Na(+)-induced Ca(2+) influx through reverse mode of Na(+)-Ca(2+) exchanger in mouse ventricular cardiomyocyte

BACKGROUND: Dobutamine is commonly used for clinical management of heart failure and its pharmacological effects have long been investigated as inotropics via β–receptor activation. However, there is no electrophysiological evidence if dobutamine contributes inotropic action due at least partially to the reverse mode of Na(+)-Ca(2+) exchanger (NCX) activation. METHODS: Action potential (AP), voltage-gated Na(+) (I(Na)), Ca(2+) (I(Ca)), and K(+) (I(to) and I(K1)) currents were observed using whole-cell patch technique before and after dobutamine in ventricular cardiomyocytes isolated from adult mouse hearts. Another sets of observation were also performed with Kb-r7943 or in the solution without [Ca(2+)](o). RESULTS: Dobutamine (0.1–1.0 μM) significantly enhanced the AP depolarization with prolongation of AP duration (APD) in a concentration-dependent fashion. The density of I(Na)was also increased concentration-dependently without alternation of voltage-dependent steady-status of activation and inactivation, reactivation as well. Whereas, the activities for I(Ca), I(to), and I(K1) were not changed by dobutamine. Intriguingly, the dobutamine-mediated changes in AP repolarization were abolished by 3 μM Kb-r7943 pretreatment or by simply removing [Ca(2+)](o) without affecting accelerated depolarization. Additionally, the ratio of APD(50)/APD(90) was not significantly altered in the presence of dobutamine, implying that effective refractory period was remain unchanged. CONCLUSION: This novel finding provides evidence that dobutamine upregulates of voltage-gated Na(+) channel function and Na(+) influx-induced activation of the reverse mode of NCX, suggesting that dobutamine may not only accelerate ventricular contraction via fast depolarization but also cause Ca(2+) influx, which contributes its positive inotropic effect synergistically with β-receptor activation without increasing the arrhythmogenetic risk.