Calcium and the Control of Contraction and Relaxation in a Molluscan Catch Muscle

The anterior byssal retractor muscle (ABRM) is a molluscan smooth muscle that when appropriately stimulated can be made to perform two distinct types of contraction: (1) a phasic contraction that relaxes upon cessation of stimulation, and (2) a contraction known as "catch " that continues long after stimulation has ended. The latter type is marked by an energy output much smaller than that during phasic contraction (Nauss and Davies, 1969; Baguet and Gillis, 1968); during catch there are neither signs of active state as measured by quick-release xperiments (Jewell, 1959; Johnson and Twarog, 1960) nor electrical activity as measured by intracellular or extracellular experi-ments (Twarog, 1967a). Catch can be relaxed by neural stimulation or by relaxing agents, 5-hydroxytryptamine (5-HT) or dopamine. Either or both of these are probably mediators released by the catch-relaxing nerves (Muneoka and Twarog, 1972). Relaxing nerves are in no sense inhibitory; when stimulated they selectively "shut off " catch tension and increase active tension. Figure 1 shows the increased phasic response to repetitive stimulation following stimu-lation of the relaxing nerves. Figure 2 shows that active tension, in response to ACh, is increased even though relaxation is speeded (York and Twarog, 1972). The asterisk on the tension diagram of Fig. 3 designates the point at which catch begins and indicates the occurrence of an unknown process that controls the transition from active state to catch. The cross-bridges between the interacting proteins become fixed and they no longer turn over and use energy, yet tension levels remain high. The tension substrate has been a continuing source of dispu-tation: Lowy et al. (1964) and Nauss and Davies (1966) argue that catch tension is maintained by interaction between actin and myosin, whereas Johnson (1962), Rfiegg (1971), and Heumann and Zebe (1968) insist that the interaction is among paramyosin filaments and is independent of the actin-myosin contractile system. However the accumulation of evidence favoring the actin-myosin system (Szent-GySrgyi et al., 1971) holds promise of a conclusive nd to the argument in the near future. AND YOJIRO MUNEOKA