Ca2+ control of electrolyte permeability in plasma membrane vesicles from cat pancreas

Summary The influence of Ca²⁺ and other cations on electrolyte permeability has been studied in isolated membrane vesicles from cat pancreas.Ca²⁺ in the micromolar to millimolar concentration range, as well as Mg²⁺, Sr²⁺, Mn²⁺ and La³⁺ at a tested concentration of 10^–4 m, increased Na⁺ permeability when applied at the vesicle inside. When added to the vesicle outside, however, they decreased Na⁺ permeability. Ba²⁺ was effective from the outside but not from the vesicle inside.When Ca²⁺ was present at both sides of the membrane, Na⁺ efflux was not affected as compared to that in the absence of Ca²⁺. Monovalent cations such as Rb⁺, Cs⁺, K⁺, Tris⁺ and choline⁺ decreased Na⁺ permeability when present at the vesicle outside at a concentration range of 10 to 100mm. Increasing Na⁺ concentrations from 10 to 100mm at the vesicle inside increased Na⁺ permeability.The temperature dependence of Na⁺ efflux revealed that the activation energy increased in the lower temperature range (0 to 10°C) when Ca²⁺ was present at the outside or at both sides, but not when present at the vesicle inside only or in the absence of Ca²⁺.The results suggest that the Ca²⁺ outside effect is due to binding of calcium to negatively charged phospholipids with a consequent reduction of both fluidity and Na⁺ permeability of the membrane. The Ca²⁺-inside effect most likely involves interaction with proteins with consequent increase in Na⁺ permeability.The data are consistent with current hypotheses on secretagogue-induced fluid secretion in acinar cells of the pancreas according to which secretagogues elicit NaCl and fluid secretion by liberating Ca²⁺ from cellular membranes and by stimulating Ca²⁺ influx into the cell. The increased intracellular Ca²⁺ concentration in turn increases the contraluminal Na⁺ permeability which leads to NaCl influx. The luminal sodium pump finally transports Na⁺ ions into the lumen.