Regulatory volume increase in Ehrlich ascites tumor cells is mediated by the 1Na:1K:2Cl cotransport system.

After swelling in hyposmotic solution, Ehrlich ascites tumor cells shrink towards their original volume. Upon restoration of isosmolality (300 mOsM) the cells initially shrink but subsequently recover volume. This regulatory volume increase (RVI) is completely blocked when [Na+]o or [Cl-]o is reduced by 50% in the presence of normal [K+]o. With normal [NaCl]o but less than 2 mM [K+]o, not only is volume recovery blocked but the cells lose KCl and shrink. When [K+]o is increased to 5 mM there is a rapid net uptake of K+ and Cl- which results in volume recovery. This suggests that the reswelling phase requires the simultaneous presence of Na+, K+, and Cl-. Although ouabain has no effect on volume recovery, bumetanide completely blocks RVI by inhibiting a cotransport pathway that mediates the net uptake of Na+, K+ and Cl- in the ratio of 1Na:1K:2Cl. Na+ that accumulates is then replaced by K+ via the Na/K pump.