Effects of Hyperosmolarity on the Volume of Vascular Smooth Muscle Cells and the Relation between Cell Volume and Muscle Activity

Abstract

Abstract Electrical and mechanical responses of vascular smooth muscle to variations in extracellular osmolarity were analysed in a previous study using isolated preparations of rat portal vein (Johansson and Jonsson 1968). The purpose of the present investigation has been to measure the changes in the volume of the cells of this smooth muscle when exposed to hyperosmolarity and to elucidate further the previously postulated relation between cell volume and muscle activity. Distribution of water, urea‐ 14 C and sucrose‐ 14 C in the isolated portal vein has been determined. Urea‐ 14 C rapidly equilibrates in a volume of 77.9 ml/100 g wet tissue weight which agrees satisfactorily with the total water content of the muscle (79.0 ml/100 g). The smooth muscle cells thus show a high membrane permeability for urea. When the muscle is incubated in isoosmotic Krebs solution the distribution volume of sucrose‐ 14 C reaches 45.4 ml/100 g within 15 min and then increases very slowly to 49.4 ml/100 g within 120 min. The former value has been taken to represent the extracellular space. After 20 min in a hyperosmotic medium (normal Krebs solution +150 mmoles sucrose/l) there is a decrease of total wet tissue weight to approximately 86 per cent of the control value and, simultaneously, a relative increase of extracellular space from 45.4 to 48.5 ml/100 g. Calculations show that the intracellular fluid volume has then decreased by about 34 per cent indicating an almost perfect osmotic behaviour of the smooth muscle cells in response to hyperosmolarity of this degree. The changes in muscle activity observed during and after exposure to hyperosmotic urea‐ and sucrose‐Krebs solutions are discussed in the light of the changes in cell volume and the time course of the uptake of urea‐ 14 C and sucrose‐ 14 C. The present results support the existence of a relation between cell volume and activity in vascular smooth muscle.