PH sensitivity of the Na+:HCO3- cotransporter in basolateral membrane vesicles isolated from rabbit kidney cortex.

Abstract

HCO3- exit across the basolateral membrane of the kidney proximal tubule cell is mediated via an electrogenic Na+:HCO3- cotransporter. We have studied the effect of pH on the activity of this cotransport system in basolateral membrane vesicles isolated from rabbit renal cortex. At constant internal pH 6.0, increasing the external pH and [HCO3-] increased the rate of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid-sensitive 22Na+ influx into the vesicles. To determine the role of internal pH on the activity of the Na+:HCO3- cotransport system, the influx of 22Na+ via HCO3-dependent Na(+)-Na+ exchange was measured in the absence of an initial pH and [HCO3-] gradient (pH(i) = pH(o), 5% CO2). Increasing the pH from 6.8 to 7.2 increased whereas, increasing the pH from 7.4 to 8.0 decreased the rate of 22Na+ influx via this exchange. Increasing pH at constant [HCO3-] (pH(i) = pH(o) = 8.0, 1.5% CO2 versus pH(i) = pH(o) = 7.2, 10% CO2) reduced the influx of 22Na+ via HCO3-dependent Na(+)-Na+ exchange. Increasing pH at constant [CO3(2-)](pH(i) = pH(o) = 8.0, 1.5% CO2 versus pH(i) = pH(o) = 7.2, 60% CO2) was associated with reduced 22Na+ uptake. Decreasing the pH (pH(i) = pH(o) = 6.3, 60% CO2 versus pH(i) = pH(o) = 7.2, 5% CO2) was associated with a reduced rate of HCO3(-)-dependent Na(+)-Na+ exchange. We conclude that the Na+:HCO3- cotransporter displays a significant pH sensitivity profile with the cotransporter being more functional at pH 7.0-7.4 and less active at more acid or alkaline pH. In addition, the results suggest that the pH sensitivity arises at the inner surface of the basolateral membrane.