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 (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.
Warden
D H
DH
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City 52242.
Hayashi
M
M
Schuster
V L
VL
Stokes
J B
JB
D000042:Absorption; D000818:Animals; D001464:Barium / Q000494:pharmacology; D001692:Biological Transport; D007684:Kidney Tubules / Q000378:metabolism*; D007685:Kidney Tubules, Collecting / Q000378:metabolism*; D010477:Perfusion; D010539:Permeability; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012413:Rubidium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000032:analysis
D001464:Barium; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D012413:Rubidium; D011188:Potassium