Potassium permeability of the mesothelium of the frog mesentery

The mesothelium of the mesentery is a single cell layer described to offer no or every little hindrance to diffusion of small water soluble solutes. We have measured the potassium permeability of the mesothelium of the frog mesentery both in vivo and in vitro. The permeability measured in vivo--using K+--sensitive microelectrodes is 2.4 X 10(-5) cm s-1. In vitro measurements using conventional tracer flux technique on isolated mesentery yields a K+-permeability of 5.7 X 10(-5) cm s-1. These values are 15--30 times smaller than values previously reported from in vitro experiments on rat and rabbit mesentery. Also, the permeability is 12--15 times lower than the K+-permeability of the capillary wall determined on single capillaries in the frog mesentery. In the frog mesentery the mesothelium thus represents an important diffusion barrier compared to the capillary wall. This may be critical in experiments where filtration and reflexion coefficients of the capillaries are determined from measurements of fluid exchange across the capillary wall in response to application of hypertonic solutions on the surface of the mesentery.