VASOPRESSIN REGULATES APICAL AND BASOLATERAL NA+-H+ ANTIPORTERS IN MOUSE MEDULLARY THICK ASCENDING LIMBS

We assessed in isolated perfused mouse medullary thick ascending limb (MTAL) segments Na+-H+ antiporter activity in both apical and basolateral membranes and the effects of arginine vasopressin (AVP) on the activities of these antiporters under isotonic conditions using 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein to monitor intracellular pH (pH(i)). When the apical Na+-H+ antiporter was inhibited in the absence of AVP with removal of luminal Na+ plus addition of 0.5 mM amiloride, a small but significant increase in pH(i) was observed after luminal NH4Cl-induced acidification of MTAL cells to pH(i) < 6.7. This increase in pH(i) was dependent on basolateral Na+ and inhibited with 0.5 mM basolateral amiloride, consistent with the function of a basolateral Na+-H+ antiporter. Basolateral AVP (100-mu-U/ml) enhanced the rate of pH(i) recovery due to the basolateral Na+-H+ antiporter by more than twofold. In contrast, AVP decreased the apical Na+-H+ antiporter activity by 50%. In the absence of AVP, addition of 0.5 mM amiloride to the luminal perfusate reduced steady-state pH(i) by 0.40 +/- 0.07 units, whereas exposure of the basolateral membrane to the same concentration of amiloride had no effect on pH(i) (DELTA-pH(i) = 0.01 +/- 0.01 units). AVP reduced the magnitude of cell acidification on exposure of apical membranes to amiloride (DELTA-pH(i) = 0.16 +/- 0.03) but increased the pH(i) response to basolateral amiloride (DELTA-pH(i) = 0.09 +/- 0.00). Thus Na+-H+ antiporters are present on both apical and basolateral membranes of the mouse MTAL in the absence of AVP. AVP stimulates the basolateral, while inhibiting the apical, Na+-H+ antiporter. These effects of AVP may account, at least in part, for the AVP requirement for a functional regulatory volume increase response in this nephron segment.