Absorption of biliary calcium from the canine gallbladder: Protection against the formation of calcium-containing gallstones

Calcium salts are the major components of pigment gallstones. The calcium species in bile that is critical for Ca⁺⁺ precipitation is free ionized calcium, [Ca⁺⁺]. Factors that regulate biliary [Ca⁺⁺] in bile are therefore of great importance in the pathogenesis of pigment gallstones. The fate of biliary Ca⁺⁺ on entry into the gallbladder has not previously been studied. We here report that a minimum of 51.3% ± 8.8% (SEM) of Ca⁺⁺ is absorbed from the canine gallbladder on concentration of bile during a 24-hour fast. In addition, there was absorption of least 70.9% ± 6.2% of Na⁺, 56.5% ± 8.6% of K⁺, and nearly all (>98%) of Cl^-. Absorption, neutralization, or both, of HCO₃^- was also nearly complete (>98%). During concentration of bile by the gallbladder, the concentrations of all biliary cations increased as total bile salt concentration increased, whereas anion concentrations declined. These results are consistent with a Gibbs-Donnan effect induced by impermeable, negatively charged bile salt molecules. Comparison of bile/plasma [Ca⁺⁺] ratios with those for [K⁺], a passively distributed Ion, was also consistent with, although not proof of, the passive absorption and distribution of Ca⁺⁺ across the gallbladder epithelium. The absorption of Ca⁺⁺ by the gallbladder may be a factor in the prevention of pigment gallstones, because it limits free Ca⁺⁺ ion in bile, and thus decreases the likelihood of precipitation of calcium.