In skinned rat cardiac cells, increasing free magnesium concentration increases the capacity and rate of calcium binding by the sarcoplasmic reticulum, altering contractile activation.
In the presence of a slight buffering of the free Ca2+ with 0.050 mM total EGTA cyclic contractions were induced by a Ca2+-triggered release of Ca2+ on skinned (sarcolemma-free) segments of single cardiac cells from rat ventricle. The threshold of the free Ca2+ trigger was elevated when the free Mg2+ was increased. 2. At a suprathreshold free Ca2+ increasing the free Mg2+ resulted in a decrease in frequency and in an increase in amplitude of the phasic contractions. Addition of caffeine at a specified interval after a cyclic contraction produced a larger contraction when free Mg2+ was higher. It was concluded that an increase of free Mg2+ increased the capacity and the rate of binding for Ca2+ by the sarcoplasmic reticulum (SR). 3. Small skinned fibres of skeletal muscle which were perfused with 10 mM caffeine yielded results similar to those obtained in skinned cardiac cells. It was concluded that the mechanism of action of free Mg2+ was similar in both preparations, but that the SR of skeletal muscle had a higher capacity and rate of binding for Ca2+ than the cardiac SR. 4. With a strong buffering of the free Ca2+ with 4-0 mM total EGTA, a smaller tonic tension was developed for a given pCa in the presence of a higher free Mg2+. This result was nearly identical in skinned cells from cardiac and skeletal muscle tissue. 5. A decrease of the MgATP2- produced a tension in the skinned cardiac cells that were perfused in Ca2+ free media. The maximum tension was observed for MgATP2- 10(-5-50)M as in skinned fibres of skeletal muscle. A further decrease of MgATP2- resulted in a decrease of tension.
Fabiato et al. (1975) studied this question.