The adductor muscles of bivalve molluscs exhibit a prolonged tonic contraction with a very low rate of energy expenditure. This state is called the catch state, and a muscle exhibiting the catch state is called the catch muscle. Among the catch muscles, the anterior byssus retractor muscle (ABRM) of Mytilus has been used most frequently to study the catch state. The ABRM is innervated by both cholinergic and serotonergic nerves. Cholinergic nerve excitation produces an increase in intracellular [Ca2+] and active force development as with all other kinds of muscle. In the catch state, however, the tension decays extremely slowly despite a decrease in [Ca2+]i almost to its resting level. The catch tension is made to relax rapidly by serotonergic nerve activation to cause an increase in cAMP and activation of Protein kinase A (PKA). During the catch state, the ABRM neither shotens actively nor redevelops tension after a quick release, indicationg that the catch tension is only passively maintained. Two different hypotheses have been proposed to account for the catch state. One is the linkage hypothesis, in which the catch tension is associated with an extremely slow dissociation rate of actin-myosin linkages. The other is the parallel hypothesis, which assumes, in addition to the actin-myosin linkages, formation of some specialized structures to maintain the catch force. Recently, evidence has been accumulating that seems to favour the parallel hypothesis: (1) the catch tension is associated with phosphorylation of a high molecular protein, twitchin, (2) resistance of the catch tension against external load is too large to be explained by the actin-myosin linkages, and (3) electron microscopic studies indicate interconnections between the thick filaments during the catch state. It is of interest to examine possible role of twitchin in the formation of interconnections between the thick filaments.
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