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

通过分析Ca^<2+>对Na通道的电压依赖性阻断，探讨Ca^<2+>通过心脏Na通道渗透的可能性（“滑移模式电导”）。生理离子环境（140mM-Na^+和移液溶液中1 ~ 10mM-Ca^<2+>）下细胞贴壁单通道记录时，大鼠和豚鼠心室肌细胞中Ca^<2+>通道阻滞明显。增加移液管溶液中Ca^<2+>的外部浓度（[Ca^<2+>]_0），以负电位为主降低了单一电流幅值。当[Ca^<2+>]_0>1mM时，尽管驱动力增强，但在负电位至-40mV时，单一电流幅值没有增加。5μ m异丙肾上腺素可增强保持电位为120mv的去极化脉冲引起的单通道活性，表明所观察的斑块中的通道受到了蛋白激酶A （PKA）刺激的修饰。缬草碱修饰的钠通道也证实了活性增加，其中通道开口明显延长。在这两种情况下，异丙肾上腺素诱导的增强都没有降低或改变Ca^<2+>阻断心脏Na通道的特性，这一点可以从-20和-60mV之间的稳定的单一电流幅值中得到证明。这些结果表明，Na^+, Ca^<2+>与通道分子之间的相互作用在渗透特性方面没有改变。因此，他们反对经典心脏Na通道的“滑移模式”概念，如果离子通过“多离子孔”渗透的一般概念适用于确定Na通道的离子选择性。

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.