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 (greater than 98%) of Cl-. Absorption, neutralization, or both, of HCO3- was also nearly complete (greater than 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.