Convective movement of Ca²⁺ across guinea pig gallbladder epithelium

Recently, much interest has developed in biliary calcium because of its importance in the pathogenesis and composition of gallstones. While much progress has been made in understanding the thermodynamic factors that control biliary calcium concentrations, little is known about the kinetic factors that control the movement of calcium across the gallbladder epithelium. These studies measure guinea pig gallbladder epithelial permeability to Ca²⁺ during in vivo convective water movement across the membrane. Water movement, ranging from -15.2 (absorption) to 6.3 μl · min^{- 1} · cm^-2 (water entry), was induced by placing hypotonic, isotonic, and hypertonic solutions into the gallbladder lumen. Calcium movement was found to be directly and linearly related to water flow, indicating that Ca²⁺ moved with the convective water flow, presumably across paracellular channels. The slope of this relationship (0.602), representing the concentration of calcium in the fluid translocated across the gallbladder epithelium, was only about half that of plasma or luminal contents, indicating that calcium movement across the membrane was restricted. The mean sieving coefficient (1 - r) of guinea pig gallbladder, calculated from this slope, was ~0.5, indicating that the epithelium is only moderately permeable to Ca²⁺. The results suggest that intraluminal chelation of Ca²⁺ for the possible prevention and/or treatment of calcium-containing gallstones is a potentially feasible therapeutic modality.