A Ca2+ Influx in Response to Hypo-osmotic Stress May Alter Osmolyte Permeability by a Phenothiazine-sensitive Mechanism

The phenomenon of cell volume recovery following a hypo-osmotic stress mediated by intracellular osmolyte regulation is well known. In many, perhaps all, cell types, the osmolytes involved are usually inorganic ions and amino acids. The details of the regulatory mechanisms for the organic-type osmolytes are not well known. We have found that an immediate influx of external Ca2+ occurs coincident with the application of a hypo-osmotic stress into red cells of two invertebrate species. In both, the influx is initiated by the osmotic stress, not the concomitant ionic decrease. Volume recovery in clam red blood cells is blocked by phenothiazines. In addition, the effect of the phenothiazines is to reduce the amino acid efflux; the ionic portion of the volume response is unaffected. In contrast, the phenothiazines potentiate the volume recovery in worm red coelomocytes. A23187 also potentiates the volume recovery of the worm red cells. The results suggest that the Ca2+ influx is involved in the mechanism that alters cell membrane permeability permitting the amino acid efflux by a mechanism that may involve calmodulin.