Abstract
Background/Aims: Monensin, an Na ionophore, increases intracellular Na ([Na]i). Alteration of [Na]i influences ion transport through the sarcolemmal membrane. So far, the effects of monensin on ventricular myocytes have not been examined in detail. The main objective of this study was to elucidate the mechanism via which monensin-evoked increases in [Na]i affect the membrane potential and currents in ventricular myocytes of guinea pigs. Methods: Membrane potentials and currents were measured using the whole-cell patch-clamp technique in single myocytes. The concentration of intracellular Ca ([Ca]i) was evaluated by measuring fluorescence intensity of Fluo-4. Results: Monensin (10<sup>−5</sup>M) shortened the action potential duration (APD) and reduced the amplitude of the plateau phase. In addition, monensin decreased the sodium current (I<sub>Na</sub>) and shifted the inactivation curve to the hyperpolarized direction. Moreover, it decreased the L-type calcium current (I<sub>Ca</sub>). However, this effect was attenuated by increasing the buffering capacity of [Ca]i. The Na-Ca exchange current (I<sub>Na-Ca</sub>) was activated particularly in the reverse mode. Na-K pump current (I<sub>Na-K</sub>) was also activated. Notably, the inward rectifying K current (I<sub>K1</sub>) was not affected, and the change in the delayed outward K current (I<sub>K</sub>) was not evident. Conclusion: These results suggest that the monensin-induced shortened APD and reduced amplitude of the plateau phase are primarily due to the decrease in the I<sub>Ca</sub>, the activation of the reverse mode of I<sub>Na-Ca</sub>, and the increased I<sub>Na-K</sub>, and second due to the decreased I<sub>Na</sub>. The I<sub>K</sub> and the I<sub>K1</sub> may not be associated with the abovementioned changes induced by monensin. The elevation of [Na]i can exert multiple influences on electrophysiological phenomena in cardiac myocytes.
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