Nonaka T, Warden D H, Stokes J B
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City 52242.
Am J Physiol. 1992 Jan;262(1 Pt 2):F86-97. doi: 10.1152/ajprenal.1992.262.1.F86.
We studied the cellular pathways of K+ transport by the rabbit cortical collecting duct that was stimulated to absorb Na+ and to secrete K+. The vast majority of K+ secretion (into the lumen) was inhibited by benzamil, a blocker of epithelial Na+ channels. The residual K+ secretion was completely inhibited by ouabain. Thus all active K+ secretion was dependent on Na+ transport by the Na(+)-K+ pump. The passive pathways of K+ transport were further examined using tracer and electrophysiological measurements. K+ transfer across the apical membrane was predominantly or exclusively conductive; the apical K+ conductance was 31 mS/cm2. The basolateral membrane contained two pathways for K+ tracer translocation. The (barium-sensitive) conductive pathway accounted for a relatively small (12-20%) portion of the tracer permeation. A larger pathway appeared to be via K(+)-K+ exchange on the Na(+)-K+ pump. The magnitude of the Ba2(+)-sensitive (basolateral) K+ conductance predicted a substantially larger tracer flux than was actually measured. The best explanation for this difference is the presence of single-file diffusion through K+ channels on the apical and basolateral membranes. An analysis of the electrically silent K+ transport from lumen to bath suggests that the Na(+)-K+ pump can vary the ratio of its Na(+)-K+ and K(+)-K+ modes of operation. When the tubule is actively transporting Na+ and K+, the Na(+)-K+/K(+)-K+ turnover ratio is greater than 7. When Na+ transport is limited by inhibiting Na+ entry across the apical membrane, the ratio falls to less than 1. A major factor determining this ratio is probably the availability of Na+ to the cytoplasmic side of the pump.
我们研究了兔皮质集合管中钾离子转运的细胞途径，该集合管被刺激吸收钠离子并分泌钾离子。绝大多数钾离子分泌（进入管腔）受到苯扎明的抑制，苯扎明是上皮钠离子通道的阻滞剂。残余的钾离子分泌被哇巴因完全抑制。因此，所有主动钾离子分泌都依赖于钠钾泵的钠离子转运。使用示踪剂和电生理测量进一步研究了钾离子转运的被动途径。钾离子跨顶端膜的转运主要或完全是通过传导进行的；顶端钾离子电导率为31 mS/cm²。基底外侧膜包含两条钾离子示踪剂转运途径。（对钡敏感的）传导途径占示踪剂渗透的相对较小部分（12 - 20%）。一条更大的途径似乎是通过钠钾泵上的钾离子 - 钾离子交换。钡离子敏感的（基底外侧）钾离子电导率预测的示踪剂通量比实际测量的要大得多。对此差异的最佳解释是顶端和基底外侧膜上存在通过钾离子通道的单通道扩散。对从管腔到浴液的电沉默钾离子转运的分析表明，钠钾泵可以改变其钠离子 - 钾离子和钾离子 - 钾离子运作模式的比例。当肾小管积极转运钠离子和钾离子时，钠离子 - 钾离子/钾离子 - 钾离子周转比大于7。当通过抑制钠离子跨顶端膜进入来限制钠离子转运时，该比例降至小于1。决定该比例的一个主要因素可能是泵细胞质侧钠离子的可用性。
