Responses of the Na+/H+ exchanger of European flounder red blood cells to hypertonic, beta-adrenergic and acidotic stimuli

The transport pathways mediating regulatory volume increase (RVI) and beta- adrenergic responses in red cells of the European flounder Platichthys fles us have been investigated, Hypertonic treatment under a low-P-O2 atmosphere led to a complete RVI and to a three- to fourfold increase in Na+ influx, The RVI and the activated Na+ influx were blocked by the transport inhibito rs amiloride and 3,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) , both at a concentration of 10(-4) mol l(-1), and the RVI was abolished in a Na+-free saline, indicating the involvement of a hypertonically induced Na+/H+ exchanger and an accompanying Cl-/HCO3- exchanger. Both the hyperton ically induced Na+ influx and the RVI were blocked by oxygenation of shrunk cells. The beta-adrenergic agonist isoproterenol also strongly activated a Na+ influx and caused cell swelling, This response was also inhibited by a miloride and DIDS hut was unaffected by oxygenation. Simultaneous applicati on of isoproterenol and hypertonic shrinkage did not lead to additive Na+ i nfluxes, suggesting that both responses were mediated by the same pool of e xchangers. Mild cell acidification activated a Na+ influx under iso-osmotic conditions; amiloride caused partial inhibition of this influx, but oxygen ation had no effect, Acid-induced and isoproterenol-induced Na+ fluxes were again non-additive. Thus, the Na+/H+ exchanger of flounder red cells is st rongly activated by three physiological stimuli: hypertonic shrinkage, beta -adrenergic hormones and cell acidification. Of these responses, only the f irst is affected by oxygenation, indicating some differentiation of their r espective transduction mechanisms. These characteristics contrast with thos e of the corresponding exchangers from rainbow trout and eel red cells.