T he outer medullary collecting duct (OMCD) absorbs HCO3- at high rates, but it is not clear if it responds to metabolic acidosis to increase H+ secretion. We measured net HCO3- transport in isolated perfused OMCDs taken from deep in the inner stripes of kidneys from control and acidotic (NH4Cl-fed for 3 d) rabbits. We used specific inhibitors to characterize the mechanisms of HCO3- transport: 10 microM Sch 28080 or luminal K+ removal to inhibit P-type H+,K+-ATPase activity, and 5-10 nM bafilomycin A1 or 1-10 nM concanamycin A to inhibit H+-ATPase activity. The results were comparable using either of each pair of inhibitors, and allowed us to show in control rabbits that 65% of net HCO3- absorption depended on H+-ATPase (H flux), and 35% depended on H+,K+-ATPase (H,K flux). Tubules from acidotic rabbits showed higher rates of HCO3- absorption (16.8+/-0.3 vs. 12.8+/-0.2 pmol/min per mm, P < 0.01). There was no difference in the H,K flux (5.9+/-0.2 vs. 5.8+/-0.2 pmol/min per mm), whereas there was a 61% higher H flux in segments from acidotic rabbits (11.3+/-0.2 vs. 7.0+/-0.2 pmol/min per mm, P < 0.01). Transport was then measured in other OMCDs before and after incubation for 1 h at pH 6.8, followed by 2 h at pH 7.4 (in vitro metabolic acidosis). Acid incubation in vitro stimulated HCO3- absorption (12.3+/-0.3 to 16.2+/-0.3 pmol/min per mm, P < 0.01), while incubation at pH 7.4 for 3 h did not change basal rate (11.8+/-0.4 to 11.7+/-0.4 pmol/min per mm). After acid incubation the H,K flux did not change, (4.7+/-0.4 to 4.6+/-0.4 pmol/min per mm), however, there was a 60% increase in H flux (6.6+/-0.3 to 10.8+/-0.3 pmol/min per mm, P < 0.01). In OMCDs from acidotic animals, and in OMCDs incubated in acid in vitro, there was a higher basal rate and a further increase in HCO3- absorption (16.7+/-0.4 to 21.3+/-0.3 pmol/min per mm, P < 0.01) because of increased H flux (11.5+/-0.3 to 15.7+/-0.2 pmol/min per mm, P < 0.01) without any change in H,K flux (5.4+/-0.3 to 5.6+/-0.3 pmol/min per mm). These data indicate that HCO3- absorption (H+ secretion) in OMCD is stimulated by metabolic acidosis in vivo and in vitro by an increase in H+-ATPase-sensitive HCO3- absorption. The mechanism of adaptation may involve increased synthesis and exocytosis to the apical membrane of proton pumps. This adaptation helps maintain homeostasis during metabolic acidosis.