Effects of vanadate, phosphate and 2,3-butanedione monoxime (BDM) on skinned molluscan catch muscle

The effects of orthovanadate (Vi), inorganic phosphate (Pi) and 2,3-butanedione monoxime (BDM) on tension, force transients and the catch state (passive tension maintenance) were investigated in saponin-skinned fibre bundles of the anterior byssus retractor muscle (ABRM) of the bivalve mollusc Mytilus edulis at pH 6.7. During maximal Ca2+ activation isometric force was depressed by Vi (0.03-10 mM), Pi (10 mM) and BDM (50 mM). Force transients following quick stretches (0.1-0.3% of fibre length) were accelerated substantially by 1 mM Vi, 10 mM Pi or 50 mM BDM. These compounds also accelerated force responses in experiments in which ATP was released rapidly from caged ATP by flash photolysis at both pCa 4.7 (force rise) and at pCa > 8 (force decline). The effects on the catch state were investigated in two types of experiments: (1) Ca2+ removal after maximal Ca2+ activation and (2) rapid ATP release during high-force rigor at pCa > 8. In both cases rapid relaxation was followed by slow relaxation (slower than 2% of initial force per min). This later slow relaxation (catch) was insensitive to Vi (1-10 mM), Pi (10 mM) and BDM (50 mM) but was accelerated by 0.12 mM cAMP. Complete relaxation to almost zero force was attained by changing pH from 6.7 to 7.7 (pCa > 8). We conclude that catch depends on cAMP- and pH-sensitive structures linking the myofilaments and not on the force-generating actomyosin cross-bridges that are sensitive to Vi, Pi and BDM.