The Taurine Efflux Portal Used to Regulate Cell Volume in Response to Hypoosmotic Stress Seems to Be Similar in Many Cell Types: Lessons to Be Learned from Molluscan Red Blood Cells1

Abstract

SYNOPSIS. The control of cell volume in all cell types is accomplished by the regulation of two general categories of osmolytes: inorganic ions, most commonly K 1 and Cl 2 , and small molecular weight organic compounds, usually certain amino acids and certain quaternary ammonium compounds. The difference in who regulates what does not depend phylogeny, but instead upon the type of osmotic environment that a cell expects (in an evolutionary sense) to encounter. Cells that exist in extracellular osmotic concentrations up to 300‐400 mosmol/kg (mosm) rely primarily on inorganic osmolytes for volume control, while cells that exist at greater osmotic concentrations rely more on organic osmolytes for volume control. Usually, strange or unique volume regulatory mechanisms are found in cells that exist in particularly demanding osmotic conditions. In order to provide further support the foregoing generalizations, the following paper will focus on comparisons between the hypoosmotically induced mechanism of taurine efflux regulation by red blood cells of the bivalve, Noetia ponderosa, probably the best understood ‘‘invertebrate’’ cell type in this regard, and taurine efflux from a variety of ‘‘vertebrate’’ cells.