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

To give information about the catch state, i.e. a prolonged tonic contracti on maintained with very little energy expenditure, sinusoidal vibrations (p eak-to-peak amplitude, 0.5% of L-o, frequency, 10 Hz) and ramp releases (le ss than or equal to 2% of slack length, L-o, 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 wi th 10(-3) M acetylcholine (ACh), but remained unchanged during the catch st ate attained after removal of ACh and during the accelerated relaxation wit h 10(-6) M 5-hydroxytryptamine (5-HT). Muscle fiber stiffness, determined f rom the amplitude of the force response to sinusoidal vibration, approached a finite value (20-25% of the maximum value) as the isometric force decrea sed towards zero during the catch state. The force-extension curves of the series elastic component (SEC) during the catch state was steeper at low fo rce levels (less than or equal to 50% of the maximum isometric force) than those during the force development and the accelerated relaxation. These re sults can be taken as evidence for the load-bearing system specialized for the catch state. (C) 1999 Elsevier Science Inc. All rights reserved.