Specific protein phosphorylation occurs in molluscan red blood cell ghosts in response to hypoosmotic stress.

The regulation of cellular volume upon exposure to hypoosmotic stress is accomplished by specific plasma membrane permeability changes that allow the efflux of certain intracellular solutes (osmolytes). The mechanism of this membrane permeability regulation is not understood; however, previous data implicate Ca2+ as an important component in the response. The regulation of protein phosphorylation is a pervasive aspect of cellular physiology that is often Ca2+ dependent. Therefore, we tested for osmotically induced protein phosphorylation as a possible mechanism by which Ca2+ may mediate osmotically dependent osmolyte efflux. We have found a rapid increase in 32Pi incorporation into two proteins in clam blood cell ghosts after exposure of the intact cells to a hypoosmotic medium. The osmotic component of the stress, not the ionic dilution, was the stimulus for the phosphorylations. The osmotically induced phosphorylation of both proteins was significantly inhibited when Ca2+ was omitted from the medium, or by the calmodulin antagonist, chlorpromazine. These results correlate temporally with cell volume recovery and osmolyte (specifically free amino acid) efflux. The two proteins that become phosphorylated in response to hypoosmotic stress may be involved in the regulation of plasma membrane permeability to organic solutes, and thus, contribute to hypoosmotic cell volume regulation.