THE OSMOLALITY-SENSITIVE TAURINE CHANNEL IN FLOUNDER ERYTHROCYTES IS STRONGLY STIMULATED BY NORADRENALINE UNDER HYPOOSMOTIC CONDITIONS

Stimulation of flounder erythrocytes by noradrenaline under isosmotic conditions (330 mosmol kg-l) and physiological Nat concentration (113 mmol l(-1)) caused swelling of the cells. The EC(50) of this cell swel ling was 0.65 mu mol l(-1) noradrenaline, The effect of the noradrenal ine-induced cell swelling on the taurine channel under isosmotic condi tions was negligible. However, when the cells were stimulated by norad renaline (1.0 mu mol l(-1)) before, simultaneously with or after reduc tion of osmolality (255 mosmol kg(-1)), the volume regulatory efflux o f taurine mediated by the taurine channel was transiently accelerated, The rate coefficient for taurine efflux was more than four times high er than in osmolality-stimulated cells not exposed to noradrenaline, T he present paper deals with the accelerating effect of noradrenaline o n the taurine channel under hypo-osmotic conditions and the lack of ef fect of noradrenaline-induced cell swelling on the channel under iso-o smotic conditions. Noradrenaline initiated the cell swelling by intera cting with beta-receptors which appeared to be more related to the mam malian beta(1)-receptors than to the beta(2)-receptors, The receptor i nteraction activated the adenylate cyclase system and, in the presence of 1.0 mu mol l(-1) noradrenaline, the cellular cyclic AMP concentrat ion increased about 23 times, Noradrenaline also stimulated the Na+/H and Cl-/HCO3- antiporters and this affected the extracellular pH as w ell as the cell volume, Depending on the extracellular Na+ concentrati on, the incubation medium was acidified (113 mmol l(-1) Na+) or alkali zed (2.7 mmol l(-1) Na+), Under these two conditions, the accelerating effects of noradrenaline on the taurine efflux were of similar magnit ude, Similar effects on the cell volume, the extracellular pH and the volume regulatory taurine efflux were obtained in the presence of the cyclic AMP analogue 8-bromo-cyclic AMP, Under hypo-osmotic conditions in the absence of noradrenaline, the cellular level of cyclic AMP was not elevated, There was no significant positive correlation between th e water content of the cells (cell volume) under different conditions in the presence or absence of noradrenaline and the state of activatio n of the osmolality-sensitive taurine channel. We conclude that the me chanism(s) which activate(s) the osmolality-sensitive taurine channel in flounder erythrocytes is transiently and strongly accelerated by no radrenaline, but not triggered by the noradrenaline-induced events, Th e acceleration does not appear to be due to increased activity of the antiporters, but to increased cellular levels of cyclic AMP.