K⁺ and Rb⁺ transport by the rabbit CCD: Rb⁺ reduces K⁺ conductance and Na⁺ transport

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 (G(T)) 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 G(T) 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⁺ (~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.