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This study sought to characterize the mechanism of Na transport across basolateral membrane vesicles of rat distal colon. Both an outward proton gradient and an inward bicarbonate gradient stimulated 22Na uptake. Proton gradient-stimulated 22Na uptake was activated severalfold by the additional presence of an inward bicarbonate gradient, and bicarbonate gradient-stimulated 22Na uptake was significantly enhanced by an imposed intravesicular membrane positive potential. 0.1 mM amiloride inhibited both proton gradient- and bicarbonate gradient-stimulated 22Na uptake by 80 and 95%, respectively, while 1 mM 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) inhibited both proton gradient- and bicarbonate gradient-stimulated 22Na uptake by 40 and 80%, respectively. Both proton gradient- and bicarbonate gradient-stimulated 22Na uptake saturated as a function of increasing Na concentration: the …