Factors influencing contraction and catch in Mytilus smooth muscle.

1. Conditions are defined which determine the level of catch after acetylcholine stimulation of Mytilus muscle.2. Catch tension in dissected muscle is absent when connexions with ganglia are intact.3. Catch tension is absent at temperatures above 30 degrees C.4. Catch tension decreases when intervals between stimuli are increased.5. Increasing concentrations of 5-hydroxytryptamine (5-HT) from 10(-8)M to 10(-6)M quantitatively decreases catch tension.6. The length-tension curve of ganglion-free Mytilus muscle bundles suggests that catch tension varies in proportion to the tension developed in contraction.7. External Ca(2+) concentration has no selective influence on catch.8. All factors which reduce catch also increase muscle excitability, suggesting that catch may depend on a mechanism controlling the intracellular concentration of an activator such as Ca(2+).