The effect of membrane potential and various extracellular monovalent cations on the Na+ pump current (Ip) was studied on isolated, single Purkinje cells of the rabbit heart by means of whole-cell recording. 2. Ip was identified as current activated by external K+ or its congeners NH4+ and Tl+. The current was blocked by dihydroouabain (1-5 x 10(-4) M) over the whole range of membrane potentials tested. 3. In Na(+)-containing solution half-maximum Ip activation (K0.5) occurred at 0.4 mM-Tl+, 1.9 mM-K+ and 5.7 mM-NH4+ (holding potential, -20 mV). 4. The pump current (Ip)-voltage (V) relationship of the cells in Na(+)-containing media with K+ or its congeners at the tested concentrations greater than K0.5 displayed a steep positive slope at negative membrane potentials between -120 and -20 mV. Little voltage dependence of Ip was observed at more positive potentials up to +40 mV. At even more positive potentials Ip measured at 2 and 5.4 mM-K+ decreased. 5. Lowering the concentration of K+ or its congeners below the K0.5 value in Na(+)-containing solution induced a region of negative slope of the Ip-V curve at membrane potentials positive to -20 mV. 6. The shape of the Ip-V relationship remained unchanged when the K+ concentration (5.4 mM) of the Na(+)-containing medium was replaced by NH4+ or Tl+ concentrations of similar potency to activate Ip (20 mM-NH4+ or 2 mM-Tl+). 7. In Na(+)-free, choline-containing solution half-maximum Ip activation occurred at 0.13 mM-K+ (holding potential, -20 mV). 8. At negative membrane potentials the positive slope of the Ip-V curve was flatter in Na(+)-free than in Na(+)-containing media. A reduced voltage dependence of Ip persisted, regardless of whether choline ions or Li+ were used as a Na+ substitute. 9. Lowering the K+ concentration of the Na(+)-free, choline-containing solution to 0.05 mM evoked an extended region of negative slope in the Ip-V relationship at membrane potentials between -40 and +60 mV. 10. It is concluded that the apparent affinity of the Na(+)-K+ pump towards K+ in cardiac Purkinje cells depends on both the membrane potential and the extracellular Na+ concentration. 11. The region of negative slope of the Ip-V curve observed in cells which were superfused with media containing low concentrations of K+ or its congeners strongly suggests the existence of at least two voltage-sensitive steps in the cardiac Na(+)-K+ pump cycle.(ABSTRACT TRUNCATED AT 400 WORDS)
摘要
采用全细胞记录法，在兔心脏分离的单个浦肯野细胞上研究了膜电位和各种细胞外单价阳离子对钠泵电流（Ip）的影响。2. Ip被确定为由细胞外钾离子或其类似物铵离子和铊离子激活的电流。在测试的整个膜电位范围内，该电流被二氢哇巴因（1 - 5×10⁻⁴ M）阻断。3. 在含钠溶液中，Ip激活的半数最大值（K0.5）在0.4 mM - 铊离子、1.9 mM - 钾离子和5.7 mM - 铵离子时出现（钳制电位，-20 mV）。4. 在含钠培养基中，当钾离子或其类似物浓度大于K0.5时，细胞的泵电流（Ip）-电压（V）关系在 - 120至 - 20 mV的负膜电位处呈现陡峭的正斜率。在高达 + 40 mV的更正电位下，Ip对电压的依赖性很小。在甚至更正的电位下，在2和5.4 mM - 钾离子时测量的Ip降低。5. 在含钠溶液中，将钾离子或其类似物的浓度降低到K0.5值以下，在膜电位高于 - 20 mV时，Ip - V曲线出现负斜率区域。6. 当含钠培养基中的钾离子浓度（5.4 mM）被激活Ip效力相似的铵离子或铊离子浓度（20 mM - 铵离子或2 mM - 铊离子）取代时，Ip - V关系的形状保持不变。7. 在无钠、含胆碱的溶液中，Ip激活的半数最大值在0.13 mM - 钾离子时出现（钳制电位，-20 mV）。8. 在负膜电位下，Ip - V曲线的正斜率在无钠培养基中比在含钠培养基中更平缓。无论使用胆碱离子还是锂离子作为钠替代物，Ip对电压的依赖性都降低。9. 将无钠、含胆碱溶液中的钾离子浓度降低到0.05 mM，在 - 40至 + 60 mV的膜电位下，Ip - V关系中出现了一个扩展的负斜率区域。10. 得出结论，心脏浦肯野细胞中钠钾泵对钾离子的表观亲和力取决于膜电位和细胞外钠离子浓度。11. 在用含低浓度钾离子或其类似物的培养基灌流的细胞中观察到的Ip - V曲线负斜率区域强烈表明，心脏钠钾泵循环中至少存在两个电压敏感步骤。（摘要截于Word） 400字）
