Ionic transport mechanisms underlying fluid secretion by the pancreas

Abstract

The pancreas is a ‘leaky’ epithelium and secretes a juice in which sodium and potassium have concentrations similar to those of plasma. The characteristic features of the secretion are its isosmolality and its high bicarbonate concentration. It is the latter that has attracted considerable attention. Secretion in the isolated cat pancreas is directly proportional to the bicarbonate concentration in the nutrient fluid. The ability of the gland to secrete weak acids has led to the view that because of the very different chemical nature of the anions, it is most likely that it is a component common to all buffers, the proton, that is subject to active transport. This is supported by the decrease in pH and the increase in p co 2 of the venous effluent when secretion occurs and the sensitivity of secretion to the pH of the nutritional extracellular fluid. It is proposed that the cellular mechanisms are as follows: CO 2 diffuses into the cell and is hydrated to carbonic acid under the influence of carbonic anhydrase. The bicarbonate ion so formed diffuses into the ductular lumen and the proton is transported backwards through the epithelium with a proton pump (Mg 2+ -ATPase) provisionally located in the luminal membrane and a hydrogen-sodium exchange carrier located in the basolateral membrane. Energy for the latter process is derived from the sodium gradient between extracellular fluid and cell. This gradient is maintained by a (Na + +K + )-ATPase also located in the basolateral membrane. Chloride appears to be transported partly through a chloride-bicarbonate exchange mechanism, but largely passively together with a large sodium and potassium com ponent through the paracellular pathway. Osmotic equilibrium is likely to occur in the small ductules.