Hoshiko T, Grossman R A, Machlup S
Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, OH 44106.
Biochim Biophys Acta. 1988 Jul 7;942(1):186-98. doi: 10.1016/0005-2736(88)90288-x.
In a previous study, the amiloride-induced corner frequency (fc) was found to decrease as apical sodium was increased. This effect was small or absent when the basolateral surface was exposed to high potassium. It has been suggested that the apical sodium effect may be indirect, due either to increased intracellular [Na+] which repelled amiloride or to an increased potential at the apical surface which reduced amiloride affinity. High basolateral K+ might then suppress the sodium effect either by preventing intracellular [Na+] from increasing or by allowing a better clamp of the apical membrane potential by reducing basolateral membrane resistance and potential. We checked the effects of basolateral [K+], of cyanide and of ouabain at concentrations known to increase intracellular [Na+]. We found only negligible effects on fc. In addition, amphotericin B added to the basolateral bathing solution either in 115 mM Na+ or in 120 mM K+ had no significant effect on fc. We found that relatively wide variation in clamp potential under all conditions, even with active transport severely inhibited, left fc virtually constant. Since the amiloride kinetics were independent of clamp potential, we were able to measure paracellular and transcellular conductances separately by examining the voltage dependence of clamp current (linear) and amiloride noise power (quadratic). This made possible estimation of channel density and single-channel current.
在先前的一项研究中，发现随着顶端钠浓度升高，氨氯吡咪诱导的转角频率（fc）会降低。当基底外侧表面暴露于高钾环境时，这种效应很小或不存在。有人提出顶端钠效应可能是间接的，要么是由于细胞内[Na⁺]增加排斥了氨氯吡咪，要么是由于顶端表面电位增加降低了氨氯吡咪的亲和力。高基底外侧K⁺可能通过阻止细胞内[Na⁺]增加，或者通过降低基底外侧膜电阻和电位从而更好地钳制顶端膜电位来抑制钠效应。我们检查了已知会增加细胞内[Na⁺]的基底外侧[K⁺]、氰化物和哇巴因的作用。我们发现它们对fc的影响微乎其微。此外，添加到115 mM Na⁺或120 mM K⁺的基底外侧浴液中的两性霉素B对fc没有显著影响。我们发现在所有条件下钳制电位的相对较大变化，即使主动转运受到严重抑制，fc实际上仍保持恒定。由于氨氯吡咪动力学与钳制电位无关，我们能够通过检查钳制电流（线性）和氨氯吡咪噪声功率（二次方）的电压依赖性分别测量细胞旁和跨细胞电导。这使得估计通道密度和单通道电流成为可能。
