Effects of pimobendan on the L-type Ca2+ current and developed tension in guinea-pig ventricular myocytes and papillary muscle: Comparison with IBMX,milrinone, and cilostazol

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