ABSTRACT
In terms of physics, holding a load without displacement should not require energy consumption. However, our muscles usually use energy for holding functions as this allows them to remain ready for movement. In contrast, certain specialized muscles are able to reduce energy consumption greatly when only holding but not movement is required. The saving of energy is most distinct in the so-called “catch” muscles of mollusks. For instance, when the shells of mussels are held closed, the responsible muscles enter in the “catch” state with very high holding efficiency. In experiments with isolated catch muscles of the mussel Mytilus, the neurotransmitter acetylcholine causes contraction; removal of acetylcholine is followed by only a slow or even no relaxation. This state of slowly decaying force in the absence of stimulation represents the catch state. Rapid relaxation and, thus, the termination of the catch state are attained by serotonin. Force and
even more the resistance to stretch are high in the catch state, but the consumption of ATP and the cytosolic free Ca2+concentration are low as at resting condition. The catch state resembles the rigor state; however, unlike the rigor state the ATP concentration is high. Despite of more than one century of research the mechanism of catch is still unclear. The knowledge about molecular aspects has increased during the last decades; however, a convincing theory which satisfies the findings of various experimental approaches is still missing. This chapter gives a basic overview about the current state of research. 14.1 BackgroundCatch muscles have already attracted attention more than a century ago when the closing mechanism of the shells of mussels were investigated. The muscles responsible for permanent closing exert a great deal of pressure on a piece of wood inserted between the shells, but they are unable to shorten and close the shells when the piece of wood is removed (Grützner, 1904; von Uexküll, 1912). Later, the anterior byssus retractor muscle (ABRM) of the mussel Mytilus became a popular preparation for studying the phenomenon of catch. Measurements of pulling force of isolated ABRMs showed that force decays only slowly or not at all after removing of the excitatory neurotransmitter acetylcholine from the bath solution. The remaining force (catch force) is rapidly relaxed after application of serotonin (Twarog, 1954; Fig. 14.1a). Neuronal activity is almost absent during the catch state (Flechter, 1937) and oxygen consumption is very low (Brecht et al., 1955; Baguet and Gillis, 1968). The catch state resembles the rigor state; however, unlike the rigor state the ATP concentration is high. Thus, the ATP level and the ATP/ADP ratio do not differ in catch and in other contractile states (Rüegg and Strassner, 1963, Rüegg, 1965). Direct measurements of intracellular Ca2+ showed that the free cytosolic Ca2+ concentration is at resting levels during catch (Ishii et al., 1989).A catch-like state was also observed in skinned preparations of catch muscles (permeable cell membranes; Fig. 14.1b). Removal of Ca2+ after Ca2+ induced contractions is followed by incomplete
relaxation. The remaining force decays only very slowly; addition of cAMP or the catalytic unit of protein kinase A (PKA) induces fast relaxation (Achazi et al., 1974; Cornelius, 1982; Pfitzer and Rüegg, 1982).
