Regulation of Ca2+-activated tension in limulus striated muscle.

Skinned tail and leg muscle fibers of the limulus were used to study the mechanism of Ca2+ regulation of contraction. Although a Ca2+-sensitive 31,000 dalton protein phosphorylation could be observed in the presence of [gamma-32P] ATP no such phosphorylation occurred in the presence of [gamma-32P] ITP. Ca2+-activated tension occurred equally as well in ATP and ITP. For this reason we eliminated the possibility that a Ca2+-sensitive myosin light chain kinase/phosphatase system is the mechanism responsible for the Ca2+-activated tension. Other agents known to affect a myosin light chain kinase/phosphatase system showed negative results (ATP gamma S, trifluoperazine, catalytic subunit of the cyclic adenosine 3',5'-monophosphate dependent protein kinase and calmodulin). Troponin I reversibly inhibits Ca2+-activated tension. These results are consistent with thin filament regulation being responsible for Ca2+-activated tension in skinned fibers.