REGULATION OF CATCH MUSCLE BY TWITCHIN PHOSPHORYLATION - EFFECTS ON FORCE, ATPASE, AND SHORTENING

Recent experiments on permeabilized anterior byssus retractor muscle ( ABRM) of Mytilus edulis have shown that phosphorylation of twitchin re leases catch force at pCa > 8 and decreases force at suprabasal but su bmaximum [Ca2+]. Twitchin phosphorylation decreases force with no dete ctable change in ATPase activity, and thus increases the energy cost o f force maintenance at subsaturating [Ca2+]. Similarly, twitchin phosp horylation causes no change in unloaded shortening velocity (Vo) at an y [Ca2+], but when compared at equal submaximum forces there is a high er Vo when twitchin is phosphorylated. During calcium activation, the force-maintaining structure controlled by twitchin phosphorylation adj usts to a 30% Lo release to maintain force at the shorter length. The data: suggest that during both catch and calcium-mediated submaximum c ontractions, twitchin phosphorylation removes a structure that maintai ns force with.. very low ATPase, but which can slowly cycle during sub maximum calcium activation. A quantitative cross-bridge model of catch is presented that is based on modifications of the Hai and Murphy (19 88. Am. J. Physiol. 254:C99-C106) latch bridge model for regulation of mammalian smooth muscle.