Larger late sodium current density as well as greater sensitivities to ATX II and ranolazine in rabbit left atrial than left ventricular myocytes

An increase of cardiac late sodium current (I-Na.L) is arrhythmogenic in atrial and ventricular tissues, but the densities of I-Na.L and thus the potential relative contributions of this current to sodium ion (Na+) influx and arrhythmogenesis in atria and ventricles are unclear. In this study, whole-cell and cell-attached patch-clamp techniques were used to measure I-Na.L in rabbit left atrial and ventricular myocytes under identical conditions. The density of I-Na.L was 67% greater in left atrial (0.50 +/- 0.09 pA/pF, n = 20) than in left ventricular cells (0.30 +/- 0.07 pA/pF, n = 27, P < 0.01) when elicited by step pulses from -120 to -20 mV at a rate of 0.2 Hz. Similar results were obtained using step pulses from -90 to -20 mV. Anemone toxin II (ATX II) increased I-Na.L with an EC50 value of 14 +/- 2 nM and a Hill slope of 1.4 +/- 0.1 (n = 9) in atrial myocytes and with an EC50 of 21 +/- 5 nM and a Hill slope of 1.2 +/- 0.1 (n = 12) in ventricular myocytes. Na+ channel open probability (but not mean open time) was greater in atrial than in ventricular cells in the absence and presence of ATX II. The I-Na.L inhibitor ranolazine (3, 6, and 9 mu M) reduced I-Na.L more in atrial than ventricular myocytes in the presence of 40 nM ATX II. In summary, rabbit left atrial myocytes have a greater density of I-Na.L and higher sensitivities to ATX II and ranolazine than rabbit left ventricular myocytes.