Regulation of intracellular pH in two cell populations of inner stripe of rabbit outer medullary collecting duct.

The inner stripe of the outer medullary collecting duct (OMCDis) is a major site of HCO3- reabsorption and urinary acidification. Whether this nephron segment consists of a single or multiple cell types remains unclear. Apical incubation of rabbit OMCDis via luminal perfusion with 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester resulted in heterogeneous fluorescence, suggesting two cell types. This heterogeneity was not prevented by inhibition of either carbonic anhydrase or organic anion transport. Subsequent studies were directed at characterizing the major intracellular pH (pHi) regulatory transporters in these two cell populations. Both cell populations demonstrated similar rates of Na+/H+ exchange, as assessed by peritubular Na(+)-dependent, amiloride-sensitive pHi recovery from an intracellular acid load. In contrast, Na(+)-independent, HCO3(-)-independent pHi recovery from an acid load was present in both cell populations but had two to three times greater activity in a minority cell population. In vivo deoxycorticosterone acetate administration increases this rate in both populations but to a greater extent in the minority cell population. In CO2/HCO3(-)-containing solutions, Cl- removal from the peritubular solution caused 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive alkalinization of all cells. Again, the magnitude and rate of alkalinization were significantly greater in the minority cell population. These studies demonstrate that the OMCDis consists of qualitatively similar cells in different states of functional activity. Although they are similar in most characteristics, a minority of cells more actively secrete H+ (independent of Na+) and reabsorb HCO3-.