Increased phosphate efflux from acinar cell during protein secretion.

The permeability of the pancreatic epithelium to two water soluble molecules, sucrose and inulin, increases when protein secretion is augmented by a cholinergic agonist. An increase in the permeability of passive paracellular shunts (3) has been proposed to account for these observations. In the present experiments we have measured the distribution of another molecule, phosphate ion, across the epithelium by following its secretion from the cannulated duct of whole-rabbit pancreas in short-term organ culture. A cholinergic stimulant increases phosphate ion concentration in secretion in a similar fashion to that seen for the watersoluble nonelectrolytes. However, both the unstimulated rate of phosphate secretion and the increase observed with cholinergic stimulation were not dependent on the presence of the ion in the medium, and therefore its secretion in both cases reflects phosphate efflux from the cell and not its paracellular transport. The results indicate that either the phosphate ion is excluded from paracellular shunts or that such shunts do not contribute substantially to the transpancreatic passage of molecules of this size.