Mechanism of proximal tubule bicarbonate absorption in NHE3 null mice

NHE3 is the predominant isoform responsible for apical membrane Na+/H+ exch ange in the proximal tubule. Deletion of NHE3 by gene targeting results in an NHE3(-/-) mouse with greatly reduced proximal tubule HCO3- absorption co mpared with NHE3(+/+) animals (P. J. Schultheis, L. L. Clarke, P. Meneton, M. L. Miller, M. Soleimani, L,. R. Gawenis, T. M. Riddle, J. J. Duffy, T. D oetschman, T. Wang, G. Giebisch, P. S. Aronson, J. N. Lorenz, and G. E. Shu ll. Nature Genet. 19: 282-285, 1998). The purpose of the present study was to evaluate the role of other acidification mechanisms in mediating the rem aining component of proximal tubule HCO3- reabsorption in NHE3(-/-) mice. P roximal tubule transport was studied by in situ microperfusion. Net rates o f HCO3- (J(HCO3)) and fluid absorption (J,) were reduced by 54 and 63%, res pectively, in NHE3 null, mice compared with controls. Addition of 100 mu M ethylisopropylamiloride (EIPA) to the luminal perfusate caused significant inhibition of J(HCO3) and J(v) in NHE3(+/+) mice but failed to inhibit J(HC O3) OL. Jv in NHE3-/- mice, indicating lack. of activity of NHE2 or other E IPA-sensitive NHE isoforms in the null mice. Addition of 1 mu M bafilomycin caused a similar absolute decrement in J(HCO3) in wild-type and NHE3 null mice, indicating equivalent rates of HCO3- absorption mediated by H+-ATPase . Addition of 10 mu M Sch-28080 did not reduce J(HCO3) in either wild-type or NHE3 null mice, indicating lack of detectable H+-K+-ATPase activity in t he proximal tubule. We conclude that, in the absence of NHE3, neither NHE2 nor any other EIPA-sensitive NHE isoform contributes to mediating HCO3- rea bsorption in the proximal tubule. A significant component of HCO3- reabsorp tion in the proximal tubule is mediated by bafilomycin-sensitive H+-ATPase, but its activity is not significantly upregulated in NHE3 null mice.