The effect of hypertonic solutions on the rate of relaxation of contracture tension in Mytilus smooth muscle.

1. The effect of the bathing solution tonicity on the mechanical properties of the anterior byssal retractor muscle (ABRM) of Mytilus edulis was examined. 2. The rate of relaxation of contracture tension produced by acetylcholine (ACh) was greatly reduced when the bathing solution tonicity was increased by adding NaCl, KCl or LiCl after the removal of ACh, whereas a decreased tonicity increased the rate of relaxation. 3. The contracted ABRM in hypertonic solutions showed no active shortening after an isotonic release and barely redeveloped active tension after a quick release. 4. The resistance to stretch increased with increasing tonicity of the bathing solution. 5. The wet weight of the ABRM decreased much more markedly in sucrose-hypertonic solution than in Na-, K- or Li-hypertonic solutions, but the decrease in the rate of relaxation was less marked in the former, indicating that there may be little relation between the rate of relaxation and the degree of osmotic deformation of the ABRM fibres. 6. It is suggested that the elevated ionic strength in the myoplasm may be related to a reduction in the rate of detachment of the cross-linkages between the thick and thin filaments.