Effects of magnesium, ouabain and bumetanide on 86rubidium uptake in a human atrial cell line

1. The effects of extracellular magnesium concentrations (0, 0.6, 1.2 mM) on 86Rb (used as an analogue of potassium) uptake were investigated in the Girardi human atrial cell line in the presence and absence of drugs. 2. Increasing extracellular magnesium resulted in significantly higher 86Rb uptake. Compared to uptake in 0.6 mM (the physiological extracellular magnesium concentration), uptake of 86Rb was significantly higher in the 1.2 mM magnesium medium and significantly lower in the magnesium-free medium. 3. Ouabain (10(-3)M) and bumetanide (10(-4)M) were added to inhibit, respectively, the Na-K-ATPase and the Na-K-Cl co-transport system in the media containing the three magnesium concentrations. The ouabain-sensitive, bumetanide-sensitive and residual transport were found to be 58%, 29% and 13% of the total uptake in the medium containing 0.6 mM magnesium. 4. The ouabain-sensitive 86Rb uptake was inhibited significantly by reducing the magnesium concentrations to zero whereas the bumetanide-sensitive and residual uptake were not significantly affected by different magnesium concentrations. 5. At three different ouabain concentrations (10(-7) M, 10(-5) M, 10(-3) M) studied there was significantly greater uptake of 86Rb in 1.2 mM magnesium compared to uptake in 0 mM magnesium. 6. The present findings indicate that extracellular magnesium is important for 86Rb (potassium) transport in cardiac cells, and suggest that the main effect is on the Na-K-ATPase component of transport.