Evidence that a Na⁺/Ca²⁺ antiport system regulates murine erythroleukemia cell differentiation

The Na⁺ and Ca²⁺ transport properties of cultured murine erythroleukemia (MEL) cells have been investigated. We have previously shown that amiloride prevents dimethyl sulfoxide-induced MEL cell differentiation via inhibition of an essential Ca²⁺ influx. Here we show that external Na⁺ inhibits Ca²⁺ influx and stimulates Ca²⁺ efflux from uninduced MEL cells. Increasing the internal Na⁺ concentration by a brief incubation of cells with ouabain stimulates the rate of ⁴⁵Ca²⁺ influx. Amiloride (40 μM) completely blocks the external Na⁺-stimulated ⁴⁵Ca²⁺ efflux and external Na⁺-inhibitable ⁴⁵Ca²⁺ influx. The same concentration of amiloride had no significant effect on net Na⁺ uptake. These results suggest that a significant fraction of Ca²⁺ flux across the MEL cell plasma membrane occurs via a Na⁺/Ca²⁺ antiport system and that amiloride prevents differentiation by blocking Ca²⁺ influx through this system. The importance of a Na⁺/Ca²⁺ antiport system for MEL cell differentiation is supported by the following observation: increasing the cellular Na⁺ level by a brief treatment with ouabain plus monensin accelerates MEL cell commitment as effectively as adding the Ca²⁺ ionophore A23187. We suggest that dimethyl sulfoxide induces MEL cell differentiation by inhibiting the Na⁺ pump and consequently allowing Ca²⁺ influx through the Na⁺/Ca²⁺ antiport.