EFFECT OF LEMAKALIM ON ACTION-POTENTIALS, INTRACELLULAR CALCIUM, AND CONTRACTION IN GUINEA-PIG AND HUMAN CARDIAC MYOCYTES

Objective: The aim was to investigate the effects of lemakalim on action potential duration, intracellular free calcium ([free Ca2+](i)), and cell contraction in human and guinea pig cardiac myocytes. In addition, the possible modulation by pH of lemakalim induced activation of ATP sensitive potassium (K-ATP) channels was assessed. Methods: Single ventricular myocytes were enzymatically dissociated from adult male guinea pigs (300-600 g). Single myocytes were isolated from human ventricular tissues. Cells were loaded with the acetoxymethyl ester form of fura-2 to monitor changes in [free Ca2+](i) and subjected to conventional electrophysiological techniques. Results: In guinea pig cells, lemakalim (3, 10, 30 mu M) reduced action potential duration in a concentration dependent manner. This decrease was accompanied by hyperpolarisation of the resting membrane potential. Lemakalim (3, 10, 30 mu M) reduced the systolic fura-2 fluorescence ratio without having a significant effect on diastolic fluorescence and also reduced the cell contraction in concentration dependent manner. Glibenclamide (1 mu M), a specific inhibitor of K-ATP channels, did not affect action potential duration, fura-2 fluorescence ratio, or cell contraction in the absence of lemakalim. However, the same dose of glibenclamide markedly inhibited the lemakalim induced decrease in action potential duration, fura-2 fluorescence ratio, and cell contraction. Reducing extracellular pH enhanced the decrease in action potential duration induced by lemakalim. In human ventricular myocytes, lemakalim (3, 10 and 30 mu M) caused a decrease in action potential duration and systolic fura-2 fluorescence ratio. The reduction in action potential duration and fura-2 fluorescence ratio was also reversed by glibenclamide (1 mu M). Conclusions: These results suggest that lemakalim reduces systolic [free Ca2+](i) by activating ATP sensitive potassium channels which results in a decrease of action potential duration in guinea pig and human ventricular myocytes. The reduction in [free Ca2+](i) mediates the negative inotropic effect induced by lemakalim. In addition, pH may modulate the K-ATP channel activation by the channel opener.