Potassium secretion by cortical collecting tubule: relation to sodium absorption, luminal sodium concentration, and...

Sodium absorption and K secretion were measured in isolated perfused rabbit cortical collecting tubules. To increase the range of transport rates some tubules were dissected from rabbit pretreated with DOcA. K secretin was unaffected by varying axial volume flow from 4 to 15 nl/min. K secretion was, however, correlated with transepithelial voltage and Na absorption. The Na:K transfer ratio was 1.35. In tubules dissected from normal rabbits, K secretion was almost eliminated by reducing Na concentration in the lumen to near 0 and was unaffected by varying luminal Na concentration from 30 to 145 mM. These data, together with other experimental results reported by several investigators, are consistent with a model for K secretion that places the rate-limiting step at the Na-K pump located on the basolateral membrane. The apical (luminal) membrane appears to be highly permeable to K.