H-K-ATPASE ENHANCEMENT OF RB EFFLUX BY CORTICAL COLLECTING DUCT

Previous studies suggest that enhancement of rubidium tracer (Rb-86) lumen-to-bath efflux following removal of luminal Na is mediated in part by a Ba-insensitive pathway. To determine the role of a primary active K pump in this response, we examined the action of known inhibitors of H-K-ATPase (Sch 28080) and Na-K-ATPase (ouabain) on the Rb-86 lumen-to-bath efflux coefficient (K(Rb)). Luminal Sch 28080 (10-mu-M) significantly reduced K(Rb) by 39 +/- 8.0% (P < 0.05), whereas luminal ouabain (0.1 mM) reduced K(Rb) by 10 +/- 14% (P = not significant), suggesting that a luminal H-K-ATPase mediates Rb efflux. To examine whether H-K-ATPase mediates the increase in K(Rb) following removal of luminal Na, additional experiments were conducted to examine the effect of Sch 28080 on K(Rb) in the presence and the absence of luminal Na. In the presence of luminal Na, 10-mu-M Sch 28080 reduced K(Rb) by 15 +/- 5.0%. However, in the absence of luminal Na, 10-mu-M Sch 28080 decreased K(Rb) by 48 +/- 8.2%. The percentage inhibition of K(Rb) by Sch 28080 was significantly greater in the absence of luminal Na than in its presence (P < 0.01), suggesting that the enhancement of K(Rb) following removal of luminal Na is mediated in part by an H-K-ATPase pathway. In either case transepithelial voltage was not significantly altered. In contrast to the lack of effect of luminal ouabain, addition of 0.1 mM ouabain to the bath increased K(Rb) (69.8 +/- 11.1 vs. 95.9 +/- 18.7 nm/s, P < 0.05). However, this effect of ouabain on K(Rb) was totally abolished by perfusion of the cortical collecting duct (CCD) in the presence of 10-mu-M luminal Sch 28080 (69.8 +/- 14.0 nm/s, basal period; 69.6 +/- 14.1 nm/s, ouabain period). These data are consistent with the following conclusions, at least during K restriction: 1) H-K-ATPase is responsible for Rb (K) absorption in the rabbit CCD, 2) the presence of luminal Na reduces Rb (K) absorption by an H-K-ATPase-mediated mechanism, and 3) basolateral ouabain addition stimulates Rb (K) absorption via H-K-ATPase.