EVIDENCE FOR BASOLATERAL MEMBRANE CA2+/H+ EXCHANGE IN OUTER MEDULLARY COLLECTING DUCT

The relationship between Ca2+ and intracellular pH (pH(i) was studied in the rabbit collecting duct from the inner stripe of the outer medulla (OMCD(i)). When bath Ca2+ Was lowered [1 mM Ca2+ plus 2 mM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)] intracellular Ca2+ concentration ([Ca2+]i) decreased by 38.3 +/- 6.2% from 265 +/- 25 to 165 +/- 25 nm (n = 4). Lowering bath Ca2+ produced a biphasic response in pH(i), transient acidification (from 6.93 +/- 0.06 to 6.80 +/- 0.06) followed by a return of pH(i) to levels slightly higher than control (pH(i) = 6.97 +/- 0.06). The acidification was not blocked by bath amiloride or 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) or lumen N-ethylmaleimide (NEM). However, bath amiloride significantly reduced the initial rate of pH(i) recovery after acidification (0.70 pH U/min in control vs. 0.39 pH U/min with amiloride). Lanthanum, a Ca2+ pump inhibitor, completely inhibited the acidification. Moreover, lowering bath temperature to 20-degrees-C also inhibited the acidification. Acidification was blocked when the bath pH was raised to 8.0, whereas lowering bath Ca2+ markedly acidified cells when bath pH was lowered to 6.0 (DELTApH(i) = 0.28 +/- 0.01). We also measured the transepithelial (V(t)) and basolateral (V(bl)) membrane voltages. Lowering bath Ca2+ depolarized V(t) and V(bl) by 12.7 +/- 4.9 and 6.0 +/- 2.0 mV, respectively. In summary, 1) lowering bath Ca2+ caused a biphasic response in pH(i) of OMCD(i) cells, and 2) the acidification phase is due to H+ influx associated with the Ca2+ pump.