Rapid swelling of a CHO-K1 aspartate/glutamate transport mutant in hypo-osmotic medium

Two Chinese hamster ovary cell (CHO-K1) mutants selected for defective glutamate transport via system X-AG are also highly permeable to small neutral molecules. Light microscopy demonstrated that exposure of one of these mutants, Ed-A1, to hypo-osmotic medium led to extremely rapid swelling, presumably due to increased water flux. When placed in 20% saline, Ed-A1 cells swelled to three times their original volume within 15 sec, a sixfold larger increase than parental CHO-K1. In spite of this rapid volume increase, mutant and wild-type cells remained viable for 20 min in dilute saline. A regulatory volume decrease in Ed-A1, and the continual swelling of CHO-K1, resulted in the two cells achieving equal size after 5 min in 20% saline. The time course of these volume changes permitted analysis of large numbers of cells by a hydrodynamic technique, steric field flow fractionation (FFF). Steric FFF demonstrated the expected inhibition of osmotic swelling of human erythrocytes by the mercurial, p-chloromercuribenzenesulfonic acid (PCMBS). However, PCMBS increased the apparent swelling rate of Ed-A1 and CHO-K1, suggesting that an aquaporin-like molecule is not responsible for any significant fraction of the water fluxes into either line. PCMBS also strongly inhibited aspartate transport by system X-AG. By taking advantage of their different swelling rates in hypotonic medium, steric FFF can separate mixtures of CHO-K1 and Ed-A1.