THE NA+ H+ EXCHANGER IN TRANSITIONAL CELLS OF THE INNER-EAR/

The cytosolic pH (pH(i)) of transitional cells from the ampulla of the gerbil was measured micro-fluorometrically with the pH-sensitive dye 2',7'-bicarboxyethyl-5(6)-carboxyfluorescein (BCECF) to assess the pos sible contribution of a Na+/H+ exchanger to the regulation of pH(i). A ll experiments were conducted in virtually HCO3--free solutions. Under control conditions, pH(i) was 7.19 and addition of 10(-5) M ethylisop ropylamiloride (EIPA), a blocker of Na+/H+ exchange, caused a small bu t significant acidification of pH(i). A transient exposure to 21.4 mM NH4Cl caused a rapid cytosolic alkalinization followed by a brisk acid ification and prompt recovery of pH(i) to its control value. The cytos olic buffer capacity (B(i)) was determined in the absence of Na+ from changes in pH(i) which were elicited by [NH4+] steps. B(i) was 4.7 mM/ pH at pH(i) 7.19 and varied with pH(i). The initial net proton flux J( H), representative of Na+/H+ exchange activity, was calculated from th e product of the initial rate of alkalinization after an NH4+-prepulse and B(i) at the corresponding pH(i). J(H) was dependent on the extrac ellular Na+ with an apparent K(m) of 64 mM, sensitive to the cytosolic [H+] with an apparent K(m) of 2.7 10(-7) M (i.e. pH 6.6), and inhib ited by EIPA with an IC50 of 5 10(-7) M. These data suggest that tra nsitional cells contain in the basolateral membrane a Na+/H+ exchanger of the amiloride-sensitive subtype.