Evidence for H⁺ secretion by the in vivo canine gallbladder

In humans and most other species, a decline in pH of gallbladder contents occurs during the concentration of bile. Recent in vitro studies in rabbit, guinea pig, and Necturus gallbladders have strongly suggested mucosal H⁺ secretion during sodium reabsorption, presumably representing a Na⁺ H⁺ exchange. The present in vivo studies are the first attempt to determine whether H⁺ secretion by the gallbladder can be demonstrated in the living animal. Gallbladder bile was obtained from 27 anesthetized dogs after 12-24-h fasts; 12 samples of common duct bile were also obtained in 3 dogs during variable taurocholate infusion. In common duct bile, observed ranges were as follows: pH, 7.3-7.85; CO₂ partial pressure (Pco₂), 21-32 mmHg; total CO₂ concentration ([TCO₂]), 16.4-41.4 mM; total bile salt concentration ([TBS]), 16-93 mM; and [Na], 153-192 mM. In gallbladder bile, respective ranges were as follows: pH, 5.72-7.29; Pco₂, 36-101 mmHg; [TCO₂], 1.21-15.5 mM; [TBS], 150-305 mM; and [Na], 199-266 mM. In all samples [Na] was linearly related to [TBS]. Carbon dioxide partial pressure increased from a mean of 27.3 mmHg in common duct bile to >100 mmHg in gallbladder bile at [TBS] = 180 mM, then declined to ~36 mmHg as [TBS] increased to >300 mM. Peak Pco₂ occurred at pH ~6.4-6.6, then declined as pH decreased to ~5.7. Bile to plasma Pco₂ ratios increased from a mean of 1.08 in common duct samples to >2.0 in gallbladder samples at pH ~6.3, then declined to ~1.0 in fully concentrated bile. If the high Pco₂ values in bile were solely due to tissue CO₂ production, a sustained increase in Pco₂ throughout Na⁺ reabsorption might be expected. The results strongly suggest H⁺ secretion (HCO₃^- neutralization), as peak Pco₂ occurred when [TBS] was only about 180 mM, long before sodium absorption was complete. It is hypothesized that H⁺ secretion may have important favorable effects on calcium lithogenicity, reducing the likelihood of the formation of CaCO₃^- containing gallstones.