Pathways for HCO 3 − exit across the basolateral membrane in rat thick limbs

Abstract

We studied the pathways for [Formula: see text] transport in basolateral membrane vesicles (BLMV) purified from rat medullary thick ascending limbs (MTAL). An inward [Formula: see text]gradient in the presence of an inside-positive potential stimulated the rate of 22Na uptake minimally and did not induce a 22Na overshoot, arguing against the presence of electrogenic Na+-[Formula: see text]cotransport in these membranes. An inside-acid pH gradient stimulated to the same degree uptake of86Rb+(a K+ analog) with or without[Formula: see text]. Conversely, applying an outward K+ gradient caused a modest intracellular pH (pHi) decrease of ∼0.38 pH units/min, as monitored by quenching of carboxyfluorescein; its rate was unaffected by[Formula: see text], indicating the absence of appreciable K+-[Formula: see text]cotransport. On the other hand, imposing an inward Cl− gradient in the presence of [Formula: see text] caused a marked pHi decrease of ∼1.68 pH units/min; its rate was inhibited by a stilbene derivative. Finally, we could not demonstrate the presence of a[Formula: see text]/lactate exchanger in BLMV. In conclusion, the presence of significant Na+-, K+-, or lactate-linked[Formula: see text] transport could not be demonstrated. These and other data suggest that basolateral Cl−/[Formula: see text]exchange could be the major pathway for[Formula: see text] transport in the MTAL.