The basolateral NHE1 Na⁺/H⁺ exchanger regulates transepithelial HCO₃^- absorption through actin cytoskeleton remodeling in renal thick ascending limb

In the renal medullary thick ascending limb (MTAL), inhibiting the basolateral NHE1 Na⁺/H⁺ exchanger with amiloride or nerve growth factor (NGF) results secondarily in inhibition of the apical NHE3 Na ⁺/H⁺ exchanger, thereby decreasing transepithelial HCO₃^- absorption. MTALs from rats were studied by in vitro microperfusion to identify the mechanism underlying cross-talk between the two exchangers. The basolateral addition of 10 μM amiloride or 0.7 nM NGF decreased HCO₃^- absorption by 27-32%. Jasplakinolide, which stabilizes F-actin, or latrunculin B, which disrupts F-actin, decreased basal HCO₃^- absorption by 30% and prevented the inhibition by amiloride or NGF. Jasplakinolide had no effect on HCO₃ ^- absorption in tubules bathed with amiloride or a Na ⁺-free bath to inhibit NHE1. Jasplakinolide and latrunculin B did not prevent inhibition of HCO₃^- absorption by vasopressin or stimulation by hyposmolality, factors that regulate HCO₃^- absorption through primary effects on apical Na⁺/H⁺ exchange. Treatment of MTALs with amiloride or NGF for 15 min decreased polymerized actin with no change in total cell actin, as assessed both by fluorescence microscopy and by actin Triton X-100 solubility. Jasplakinolide prevented amiloride-induced actin remodeling. Vasopressin, which inhibits HCO₃^- absorption by an amount similar to that observed with amiloride and NGF but does not act via NHE1, did not affect cellular F-actin content. These results indicate that basolateral NHE1 regulates apical NHE3 and HCO₃^- absorption in the MTAL by controlling the organization of the actin cytoskeleton.