Transient Na+ Flux Following Hyperosmotic Shock in the Halotolerant Alga Dunaliella salina: a Response to Intracellular pH Changes

The mechanism of the transient Na+ influx that is induced by hyperosmotic shock in the halotolerant alga Dunaliella salina and its relationship to glycerol production has been investigated. It is demonstrated that: (1) The rapid Na+ influx is independent on metabolic energy but is affected by temperature, external pH and the transmembrane electrical potential. Na+ elimination depends on metabolic energy but not on external pH suggesting that it is catalyzed by a different mechanism. (2) Intracellular acidification stimulates whereas intracellular alkalinization inhibits Na+ influx following hyperosmotic shocks. Na+ influx can also be induced by artificial intracellular acidification under isoosmotic conditions. These results suggest that Na+ influx following hyperosmotic shocks may result from internal acidification. (3) Lithium and vanadate ions, inhibitors of the plasma membrane Na+/H+ antiporter and H+-ATPase, respectively, inhibit Na+ influx. These treatments as well as internal alkalinization by ammonium ions also delay glycerol synthesis and volume recovery from hyperosmotic shock. The results suggest a connection between intracellular pH and glycerol synthesis. (4) Extracellular Na+ is not obligatory for recovery from hyperosmotic shocks, and does not provide any advantage for resisting lysis by extreme osmotic changes to Dunaliella cells. It is suggested that Na+ influx following hyperosmotic shock in D. salina is a secondary response to intracellular acidification and is catalysed by a Na+/H+ antiporter at the plasma membrane. The results imply that Na+/H+ antiport is activated by internal acidification or by hyperpolarization and inhibited by internal alkalinization or by depolarization. The possible role of intracellular pH in glycerol synthesis is discussed.