Analogous to cholesterol gallstones forming in bile supersaturated with cholesterol, pigment gallstones may form in bile supersaturated with calcium bilirubinate. We tested this hypothesis in a dietary model of pigment gallstones. The concentration of ionized calcium (Ca++) and unconjugated bilirubin (UCB) was measured in 15 normal dogs and in 15 dogs with pigment gallstones induced by 6 weeks of a methionine-deficient diet. Although there was minimal change in the gallbladder's ability to acidify or concentrate bile, both [Ca++]and [UCB]markedly increased. These values were compared with equilibrium concentrations in model bile solutions. In all normal bile, the [UCB]was equal to or lower than the mean [UCB]concentration of model bile solutions with comparable [Ca++]. However, in all but one bile sample from dogs with pigment gallstones, the [UCB]exceeded this concentration and was therefore supersaturated with calcium bilirubinate. This supports the hypothesis that calcium bilirubinate precipitation is important in the formation and growth of pigment gallstones.
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