Warden D H, Hayashi M, Schuster V L, Stokes J B
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City 52242.
Am J Physiol. 1989 Jul;257(1 Pt 2):F43-52. doi: 10.1152/ajprenal.1989.257.1.F43.
We compared transport of K+ and Rb+ across the rabbit cortical collecting duct to gain insight into the mechanisms of K+ secretion. Passive tracer fluxes, active secretory rates, electrophysiological behavior, and the ability of each ion to support Na+-K+-ATPase activity were determined. When active transport was inhibited by amiloride, K+ permeability was twice the Rb+ permeability. Transepithelial conductance (GT) was half as great in solutions where 5 mM Rb+ replaced 5 mM K+. When 4 mM Ba2+ was added to the lumen, both Rb+ and K+ permeability fell to values not different from that expected for paracellular diffusion. The relationship between Ba2+-induced changes in the K+ and Rb+ permeabilities and in the simultaneously measured GT provides strong evidence that K+ transport across the apical membrane is largely, if not exclusively, conductive. We also determined that net K+ secretion is greater than net Rb+ secretion (when each is the abundant ion). The reasons for this difference probably involve several steps in the K+ secretory process and include the following: 1) reduced ATPase activity in the presence of Rb+ (approximately 80%) compared with K+, 2) reduction of Na+ absorption, and 3) partial blockade of the apical (and perhaps basolateral) K+ conductance. Although there were quantitative differences between K+ and Rb+ transport, we found no evidence suggesting that these ions are transported by different mechanisms.
我们比较了钾离子（K⁺）和铷离子（Rb⁺）跨兔肾皮质集合管的转运情况，以深入了解钾离子分泌的机制。测定了被动示踪剂通量、主动分泌速率、电生理行为以及每种离子支持钠钾ATP酶（Na⁺-K⁺-ATPase）活性的能力。当用氨氯吡脒抑制主动转运时，钾离子通透性是铷离子通透性的两倍。在5 mM铷离子取代5 mM钾离子的溶液中，跨上皮电导（GT）减半。当向管腔中加入4 mM钡离子（Ba²⁺）时，铷离子和钾离子的通透性均降至与细胞旁扩散预期值无差异的值。钡离子引起的钾离子和铷离子通透性变化与同时测量的GT之间的关系提供了强有力的证据，表明钾离子跨顶端膜的转运在很大程度上（如果不是完全）是通过离子通道进行的。我们还确定，净钾离子分泌大于净铷离子分泌（当每种离子都是丰富离子时）。这种差异的原因可能涉及钾离子分泌过程中的几个步骤，包括：1）与钾离子相比，在铷离子存在下ATP酶活性降低（约80%）；2）钠离子吸收减少；3）顶端（可能还有基底外侧）钾离子通道部分受阻。尽管钾离子和铷离子的转运存在定量差异，但我们没有发现证据表明这些离子是通过不同机制转运的。
