Furosemide stimulates K transport in HCD57 erythroid cells

We examined the influence of serum and furosemide on K movement and cell vo lume in HCD57 cells, a murine erythroleukemia cell line, which require eryt hropoietin (EPO) for survival. We found that maintenance of cell volume dep ends on the concentration of serum in the culture medium. In isotonic mediu m containing 20% serum, HCD57 cells maintain their steady-state volume. In contrast, the cells shrink progressively as medium serum content is reduced . In serum-free medium, raising external K to 75 mM prevents cell shrinkage and a further increase in K to 145 mM results in swelling, revealing a rol e for K permeability in the regulation of cell volume. Of particular intere st has been a serendipitous finding with furosemide. Below an external K co ncentration of 2.1 +/- 0.3 mM in medium containing 2% serum, furosemide inh ibits K uptake, probably stemming from its well known inhibitory action on KCl cotransport. However, above that K concentration, furosemide stimulates K uptake in a dose-dependent manner. Moreover, furosemide potentiates cell shrinkage induced by serum withdrawal. These findings suggest that the tra nsport machinery mediating cellular shrinkage, once primed by serum depleti on, becomes receptive to a second stimulus.