The reflexion coefficient as a measure of transepithelial permeability in the isolated rabbit pancreas.

1. The reflexion coefficients of a number of non-electrolytes and electrolytes have been determined in the isolated rabbit pancreas. 2. The reflexion coefficients of the following non-electrolytes were: urea, -0.02; glycerol, 0.06; erythritol, 0.11; sorbitol, 0.41; mannitol, 0.42; arabinose, 0.72; xylose, 0.74, assuming a value of 1.00 for sucrose. 3. These values are equal within the experimental error to values previously obtained with a tracer technique for the same preparation, but they are significantly lower than those reported by other investigators for the isolated perfused cat pancreas. 4. Addition of 100 mM-sucrose to the bathing medium resulted in proportionally increased Na+ and K+ concentrations in the secreted fluid. The secreted fluid remained isotonic with the bathing medium under all circumstances. 5. Addition of 10(-5) M-carbachol to the bathing medium led to a reduction in the reflexion coefficient of sucrose from 1 to 0.85, but only when 25 mM-sucrose was used. 6. The reflexion coefficients of electrolytes were: NaCl, 0.50; KCl, 0.51; NaHCO3, 0.52 and choline chloride, 1.02. 7. It is concluded that the isolated rabbit pancreas is highly permeable, both to electrolytes and to small non-electrolytes, probably being more leaky than any other epithelium studied so far.