The highly efficient holding function of the mollusc 'catch' muscle is not based on decelerated myosin head cross-bridge cycles

Certain smooth muscles are able to reduce energy consumption greatly when holding without shortening. For instance, this is the case with muscles surrounding blood vessels used for regulating blood flow and pressure. The phenomenon is most conspicuous in 'catch' muscles of molluscs, which have been used as models for investigating this important physiological property of smooth muscle. When the shells of mussels are held closed, the responsible muscles enter the highly energy-efficient state of catch. According to the traditional view, the state of catch is caused by the slowing down of the force-generating cycles of the molecular motors, the myosin heads. Here, we show that catch can still be induced and maintained when the myosin heads are prevented from generating force. This new evidence proves that the long-held explanation of the state of catch being due to the slowing down of force producing myosin head cycles is not valid and that the highly economic holding state is caused by the formation of a rigid network of inter-myofilament connections based on passive molecular structures.