Regulation by magnesium of intracellular calcium movement in skinned muscle fibers.

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Abstract
The effect of Mg on Ca movement between the sarcoplasmic reticulum (SR) and myofilament space (MFS) was studied in skinned muscle fibers of Rana pipiens by using isometric force as an indicator of MFS Ca. In Ca-loaded fibers at 20.degree. C, the large force spike induced by Ca in 1 mM Mg (5 mM ATP) was strongly inhibited in 3 mM Mg, and force development was extremely slow. After a brief Ca stimulus in 1 mM Mg, relaxation in Ca-free solution was significnatly faster in 3 mM Mg. These changes were due to altered Ca movements, since the effect of 3 mM Mg on steady force in CaEGTA [ethylene glycol bis(.beta.-aminoethyl ether) tetraacetic acid] solutions was small. Changes in Mg alone induced force transients apparently due to altered Ca movement. In relaxed fibers, decreasing the Mg to 0.25 mM caused phasic force development. In contracting fibers in Ca solutions, increasing the Mg caused a large transient relaxation. The effects of increased Mg were antagonized by 0.5 mM Cd, an inhibitor of the SR Ca transport system. Active Ca uptake by the SR in situ is apparently stimulated by Mg, and the Ca can affect local MFS [Ca++] in the presence of a substantial Ca source. An increased rate of Ca uptake in 3 mM Mg could account for inhibition of the large force spike associated with Ca-induced Ca release in skinned fibers.