Permeability Characteristics of Vascular Smooth Muscle Cells as Revealed by Their Osmotic Responses to Non‐Electrolytes

Changes of cell volume in the smooth muscle of the isolated rat portal vein, produced by variations in extracellular osmolarity, are associated with characteristic changes in the spontaneous electrical and mechanical activity of the preparation (Johansson and Jonsson 1968, Arvill, Johansson and Jonsson 1969). An analysis of such responses to anisosmotic solutions containing different non‐electrolytes was carried out in the present study in order to elucidate the osmotic influences of the test substances on the vascular smooth muscle cells. The time course of the responses revealed the following relative rates of penetration through the cell membranes: sucrose < erythritol < glycerol < urea < thiourea < ethylene glycol = propylene glycol = tetramethylene glycol = pentamethylene glycol = urethane. The importance of molecular size and lipid solubility for this order of permeability is discussed. A quantitative analysis of responses to solutions containing urea gave an approximate estimation of the osmotic reflexion coefficient for this substance. The value obtained was 0.65 which corresponds to an equivalent pore radius of 3.7 Å. This information and previous data concerning the uptake of urea‐14C by the rat portal vein could be used for a calculation of the total pore area. The result indicated that the pores occupy only a very small fraction, 7 × 10–7, of the total area of the cell membrane and that the number of pores is of the order of 103 per cell or 1.6 × 103 per cm2.