Evaluation of bicarbonate transport in rat distal tubule: effects of acid-base status.

Previous micropuncture studies utilizing indirect methods to estimate bicarbonate transport in the rat superficial distal tubule have indicated that the distal bicarbonate reabsorptive process normally operates well below the saturation level. Recent studies from our laboratory failed to demonstrate a spontaneous acid disequilibrium pH in this segment, implying that the bicarbonate reabsorptive rate was less than previously estimated. The purpose of the present experiments were 1) to measure the rate of absolute bicarbonate reabsorption by the rat superficial distal tubule while controlling bicarbonate delivery, and 2) to examine the effects of alterations in acid-base status on the rate of bicarbonate reabsorption. Five groups of rats in different states of acid-base balance were studied. No significant bicarbonate reabsorption was detected in the control hydropenic, combined respiratory acidosis-metabolic alkalosis, acute respiratory acidosis, or acute metabolic acidosis groups. In contrast, metabolic acidosis of 3 days duration resulted in a significant bicarbonate reabsorptive rate of 52.6 +/- 13.9 pmol . mm-1 . min-1. The observation of significant bicarbonate reabsorption in the distal tubule only during chronic metabolic acidosis of 3 days duration is compatible with adaptation of this normally low-capacity segment to chronic changes in systemic acid-base states.