Sodium-dependent bicarbonate absorption by cortical thick ascending limb of rat kidney

In vitro microperfusion experiments were performed to investigate the mechanism of bicarbonate absorption in the cortical thick ascending limb of the rat. Tubules were perfused at 1.0-1.5 nl·min^-1·mm^-1 and bicarbonate concentration was 25 mM in the perfusate and bath. Bicarbonate absorption rates were determined by microcalorimetry. Control tubules absorbed bicarbonate at a mean rate of 9.5 ± 0.6 pmol·min^-1·mm^-1. The limiting luminal bicarbonate concentration was approximately 5 mM for tubules perfused at slow rates with 25 mM bicarbonate in the bath. Acetazolamide (10^-4 M) in the bath reduced bicarbonate absorption by 76% without significant effect on transepithelial voltage. Removing sodium from the perfusate and bath or removing potassium from the bath reduced bicarbonate absorption and transepithelial voltage to near zero. Adding amiloride (5 x 10^-4 or 10^-3 M) to the perfusate reduced bicarbonate absorption by 60-75% without detectable effect on transepithelial voltage. Adding furosemide (10^-4 M) to the perfusate increased bicarbonate absorption significantly by 40-50% while decreasing transepithelial voltage from 17 to 1.8 mV. Thus, bicarbonate absorption by cortical thick ascending limbs requires carbonic anhydrase activity and sodium transport but is not dependent on transepithelial voltage. When considered together, the results are consistent with mediation of the bicarbonate absorption by apical membrane sodium-hydrogen exchange.