Export of Na+ from cells of the halotolerant microalga Dunaliella maritima: Na+/H+ antiporter or primary Na+-pump?

Transport of Na+ and K+ ions through the plasma membrane of intact cells of the halotolerant microalga Dunaliella maritima Massjuk was studied. Ion fluxes through the plasma membrane were induced by hyperosmotic shock (uptake of Na+ by the cells is transformed into extrusion of Na+) or by addition of K+ to a suspension of K+-deficient cells (uptake of K+ by the cells is associated with extrusion of Na+). The pathway of Na+ extrusion from the D. maritima cells does not depend on the direction or value of the proton gradient on the plasma membrane. In particular, the efficiency of Na+ extrusion was not changed at extracellular pH values varying from 6.0 to 8.0. The protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP) (20 microM) and the H+-ATPase inhibitor N,N-dicyclohexyl carbodiimide (DCCD) (25 and 100 microM) inhibited accumulation of K+ by the cells but did not influence Na+ extrusion. Significant acidification of the medium did not induce a net current of Na+ from the cells through a Na+/H+ antiporter. The data indicate that the Na+/H+ antiporter of the plasma membrane of D. maritima is not responsible for Na+ extrusion from the cells. These results can be explained by the involvement of a primary electrogenic Na+ pump (a Na+-transporting ATPase) in Na+ transfer through the plasma membrane of this alga.