FUNCTION AND STRUCTURE OF H-K-ATPASE IN THE KIDNEY

The present review summarizes recent functional and structural evidence indicating that the kidney possesses at least one and probably more than one isoform of a proton- and potassium-activated adenosinetriphosphatase (H-K-ATPase). Functional studies have examined in detail the mechanism of luminal acidification and K/Rb absorption by the outer medullary collecting duct (OMCD) from the inner stripe, a high-capacity distal site of urinary acidification. These studies indicate that the mechanism of proton secretion in this segment is similar to a model proposed for gastric acid secretion. Specifically, the profound effect of H-K-ATPase inhibitors or luminal K removal on net bicarbonate (HCO3) absorption indicates a major role for an H-K pump in luminal acidification by the OMCD. The importance of an H-K-ATPase is further supported by the finding that nanomolar concentrations of bafilomycin A(1), which specifically inhibit vacuolar-type H-ATPase, have significantly smaller effects on net HCO3 absorption than do H-K-ATPase inhibitors. Studies on the perfused inner medullary collecting duct (IMCD) and cultured IMCD cells also suggest a significant role for H-K-ATPase in luminal acidification by the IMCD. Evidence has accrued from studies in the cortical CD and OMCD that the mechanism of H-K-ATPase-mediated luminal proton secretion differs under K-replete and K-restricted conditions. In K repletion, luminal K ions transported by the pump recycle back into the lumen by a Ba-sensitive mechanism. However, in K restriction, the mechanism of the H-K-ATPase involves luminal proton secretion and K absorption that is insensitive to luminal Ba and, by inference, apical K recycling. Moreover, in K restriction, K/Rb absorption is inhibited by basolateral Ba, indicating that the pump operates to reabsorb K/Rb across the epithelium. The structural evidence reviewed here indicates the presence of mRNA within the mammalian kidney that is either identical or highly homologous to mRNAs for gastric and putative colonic H-K-ATPase alpha-subunits and gastric H-K-ATPase beta-subunit. Localization of these transcripts by in situ hybridization demonstrates gastric alpha- and beta-subunit mRNAs in intercalated cells of both the cortical and medullary CD, principal cells of the CD, and IMCD cells. Additional studies in transgenic mice indicate that regulatory sequences upstream to the H-K-ATPase beta-subunit gene direct transcription in both gastric parietal cells and the renal CD. Elucidation of the physiological function of H-K-ATPase and the first and second messenger systems responsible for H-K-ATPase activation or inhibition will remain an area of particular interest for the foreseeable future. Answers to such questions will likely provide considerable insight into whole kidney physiology and will require, of necessity, a full spectrum of physiological, biochemical, anatomical, and molecular biological techniques.