Nerve growth factor inhibits HCO₃/^- absorption in renal thick ascending limb through inhibition of basolateral membrane Na⁺/H⁺ exchange

Nerve growth factor (NGF) inhibits transepithelial HCO₃/^- absorption in the rat medullary thick ascending limb (MTAL). To investigate the mechanism of this inhibition, MTALs were perfused in vitro in Na⁺-free solutions, and apical and basolateral membrane Na⁺/H⁺ exchange activities were determined from rates of pH(i) recovery after lumen or bath Na⁺ addition. NGF (0.7 nM in the bath) had no effect on apical Na⁺/H⁺ exchange activity, but inhibited basolateral Na⁺/H⁺ exchange activity by 50%. Inhibition of basolateral Na⁺/H⁺ exchange activity with ethylisopropyl amiloride (EIPA) secondarily reduces apical Na⁺/H⁺ exchange activity and HCO₃/^absorption in the MTAL (Good, D. W., George, T., and Watts, B. A., III (1995) Proc. Natl. Acad, Sci. U.S. A. 92, 12525-12529). To determine whether a similar mechanism could explain inhibition of HCO₃/^- absorption by NGF, apical Na⁺/H⁺ exchange activity was assessed in physiological solutions (146 mM Na⁺) by measurement of the initial rate of cell acidification after lumen EIPA addition. Under these conditions, in which basolateral Na⁺/H⁺ exchange activity is present, NGF inhibited apical Na⁺/H⁺ exchange activity. Inhibition of HCO₃/^- absorption by NGF was eliminated in the presence of bath EIPA or in the absence of bath Na⁺. Also, NGF blocked inhibition of HCO₃/^- absorption by bath EIPA. We conclude that NGF inhibits basolateral Na⁺/H⁺ exchange activity in the MTAL, an effect opposite from the stimulation of Na⁺/H⁺ exchange by growth factors in other systems. NGF inhibits transepithelial HCO₃/^- absorption through inhibition of basolateral Na⁺/H⁺ exchange, most likely as the result of functional coupling in which primary inhibition of basolateral Na⁺/H⁺ exchange activity results secondarily in inhibition of apical Na⁺/H⁺ exchange activity. These findings establish a role for basolateral Na⁺/H⁺ exchange in the regulation of renal tubule HCO₃/^- absorption.