Distal tubule unidirectional HCO3 reabsorption in vivo during acute and chronic metabolic alkalosis in the rat.

During metabolic alkalosis (MA) associated with 2 days of dietary chloride restriction, there is net bicarbonate secretion by rat distal tubules in vivo, whereas after 5 wk of chloride depletion alkalosis there is net bicarbonate reabsorption. To examine unidirectional components of net bicarbonate reabsorption during chronic MA, we measured distal tubule unidirectional bicarbonate secretion (Jsec) and reabsorption (Jreab), as well as the inhibitor sensitivity of Jreab. In control, 2-day, and 7-day alkalosis, Jsec was similar. Jreab, however, was only present in 7-day MA (17 +/- 3 pmol.min-1.mm-1, P < 0.05). This Jreab was completely suppressed by perfusion with 10(-7) M bafilomycin A1, partially suppressed with 10(-5) M Schering (Sch)-28080 (4 +/- 2 pmol.min-1.mm-1, P < 0.1), and converted into a secretory flux by 3 mM amiloride. We conclude that adaptation to chloride depletion MA from the acute secretory phase to the chronic state, where plasma bicarbonate is sustained at elevated levels, does not involve suppression of distal tubule Jsec but rather enhanced Jreab, which is sensitive to bafilomycin, Sch-28080, and amiloride.