Evidence for load-bearing structures specialized for the catch state in Mytilus smooth muscle

Abstract

To give information about the catch state, i.e. a prolonged tonic contraction maintained with very little energy expenditure, sinusoidal vibrations (peak-to-peak amplitude, 0.5% of L 0, frequency, 10 Hz) and ramp releases (≤2% of slack length, L 0, complete in 4 ms) were applied to the anterior byssal retractor muscle (ABRM) fibers of a bivalve mollusc Mytilus edulis in various states. The mean force level of force responses to sinusoidal vibration decreases with time during the force development with 10 −3 M acetylcholine (ACh), but remained unchanged during the catch state attained after removal of ACh and during the accelerated relaxation with 10 −6 M 5-hydroxytryptamine (5-HT). Muscle fiber stiffness, determined from the amplitude of the force response to sinusoidal vibration, approached a finite value (20–25% of the maximum value) as the isometric force decreased towards zero during the catch state. The force–extension curves of the series elastic component (SEC) during the catch state was steeper at low force levels (≤50% of the maximum isometric force) than those during the force development and the accelerated relaxation. These results can be taken as evidence for the load-bearing system specialized for the catch state.