracellular Ca(2+) ([Ca(2+)]i) transient (133 and 117%, respectively; p < 0.05 vs. baseline), and 0.16-1.7 microM NO elicited reproducible dose-dependent increases in cell shortening. NOC18 (0.1 mM: actual NO concentration 673 nM) or SNAP (0.1 mM: actual NO concentration 285 nM) showed similar effects (shortening 215% and [Ca(2+)]i transient 160% increases, and shortening 148% and [Ca(2+)]i transient 117% increases, respectively). The NO-induced increases in cell shortening and the [Ca(2+)]i transient were inhibited by an inhibitor of soluble guanylate cyclase (ODQ, 30 microM) or by an inhibitor of cAMP-dependent protein kinase (KT5720, 0.1 microM). In the presence of an inhibitor of cGMP-inhibited cAMP-phosphodiesterase (milrinone, 10 microM), NO failed to increase both cell shortening and the [Ca(2+)]i transient. These results suggest that physiological levels of NO induce positive inotropy through a cGMP-dependent pathway.