Effects of pimobendan, a new cardiotonic agent, on contractile responses in single skeletal muscle fibres of the frog.

Effects of a new cardiotonic agent, pimobendan, on contraction were investigated in single intact skeletal muscle fibres of the frog. Pimobendan increased twitch tension in a concentration-dependent manner regardless of the presence or absence of Ca2+ without any effect on tetanic tension, the resting membrane potential and the shape of the action potential. Pimobendan caused a further increase in twitch tension potentiated by caffeine (1 mM). Adenine, an inhibitor of Ca2+-induced Ca2+ release from the sarcoplasmic reticulum, inhibited twitch tension potentiated by caffeine but not by pimobendan, suggesting that twitch potentiation by pimobendan is not attributed to increases in Ca2+-induced Ca2+ release. Pimobendan failed to increase cAMP levels in the skeletal muscle, though forskolin significantly increased it without any effect on twitch tension. Contractile responses to high concentrations of caffeine and K+ were also potentiated by pimobendan. These results suggest that the potentiating effect of pimobendan on skeletal muscle contraction is mainly due to the increase in Ca2+ sensitivity to the contractile apparatus.