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 (t(CO₂)) absorption (J(tCO₂). In these studies, we compared the J(tCO₂) in rats fed a diet that increased urine pH to the J(tCO₂) 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 t(CO₂) was 30.0 mM, and J(tCO₂) was 15.5 ± 5.3 pmol·mm^-1·min^-1. Following the ingestion of a high-protein diet the night before study, the urine pH fell to 5.6 and the plasma t(CO₂) to 28.2 mM. the J(tCO₂) in this group, Group 2, 41.1 ± 4.8 was significantly greater than Group 1, P < 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. J(tCO₂) was 23.4 ± 0.4 pmol·mm^-1·min^-1, significantly less than Group 2, P < 0.05. The transepithelial potential difference was reduced to -4.0 ± 2.3 mV, P < 0.01 vs. Group 2. We conclude that the superficial distal tube 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.