Changes in the Cell Volume of Isolated Vascular Smooth Muscle in Response to Reduced Osmolarity

The intracellular water of smooth muscle from the rat portal vein decreases on exposure to hyperosmotic solutions to an extent expected for perfect osmometers (Arvill, Johansson and Jonsson 1969). The purpose of the present study has been to investigate the variations in cell water produced by reductions in extracellular osmolarity. After 15 min of incubation at 37° C in a solution which was 105 mOsm/l hypo‐osmotic (minus 60 mmoles NaCl/1) compared to normal Krebs solution, the intracellular water increased by 14 ± 7 per cent in muscles stretched to their in situ length and by 35 ± 9 per cent in unstretched muscles. When the same procedures were performed at 1° C, a temperature that considerably reduced the mechanical response to hypo‐osmolarity, the volume of the cell water in the stretched and unstretched muscles increased to approximately the same extent (40 and 35 per cent respectively). These results suggest that the active tension development associated with excitation at low osmolarity opposed the swelling of the muscle cells in the stretched muscles at 37° C while the passive tension per se was less important for the degree of swelling. However, had the smooth muscle cells behaved like perfect osmometers the increase in cell water at this reduction in osmolarity would have been 58 per cent. The difference between this value and those obtained in situations where the active tension development was prevented might be accounted for by a loss of intracellular ions. Such a loss of ions was clearly demonstrated in stretched muscles at 37° C which showed the smallest degree of swelling. This suggests that muscle cells, when unable to adapt their intracellular osmolarity to a reduced extracellular tonicity by swelling, do so by dropping a fraction of their intracellular ions.