Characterization of Na(+)-H+ exchangers on villus cells in rabbit ileum

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Abstract
The presence of Na+-H+ exchange activity is demonstrated on both the brush-border membrane (BBM) and the basolateral membrane (BLM) of villus cells from rabbit ileum. The possibility that the Na+-H+ exchange activity on the BLM represents HCO3- cotransport is excluded. The Two Na+-H+ exchangers are then compared in terms of kinetics and substrate and inhibitor specificity. The most striking difference between the two exchangers was sensitivity to amiloride and K+. The IC50 for amiloride on the BLM was 10-fold lower than the BBM (11.2 .+-. 2.1 vs. 103 .+-. 20.9 .mu.M; P < 0.02). External K+, in concentrations as low as 10 mM, inhibited Na+-H+ exchange on the BBM but not on the BLM. The Na+ Km and proton Km were twice as high on the BLM exchanger (46.3 .+-. 3.4 vs. 28.8 .+-. 2.3 mM and 468 .+-. 9 vs. 232 .+-. 45 nM, respectively). Proton Vmax was similar, whereas Na+ Vmax was higher on the BLM. Inhibition by Li+ was similar on both membranes. These results indicate distinct differences between the two Na+-H+ exchangers. Whether these differences are due to the two different gene products or are the results of posttranslational modifications of a single gene product remains to be determined.

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