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The isolated perfused rabbit cortical collecting tubule has been shown to actively transport K from bath to lumen. The first step in this process is active uptake of K across the basolateral membrane via and Na:K exchange pump as evidenced by: 1) basolateral localization and Na:K exchange properties of the ouabain-sensitive Na,K-ATPase, 2) ouabain sensitivity of the Na and K fluxes, 3) interdependence of the Na and K fluxes, and 4) ouabain-sensitivity of 42K uptake into the cell across the basolateral membrane. At the luminal border, a significant K permeability of the apical cell membrane has been identified using electrophysiological techniques. This K permeability is insensitive to the diuretic amiloride, and, thus, differs from the pathway for Na entry, which is highly amiloride sensitive. A significant K permeability of the paracellular pathway is not apparent. It is concluded that K secretion by the rabbit cortical collecting tubule occurs via a two-step process: active uptake of K across the basolateral membrane via the Na:K exchange pump, followed by passive efflux of K across the apical membrane via an amiloride-insensitive K conductive pathway. |
