Force responses of skinned molluscan catch muscle following photoliberation of ATP.

Isometric force responses following flash photolysis of caged-ATP were measured from skinned preparations of the catch muscle anterior byssus retractor of Mytilus (ABRM). When fibres were transferred from Ca(2+)-free to Ca(2+)-containing rigor solution (pCa < 4) the force remained low, but flash photolysis produced an extended force increase (half-time, 0.30 +/- 0.07 s, n = 6). When Ca(2+)-activated fibres were transferred to a Ca(2+)-free rigor solution, their force remained at a high level. Flash photolysis produced a rapid force decay (half-time, 0.28 +/- 0.06 s, n = 9) to about 19% of the initial Ca(2+)-activated force. In the presence of 0.5 mM MgADP, the force increase was slowed down by a factor of 3 and the force decay by a factor of 5. These effects of MgADP on crossbridge kinetics are comparable to those observed in vertebrate smooth muscle and are thought to cause "latch", a catch-like state (Fuglsang et al. J Muscle Res Cell Motil 14:666-677, 1993). They are consistent with a model implicating competition between MgADP and MgATP for the nucleotide-binding site on crossbridges. Considering the relatively fast force responses induced by caged-ATP photolysis, even in the presence of MgADP, it appears unlikely that the detachment of crossbridges from the rigor state can account for catch-related processes. In view of the low myosin ATPase under maximal activating conditions (0.6 s-1, Butler et al. Biophys J 75:1904-1914, 1998), neither crossbridge attachment nor detachment of rigor crossbridges seems to be the rate-limiting processes of the crossbridge cycle.