Simasko S M
Department of Physiology, State University of New York at Buffalo 14214.
Am J Physiol. 1994 Mar;266(3 Pt 1):C709-19. doi: 10.1152/ajpcell.1994.266.3.C709.
The role of Na+ in the expression of membrane potential activity in the clonal rat pituitary cell line GH3 was investigated using the perforated patch variation of patch-clamp electrophysiological techniques. It was found that replacing bath Na+ with choline, tris(hydroxymethyl)aminomethane (Tris), or N-methyl-D-glucamine (NMG) caused the cells to hyperpolarize 20-30 mV. Tetrodotoxin had no effect. The effects of the Na+ substitutes could not be explained by effects on potassium or calcium currents. Although all three Na+ substitutes suppressed voltage-dependent calcium current by 10-20%, block of voltage-dependent calcium current by nifedipine or Co2+ did not result in hyperpolarization of the cells. There was no effect of the Na+ substitutes on voltage-dependent potassium currents. In contrast, all three Na+ substitutes influenced calcium-activated potassium currents [IK(Ca)], but only at depolarized potentials. Choline consistently suppressed IK(Ca), whereas Tris and NMG either had no effect or slightly increased IK(Ca). These effects on IK(Ca) also cannot explain the hyperpolarization induced by removing bath Na+. Choline always hyperpolarized cells yet suppressed IK(Ca). Furthermore, removing bath Na+ caused an increase in cell input resistance, an observation consistent with the loss of a membrane conductance as the basis of the hyperpolarization. Direct measurement of background currents revealed a 12-pA inward current at -84 mV that was lost upon removing bath Na+. These results suggest that this background sodium conductance provides the depolarizing drive for GH3 cells to reach the threshold for firing calcium-dependent action potentials.
利用膜片钳电生理技术的穿孔膜片变体，研究了Na⁺在克隆大鼠垂体细胞系GH3膜电位活性表达中的作用。发现用胆碱、三（羟甲基）氨基甲烷（Tris）或N-甲基-D-葡萄糖胺（NMG）替代浴液中的Na⁺会使细胞超极化20 - 30 mV。河豚毒素无作用。Na⁺替代物的作用无法用对钾电流或钙电流的影响来解释。尽管所有三种Na⁺替代物都使电压依赖性钙电流抑制了10 - 20%，但用硝苯地平或Co²⁺阻断电压依赖性钙电流并不会导致细胞超极化。Na⁺替代物对电压依赖性钾电流无作用。相反，所有三种Na⁺替代物都影响钙激活钾电流[IK(Ca)]，但仅在去极化电位时。胆碱持续抑制IK(Ca)，而Tris和NMG要么无作用，要么使IK(Ca)略有增加。这些对IK(Ca)的影响也无法解释去除浴液中Na⁺所诱导的超极化。胆碱总是使细胞超极化但抑制IK(Ca)。此外，去除浴液中的Na⁺会导致细胞输入电阻增加，这一观察结果与作为超极化基础的膜电导丧失一致。对背景电流的直接测量显示，在-84 mV时存在12 pA的内向电流，去除浴液中的Na⁺后该电流消失。这些结果表明，这种背景钠电导为GH3细胞提供去极化驱动力，使其达到触发钙依赖性动作电位的阈值。
