Isolated frog sartorii were immersed in Ringer's solution, which had a 2 mM magnesium concentration containing Mg28. The uptake of the radioactive magnesium was measured under steady state conditions. Although the biological variability was fairly large, it was observed that the uptake proceeded in three stages lasting respectively about 0.5, 30, and 300 minutes and accounting respectively for about 0.21, 0.71, and 0.67 millimole magnesium/kg. muscle. It was assumed that the first stage represents surface adsorption, the second stage represents extracellular water and connective tissue phases, and the third stage entry inside the cell. It is estimated that the maximum intracellular magnesium concentration is about 1.1 mM and that only about 0.6 millimole magnesium/liter intracellular water is exchanged per hour. The maximum energy required per hour to pump the magnesium out of the cell against the electrochemical gradient is calculated to be only 1.5 cal./kg. muscle. About 75 to 80 per cent of the magnesium in muscle is non-exchangeable and difficult to remove by diffusion. It appears from previous work that the exchangeable magnesium behaves similarly to exchangeable calcium.

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