In this study, we compared the effects of pimobendan (PIM), a putative Ca(2+)-sensitizer and phosphodiesterase (PDE) inhibitor, on the L-type Ca2+ current (ICa) of guinea-pig ventricular myocytes and contractile tension of ventricular papillary muscles with those of a nonselective PDE inhibitor, isobutylmethylxanthine (IBMX), and selective PDE-III inhibitors, that is, milrinone (MIL) and cilostazol (CIL). The efficacy (maximum attainable effect) of these drugs for increasing ICa or developed tension (DT) ranged in the order of IBMX >> MIL > PIM > CIL. This finding suggests that the positive inotropic effect of each drug is roughly proportional to its increasing effect on ICa. The additional effect of PIM (a Ca(2+)-sensitizing effect) was not identified in "intact" preparations, and the potentiating effects of PIM on the DT and ICa were virtually the same as those observed for MIL. To isolate the Ca(2+)-sensitizing effect of PIM on the DT, we studied the effects of PIM in the presence of H89, an isoquinoline derivative possessing a selective inhibitory effect on cAMP-dependent protein kinase. In the absence of H89, 50 microM PIM increased the DT by 68 +/- 11% (mean +/- SE, n = 6). However, in the presence of 20 microM H89, which completely blocked the PIM-induced increase in ICa, PIM (50 microM) significantly increased the DT by 19 +/- 6% (n = 6), thereby indicating the presence of a positive inotropic effect attributable to a mechanism other than increased intracellular cAMP, that is, a Ca(2+)-sensitizing effect. The latter notion was supported by the finding that in the presence of H89 (20 microM), the PIM-induced augmentation of DT was accompanied by a prolongation of the time to 50% relaxation of contractile tension. In contrast, MIL (50 microM) and forskolin, a direct activator of adenylate cyclase (1-10 nM), did not increase DT in the presence of 20 microM H89. These results suggest that the fraction of positive inotropic effect of PIM attributable to its Ca(2+)-sensitizing effect is masked by its potent PDE-III inhibitory effect in "intact" ventricular preparations.