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.
Galler
S
S
Institut für Zoologie, Salzburg, Austria. Stefan.Galler@sbg.ac.at
Kögler
H
H
Ivemeyer
M
M
Rüegg
J C
JC
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism; D000818:Animals; D049872:Bivalvia / Q000502:physiology*; D007537:Isometric Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology; D009132:Muscles / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology* / Q000528:radiation effects; D010782:Photolysis
D000244:Adenosine Diphosphate; C029273:P(3)-1-(2-nitro)phenylethyladenosine 5'-triphosphate; D000255:Adenosine Triphosphate