Transepithelial HCO₃^- absorption is defective in renal thick ascending limbs from Na⁺/H⁺ exchanger NHE1 null mutant mice

In the medullary thick ascending limb (MTAL) of rat kidney, inhibiting basolateral Na⁺/H⁺ exchange with either amiloride or nerve growth factor (NGF) results secondarily in inhibition of apical Na ⁺/H⁺ exchange, thereby decreasing transepithelial HCO ₃^- absorption. To assess the possible role of the Na ⁺/H⁺ exchanger NHE1 in this regulatory process, MTALs from wild-type and NHE1 knockout (NHE1^-/-) mice were studied using in vitro microperfusion. The rate of HCO₃^- absorption was decreased 60% in NHE1^-/- MTALs (15.4 ± 0.5 pmol·min^-1·mm^-1 wild-type vs. 6.0 ± 0.5 pmol·min^-1·mm^-1 NHE1^-/-). Transepithelial voltage, an index of the NaCl absorption rate, did not differ in wild-type and NHE1^-/- MTALs. Basolateral addition of 10 μM amiloride or 0.7 nM NGF decreased HCO₃^- absorption by 45-49% in wild-type MTALs but had no effect on HCO₃^- absorption in NHE1^-/- MTALs. Inhibition of HCO₃ ^- absorption by vasopressin and stimulation by hyposmolality, both of which regulate MTAL HCO₃^- absorption through primary effects on apical Na⁺/H⁺ exchange, were similar in wild-type and NHE1^-/- MTALs. Thus the regulatory defect in NHE1 ^-/- MTALs is specific for factors (bath amiloride and NGF) shown previously to inhibit HCO₃^- absorption through primary effects on basolateral Na⁺/H⁺ exchange. These findings demonstrate a novel role for NHE1 in transepithelial HCO₃^- absorption in the MTAL, in which basolateral NHE1 controls the activity of apical NHE3. Paradoxically, a reduction in NHE1-mediated H⁺ extrusion across the basolateral membrane leads to a decrease in apical Na ⁺/H⁺ exchange activity that reduces HCO₃ ^- absorption.