In vivo adaptation of bicarbonate reabsorption by rat distal tubules during acid loading.

We carried out in vivo microperfusion experiments in acid-loaded rats to characterize the adaptive response of the unidirectional components secretory flux (Jsec) and reabsorptive flux (Jreab)] of distal tubule bicarbonate reabsorption and to test the hypothesis that Jreab is dependent on bafilomycin A1-sensitive H(+)-adenosinetriphosphatase activity. During 18 h of severe acidosis there was a significant decrease in Jsec (-15 +/- 3 vs. -38 +/- 5 pmol.min-1.mm-1, P < 0.05) and a significant increase in Jreab (37 +/- 6 vs. 0 +/- 5 pmol.min-1.mm-1, P < 0.05), which was insensitive to 10(-5) M bafilomycin A1, 10(-5) M Sch-28080, and 3 mM amiloride. After 3 days of acid loading, these same inhibitors reduced Jreab by approximately 60%. However, when water flux was completely inhibited by isosmotic perfusion, a significant Jreab (15 +/- 2 pmol.min-1.mm-1) resistant to 10(-5) M bafilomycin A1 persisted, as in severe acidosis. In reabsorbing distal tubules of overnight-fasted rats, Sch-28080 elicited no inhibition, whereas bafilomycin A1 and amiloride had significant effects (28 +/- 5, 24 +/- 4, respectively, vs. 50 +/- 4 pmol.min-1.mm-1 for fasted rats, P < 0.05). Thus, although Jsec is reduced in the transition from mild to severe metabolic acidosis of 18-h duration, the predominant effect is a stimulation of bafilomycin A1-resistant Jreab.