Effect of tinctormine on contraction and Ca(2+) currents in single cardiac myocytes from dog

Effect of tinctormine (TIN) on the contractile response and Ca(2+) currents (lea) were investigated in dispersed single canine ventricular myocytes. Contraction were measured by a video-edge detector and Ca(2+) currents were recorded using the whole-cell voltage clamp technique. TIN in low concentrations (10(-7)M) selectively reduced the contractile response in single ventricular cells by 50% of the control without affecting Ica. These inhibitory effects of the contractile response showed concentration-dependency and were reversible. However, TIN in high concentrations (10(-5)M) reduced the peak Ca(2+) currents by 42% of the controls and completely inhibited the contractile response in single myocytes. These inhibitory effects on lea were concentration-dependent. These results assumed that tinctormine exerts a negative inotropic effect which is mediated in part through reduction of the peak Ca(2+) currents and possibly also due to inhibition of the Na(+)-Ca(2+) exchange process and/or inhibition of the Ca(2+) release from the sarcoplasmic reticulum (SR).