The effect of Mg2+ on the Ca2+ dependence of ATPase and tension development of fast skeletal muscle. The role of the Ca2+-specific sites of troponin C.

Conflicting reports have appeared concerning the effect of Mg2+ on muscle activity. Several groups have found that increasing Mg2+ produces a right-ward shift of the pCa-tension curve, while others have found no effect of Mg2+ on myofibrillar ATPase activity. The present study is a careful evaluation of the effect of Mg2+ on myofibrillar ATPase, skinned fiber tension development, TnCDANZ (troponin C (TnC)-labeled with 5-dimethylaminonaphthalene-1-sulfonyl aziridine) fluorescence, and simultaneous TnCDANZ fluorescence and tension development in the same fiber. A small effect of Mg2+ on both ATPase and tension development was found with an apparent association constant of about 2 X 10(2) M-1. The Ca2+ dependence of TnCDANZ fluorescence was similarly effected by Mg2+, either alone or when incorporated into TnC-depleted skinned fibers (K'Mg approximately equal to 2-3 X 10(2) M-1), suggesting that the effect of Mg2+ on activity is due to an effect of Mg2+ on Ca2+ binding to the Ca2+-specific sites of TnC. It is not yet clear whether this effect of Mg2+ is through direct competition at the binding sites or through indirect effects. In either case, the calculated effect of physiological Mg2+ is so small that the regulatory sites of TnC can still be considered "Ca2+-specific." In addition, a slightly greater effect of Mg2+ on tension development (K'Mg = 4.62 X 10(2) M-1) was observed only for very low levels of Mg2+, which might suggest an additional effect of Mg2+ on tension development which is saturated by millimolar Mg2+.