The muscular membrane and calcium activation of the contractile system of a lamellibranch smooth muscle (ABRM).

1. Thin bundles of fresh ABRM treated by EDTA solution or Triton x-100 0.1% can be brought like glycerol-extracted fibres through contraction-relaxation cycles by changing the free Ca2+ level of the bathing medium for 10-6 M to 10-9 M. The kinetics of isometric force development and relaxation has been studied in the two conditions i.e ionic strength 0.06 pH 6.5 and ionic strength 0.28, pH 7.0 which are known to induce the catch-state in glycerol extracted fibres. When the low Ca-2+ solution (10-9M) is substituted for the high Ca-2+ solution (10-6M) immediately after the maximal force has been developed, relaxation occurs at a higher rate at ionic strength 0928 and pH 6.5. In this last condition, no tension redevelops after a quick release applied during the slow relaxation. 2. During the plateau, in presence of Ca-2+ 10-6 M. a quick release applied at any time is followed by a large redevelopment of tension at ionic strength 0.06 and pH 6.5. At ionic strength 0.28 and pH 7.0, the tension redevelops only during the decreased from 10-6 to 10-9 M without any change in the time course of the tension maintained. 3. These results suggest that in EDTA and Triton X-100 treated fibres of Abrm, the catch-state is spontaneously induced after the activation of the contractile mechanism under the same conditions of ionic strengh and pH as in glycerol extracted fibres. However, in the EDTA and Triton treated fibres, the presence of a high free Ca-2+ level is not necessary to maintain the tension in the catch-state induced at ionic strength 0.28 and pH 7.0.