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Department of Physiology and Biophysics, Wright State University, School of Medicine, Dayton, OH 45401.

1. Sheep erythrocytes were treated with the divalent metal ionophore A23187 to alter the cellular magnesium (Mgi) and calcium (Cai) composition. Ouabain-sensitive Na+-K+ pump fluxes were measured using rubidium as a potassium congener in media where Cl- was replaced by NO3-. 2. A23187, per se, had no effect on ouabain-sensitive rubidium influx. However, lowering the concentration of cellular magnesium [( Mg]i) and increasing that of calcium [( Ca]i) decreased Na+-K+ pump flux. 3. Ouabain-sensitive rubidium influx was found to be a saturating function of [Mg]i in high-potassium (HK) red cells with a Hill coefficient of about 1.8 and an apparent half-activation constant (K0.5) of 0.46 mmol/(l original cells). In low-potassium (LK) cells, in the absence and presence of the Na+-K+ pump stimulatory L-antibody, ouabain-sensitive rubidium influx was also saturated with Mgi yielding Hill coefficients of close to 1.8 and K0.5 values of 0.20 and 0.30 mmol/(l original cells), respectively. 4. When [Ca]i was raised at constant [Mg]i ouabain-sensitive rubidium influx was inhibited at about 700 mumol/(l cells) in both HK, and in anti-L-treated LK red cells. 5. These data exclude the possibility that the Na+-K+ pump turnover, known to be different in HK red cells, and in LK red cells in the absence and presence of anti-L (Joiner & Lauf, 1978b), is based on differences in the activation by MgATP, and that Cai interacts with the Na+-K+ pump cycle differently in the two red cell cation types.