Osmotic phenomena in smooth muscle

¹ Department of Physiology, Ohio State University, Columbus, Ohio

Anomalous osmotic phenomena observed in the frog's stomach muscle bathed in certain solutions were analyzed. It is suggested that intracellular hydrostatic pressure is an important factor in the exchange of water in these situations. In very dilute solutions of CaCl₂ and MgCl₂ (2–8 mm) the muscle maintains a large difference in the concentration of solutes between the fibers and its medium; it swells only moderately, remains semipermeable, can contract, and becomes rigid. Osmotic balance probably is determined largely by hydrostatic pressure arising from elastic tension in internal structures, as it is in a strongly cross-linked gel. Swelling in isosmotic KCl solution is slow; the addition of 2 mm CaCl₂ causes shrinking. Uptake of water in these solutions probably is opposed by internal hydrostatic pressure produced by a sustained contracture. The force produced under various conditions correlates well with the volume changes. Shrinking in isosmotic sugar solutions to which 2 mm CaCl₂ or MgCl₂ is added probably has the same cause. The role of the cell membrane under conditions of increased hydrostatic pressure within the fibers is discussed.