ABSTRACT
The internal osmotic environment is closely monitored in vertebrates, yet most cells have retained volume-sensitive transport mechanisms that tend to return cell volume towards normal after perturbation. The situation is complicated by the fact that different cells use distinct transport pathways to control cell volume. The hypothesis that protein phosphorylation is involved in volume regulation is hardly revolutionary, given the primacy of this mechanism in the control of a plethora of biological phenomena. Protein phosphorylation and dephosphorylation are controlled by the activity of two large families of enzymes: the protein kinases and protein phosphatases. The availability of permeant and specific inhibitors of particular enzyme systems generally leads to advances in our understanding of the roles of those enzymes in physiological processes. The regulation of the avian erythrocyte cotransport system has been well studied and provides a useful and unique model, in that it exhibits very low activity under normal conditions in vitro.
