Modulation of ionic selectivity of cardiac Na channels : single channel analysis of "slip mode conductance"

通过对钙离子通道电压依赖性阻断的分析，探讨了钙离子通过心肌钠离子通道(“滑模电导”)的可能性。大鼠和豚鼠心肌细胞在生理离子环境(140 mM-Na和1-10 mM-Na；2-gt；)细胞贴壁单通道记录时，均出现明显的钙通道阻断现象。增加吸管溶液中的外加钙离子浓度([Ca^；lt；2&gt；]_0)可显著降低负电位下的单位电流幅度。对于[Ca^&lt；2]_0&gt；1 mm，尽管驱动力增大，但在-40 mV的负电位下，单位电流幅值并没有增加。应用5μM-异丙肾上腺素可增强保持电位为-120 mV的去极化脉冲所引起的单通道活动，表明该通道受蛋白激酶A的刺激而发生改变。藜芦碱修饰的钠通道的活性增加也得到证实，通道开放明显延长。在这两种情况下，异丙肾上腺素诱导的增强并不降低或改变心肌钠通道钙通道阻滞剂的特性，在-20 mV到-60 mV之间的稳定的单位电流幅度证明了这一点。这些结果表明，钠通道、钙通道分子和通道分子之间的相互作用并没有改变通透性。因此，如果离子通过“多离子孔”的一般概念适用于确定钠通道的离子选择性，他们反对经典心脏钠通道的“滑移模式”概念。

The possibility of Ca^<2+> permeation through cardiac Na channels ("slip mode conductance") was explorerd through the analysis of voltage dependent block of Na channels by Ca^<2+>. Ca^<2+> block of Na channels was evident in rat and guinea-pig ventricular myocytes during cell-attached single channel recording with physiological ionic environment (140mM-Na^+ and 1 to 10mM-Ca^<2+> in the pipette solution). Increasing external Ca^<2+> concentration ([Ca^<2+>]_0) in the pipette solution reduced the unitary current amplitude predominantly at negative potentials. With [Ca^<2+>]_0>1mM, unitary current amplitude did not increase at potentials negative to -40mV in spite of augmented driving forces. Application of 5μM-isoproterenol potentiated the single channel activity elicited by depolarizing pulses from the holding potential of-120mV, indicating that the channels in the patch under examination were modified by protein kinase A (PKA) stimulation. Increased activity was also confirmed with veratridine-modified Na channels, where channel openings were markedly prolonged. In either case, isoproterenol-induced potentiation did not reduce nor alter the properties of Ca^<2+> block of cardiac Na channels, as evidenced by the stable unitary current amplitudes at potential levels between -20 and -60mV.These results indicate that interactions among Na^+, Ca^<2+> and the channel molecule were not modified with respect to permeation properties. They therefore argue against "slip mode" concept of classical cardiac Na channel, if a general concept of ion permeation through "multi-ion pores" is applicable to determine the ionic selectivity of Na channels.