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

The effects of hypo‐osmolarity on the spontaneous activity of the smooth muscle in the isolated portal vein of the rat have been studied in solutions with varied ionic composition. A comparison between the stimulatory effects of lowered osmolarity and increased [K+]o indicated that at normal [K+]o in the control state the excitation caused by hypotonicity is not mediated only via changes in the [K+]i/[K+]o ratio. The response to hypo‐osmolarity at a higher extracellular potassium concentration is, however, more readily explained by the change in [K+]i/[K+]o which occurs in association with the swelling of the cells. This might be attributed to the fact that the membrane potential of smooth muscle is more dependent on the transmembrane concentration gradient for potassium at higher levels of external potassium concentrations. The stimulatory effects of hypo‐osmolarity are markedly reduced at a low [Na+]o but are not considerably affected by comparable reductions in [K+]o or [Cl+]o. This indicates that the transmembrane sodium ion gradient is involved in the mechanisms by which hypo‐osmolarity excites the spontaneous activity of vascular smooth muscle. One possible interpretation is that the swelling of the muscle cells in hypotonic solutions causes an increase in the permeability of the cell membrane to the relatively larger sodium ion.