Role of magnesium in contraction of glycerinated muscle fibers

The first phase of the magnesium effect, the increase in tension with increasing concentration of magnesium, up to about 25 µm, is abolished by the metal chelators DCTA and EDTA, but not by EGTA. DCTA is much stronger than EDTA in abolishing this first phase. The second phase of the magnesium effect, the decrease in tension with increasing concentration of magnesium, from about 25 µm to about 1 mm, is reduced when the concentration of ATP is decreased from 5 mm to about 0.5 mm. The third phase, the increase in tension with increasing concentration of magnesium to above 1 mm, is also reduced by metal chelators. However, EGTA is the strongest and DCTA is the weakest in reducing this third phase. From these observations, it is suggested that the first and the second phases are due to Mg (probably in the form of Mg-ATP complex) and that the third phase reflects the effect of a small amount of Ca which has been well established ( J. Biochem., Tokyo 50: 236–244, 1961 and J. Biol. Chem. 234: 2764–2769, 1959).