Role of calcium in exocrine pancreatic secreation. VI. Characteristics of the paracellular pathway for divalent cations.

(1) The transepithelial permeability for Ca2+ and Mg2+ in the isolated rabbit pancreas has been studied. (2) Values for the permeability of the unstimulated pancreas were obtained either by adding radioactive tracers to the bathing medium and measuring their concentration in the secreted fluid under steady-state conditions, or by analysis of the Ca2+ and Mg2+ concentrations in the secreted fluid after correction for protein-bound divalent cations. (3) Both methods give almost the same results: 27 and 26% for Ca2+ and 21 and 18% for Mg2+, respectively; both values being expressed as the percentage of the concentrations in the bathing medium. (4) The amounts of Ca2+ and Mg2+, appearing in the secretory fluid after correction for protein-bound cations, are linearly related to the extracellular Ca2+ and Mg2+ concentrations in the bathing medium, which indicates passive permeation. The two cations appear to pass through the paracellular route in their hydrated form. (5) Stimulation with carbachol or pancreozymin causes an increase in the paracellular permeability. This increase is approximately equal for the two divalent cations. Its time dependence and magnitude depend on the concentration of the stimulant rather than on the type of stimulant.