When the anterior byssal retractor muscle (ABRM) of Mytilus edulis was subjected to a sudden reduction in temperature during a contractionrelaxation cycle, a tension increment, the cold-induced contracture (CIC), was observed. The CIC could be obtained in a muscle stimulated by the application of acetylcholine (ACh), caffeine or high-K+ solutions, but could not be elicited from a muscle at rest or in ‘catch’. When the muscle was released from ‘catch’ by the application of 5-hydroxytryptamine (5-HT), a CIC could once again be produced. The CIC tension elicited after the application of ACh was shown to follow a log-dose relationship with respect to ACh concentration but no CIC was observed at an ACh concentration at which the initial tension was 10% or less of maximum. The CIC tension decreased with time after the application of ACh and appeared to be dependent on both the initial and final temperatures and on the size of the temperature drop. The significance of these results and possible explanations are considered.

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