Transport of magnesium by two isoforms of the Na⁺-Ca²⁺ exchanger expressed in CCL39 fibroblasts

Abstract.

Cytoplasmic concentrations of Ca²⁺ ([Ca²⁺]_i) and Mg²⁺ ([Mg²⁺]_i) were measured with fluorescent indicators in CCL39 cells, a cell line established from Chinese hamster lung fibroblasts, transfected with complementary deoxyribonucleic acid (cDNA) of the Na⁺-Ca²⁺ exchanger isolated either from canine heart (NCX1) or from rat brain (NCX3). Raising extracellular [Mg²⁺] to 10 mM increased Mg²⁺ influx and the resultant change in [Mg²⁺]_i (Δ[Mg²⁺]_i) was monitored with furaptra under Ca²⁺-free conditions. In control (vector-transfected) cells, Δ[Mg²⁺]_i at 45 min was similar with or without extracellular Na⁺ (130 mM or 0 mM) and when [Na⁺]_i was raised by 1 mM ouabain treatment. Δ[Mg²⁺]_i in NCX1-transfected cells was attenuated significantly in the presence of 130 mM Na⁺, but became comparable to (or slightly larger than) that in control cells on either removal of extracellular Na⁺ or treatment with 1 mM ouabain. Cells expressing NCX3 showed an intermediate dependence of Δ[Mg²⁺]_i on Na⁺, probably reflecting a lower degree of expression of the exchanger protein. Extracellular Na⁺-dependent changes in [Ca²⁺]_i (measured with fura-2 in the presence of extracellular Ca²⁺ and 10 µM ionomycin, a Ca²⁺ ionophore) were minimal in control cells, marked in the NCX1-transfected cells and intermediate in the NCX3-transfected cells. These results suggest that the Na⁺-Ca²⁺ exchanger (either NCX1 or NCX3) can transport Mg²⁺ and may play a role in the extrusion of magnesium from cells.