Trans-magnesium dependency of ATP-dependent calcium uptake into sarcoplasmic reticulum of skeletal muscle.

Increasing the levels of Mg2+ concentrations outside the sarcoplasmic reticulum (SR) vesicles larger than the ATP concentrations results in a decrease of the Ca2+/Mg2+-ATPase affinity to Ca2+. When SR vesicles are loaded with varying concentrations of magnesium in the inside and the same concentration on the outside, the initial rate of calcium transport into the SR is increased by up to 50% with K0.5 (Mg2+) = 0.127 mM. During active calcium transport, we found no evidence of a magnesium influx. However, the data indicate that magnesium is extruded from SR vesicles during calcium uptake, but the magnitude of magnesium efflux is too small (8%) to account for the cation counter transport of calcium.