Politis A D, Pierce S K
Department of Zoology, University of Maryland, College Park 20742.
J Membr Biol. 1991 Nov;124(2):169-77. doi: 10.1007/BF01870461.
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.
细胞在暴露于低渗应激时的体积调节是通过特定的质膜通透性变化来实现的，这种变化允许某些细胞内溶质（渗透剂）外流。这种膜通透性调节的机制尚不清楚；然而，先前的数据表明Ca2+是该反应的一个重要组成部分。蛋白质磷酸化的调节是细胞生理学中一个普遍存在的方面，通常依赖于Ca2+。因此，我们测试了渗透压诱导的蛋白质磷酸化，作为Ca2+可能介导渗透压依赖性渗透剂外流的一种可能机制。我们发现，将完整细胞暴露于低渗培养基后，蛤血细胞血影中两种蛋白质的32Pi掺入迅速增加。应激的渗透压成分而非离子稀释是磷酸化的刺激因素。当培养基中省略Ca2+或使用钙调蛋白拮抗剂氯丙嗪时，两种蛋白质的渗透压诱导磷酸化均受到显著抑制。这些结果在时间上与细胞体积恢复和渗透剂（特别是游离氨基酸）外流相关。响应低渗应激而发生磷酸化的两种蛋白质可能参与质膜对有机溶质通透性的调节，从而有助于低渗细胞体积调节。
