BASOLATERAL MEMBRANE NA+ H+ EXCHANGE ENHANCES HCO3- ABSORPTION IN RATMEDULLARY THICK ASCENDING LIMB - EVIDENCE FOR FUNCTIONAL COUPLING BETWEEN BASOLATERAL AND APICAL MEMBRANE NA+/H+ EXCHANGERS/

The role of basolateral membrane Na+/H+ exchange in transepithelial HC O3- absorption (J(HCO3)) was examined in the isolated, perfused medull ary thick ascending limb (MTAL) of the rat. In Na+-free solutions, add ition of Na+ to the bath resulted in a rapid, amiloride-sensitive incr ease in intracellular pH. In MTALs perfused and bathed with solutions containing 146 mM Na+ and 25 mM HCO3-, bath addition of amiloride (1 m M) or 5-(N-ethyl-N-isopropyl) amiloride (EIPA, 50 mu M) reversibly inh ibited J(HCO3) by 50%. Evidence that the inhibition of J(HCO3) by bath amiloride was the result of inhibition of Na+/H+ exchange included th e following: (i) the IC50 for amiloride was 5-10 mu M, (ii) EIPA was a 50-fold more potent inhibitor than amiloride, (iii) the inhibition by bath amiloride was Na+ dependent, and (iv) significant inhibition was observed with EIPA as low as 0.1 mu M. Fifty micromolar amiloride or 1 mu M EIPA inhibited J(HCO3) by 35% when added to the bath but had no effect when added to the tubule lumen, indicating that addition of am iloride to the bath did not directly inhibit apical membrane Na+/H+ ex change. In experiments in which apical Na+/H+ exchange was assessed fr om the initial rate of cell acidification following luminal EIPA addit ion, bath EIPA secondarily inhibited apical Na+/H+ exchange activity b y 46%. These results demonstrate that basolateral membrane Na+/H+ exch ange enhances transepithelial HCO3- absorption in the MTAL. This effec t appears to be the result of cross-talk in which an increase in basol ateral membrane Na+/H+ exchange activity secondarily increases apical membrane Na+/H+ exchange activity.