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 [γ-³²P] ATP no such phosphorylation occurred in the presence of [γ-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 (ATPγS, 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.