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+.
D000818:Animals; D001665:Binding Sites; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D003619:Dansyl Compounds; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects*; D011817:Rabbits; D013050:Spectrometry, Fluorescence; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
D003619:Dansyl Compounds; D014336:Troponin; D019209:Troponin C; D000252:Calcium-Transporting ATPases; D008274:Magnesium