Total CO2 absorption in the distal tubule of the rat.

By use of in vivo microperfusion methodology, we assessed proton secretion (acidification) in the superficial distal tubule of the rat by determining the rate of total CO2 (tCO2) absorption (JtCO2). In these studies, we compared the JtCO2 in rats fed a diet that increased urine pH to the JtCO2 in rats fed a high-protein diet that reduced urine pH. The effect of amiloride added to the perfusate, used in rats fed the high-protein diet, was also examined. In rats, Group 1, fed a commercial diet, urine pH was 6.9; plasma tCO2 was 30.0 mM, and JtCO2 was 15.5 +/- 5.3 pmol X mm-1 X min-1. Following the ingestion of a high-protein diet the night before study, the urine pH fell to 5.6 and the plasma tCO2 to 28.2 mM. The JtCO2 in this group, Group 2, 41.1 +/- 4.8 was significantly greater than Group 1, P less than 0.05. The late distal transepithelial potential difference was comparable in both groups, -50.3 +/- 4.3 vs. -45.2 +/- 3.1 mV, P not significant. In a third group, Group 3, amiloride (10(-4) M) was added to the perfusate of rats prepared as in Group 2. JtCO2 was 23.4 +/- 0.4 pmol X mm-1 X min-1, significantly less than Group 2, P less than 0.05. The transepithelial potential difference was reduced to -4.0 +/- 2.3 mV, P less than 0.01 vs. Group 2. We conclude that the superficial distal tubule of the rat responds to subtle stimuli to increase proton secretion and contributes to urinary acidification. The rate of acidification can be influenced by alterations in the electrical profile across the acidifying epithelium.