5-HT effect and the control of the relaxation of a molluscan smooth muscle (ABRM) of Mytilus edulis L.

Morphological differences exist between the arrangement of myofilaments in relaxed EDTA-treated fibres (induced by a 10(-9)M Ca2+ solution, ionic strength 0.28 at pH7.0) and the fibres relaxed by 5-HT added into the same relaxing solution. Complemented by a tension recording of an ABRM preparation, we find the following characteristics during 5-HT relaxation: (i) The distance between centers of thick neighboring filaments is increased. (ii) The thin filaments appear loosely dispersed between the thick ones. These observations suggest that decrease in myofilament interactions is responsible for the plasticizing effects of 5-HT. The sarcolemma appears with some granularity after 5-HT treatment; nexuses and dyads, however, are well defined.