Mn and Cd transport by the Na-Ca exchanger of ferret red blood cells

Ca influx via the Na-Ca exchanger into ferret red blood cells is easily measured from a Na-free solution; the intracellular Na concentration is normally approximately 150 mM in ferret red blood cells. We have found that Mn and Cd competitively inhibit Ca influx. Mn influx and Cd influx were a saturable function of the divalent cation concentration, consistent with a carrier mechanism. Indeed, the Km (approximately 10 microM) and the Vmax (usually 1-3 mmol.l packed cells-1.h-1) were similar for Ca, Cd, and Mn. Extracellular Na inhibited divalent cation influx, and intracellular Na stimulated influx. These results are consistent with Na-Cd and Na-Mn influx pathways in ferret red blood cells. Ca (1 mM) almost completely inhibited Mn influx and Cd influx, whereas 1 mM Mg inhibited 5-15%. These results strongly support the notion that Mn and Cd are alternative substrates for Ca on the ferret red cell Na-Ca exchanger. The similarity in the behavior of all three divalent cation places important constraints on kinetic and structural models of the exchanger.