EFFECT OF ACUTE BILE-ACID POOL DEPLETION ON TOTAL AND IONIZED CALCIUMCONCENTRATIONS IN HUMAN BILE

Although calcium salts are important components of gallstones, there a re few data on the total and ionized calcium content of human bile. Th erefore, in 14 fasting T-tube patients studied 7-11 days after cholecy stectomy, we measured bile flow, bile acid [BA], total [Ca-TOT] and fr ee ionized [Ca++] calcium concentrations, in 20-30 min bile collection s during acute BA pool depletion induced by 6-8 h of continuous bile d rainage. During washout of the BA pool there were parallel falls in bi le flow, BA output and total calcium output (correlation coefficients ranging from 0.59 to 0.99; P < 0.02-0.001). In 12 of the 14 patients, [Ca-TOT] also fell (from 1.84 +/- 0.29 to 1.32 +/- 0.34 mmol L(-1)) in parallel with [BA] (from 34.0 +/- 14.0 to 8.2 +/- 8.0 mmol L(-1) r=0. 75-0.98; P<0.005).In contrast, biliary [Ca++] remained virtually uncha nged. These data suggest that the BAs are linked to the bound, rather than to the free, ionized, fraction of biliary calcium, which is consi stent with in vivo calcium binding by BAs. A model is proposed in whic h BA-induced biliary calcium secretion results from (i) bile acid-indu ced water flow via solvent drag; and (ii) calcium binding in the bile canaliculus by bile acids, which induces paracellular diffusion of Ca+, thereby maintaining [Ca++] independent of [BA].