ROLE OF TAURINE IN OSMOREGULATION IN BRAIN-CELLS - MECHANISMS AND FUNCTIONAL IMPLICATIONS

All cells including neurons and glial cells are able to keep their vol ume within a very limited range. The volume regulatory mechanism invol ves changes in the concentration of osmolytes of which taurine appears to be of particular importance in brain cells. Swelling in brain cell s may occur as a result of depolarization or small fluctuations in osm olarity. In isolated brain cells these conditions will always lead to a release of taurine, the time course of which is superimposable on th at of the volume regulatory decrease which follows the initial cell sw elling. The mechanism responsible for taurine release associated with swelling has not been fully elucidated but a large body of evidence se ems to exclude participation of the taurine high affinity carrier. Usi ng a number of inhibitors of anion exchangers it has been demonstrated that both volume regulation and taurine release in brain cells are in hibited by these drugs, implicating an anion channel in the process. I t has been a controversial issue as to whether or not taurine release is Ca++ dependent. Recent evidence appears to suggest that the release process is not associated with Ca++ or Ca++ channels. It is, however, quite possible that the swelling process may involve the Ca++ calmodu lin system or other second messengers. Taurine also contributes to vol ume regulation after shrinkage of brain cells, in this case by increas ing its intracellular concentration. This change is accomplished by an upregulation of the Na+/taurine cotransporter, together with reduced passive fluxes and increased endogenous synthesis.