Dependence of Tension Development on Calcium and Magnesium Adenosinetriphosphates in Chemically Skinned Molluscan Smooth Muscle Fibers

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Abstract
The dependence of tension development on Ca2+ and Mg·ATP was studied in chemically skinned smooth muscle fibers of the anterior byssal retractor muscle (ABRM) of a common mussel, Mytilus edulis. The C2+ requirement for contraction of the fibers was quite similar to that of the skeletal muscle. However, the dependence of the tension development on Mg·ATP was very different from that of the skeletal muscle. The threshold concentration of Mg·ATP for contraction of the fibers was about 10−5 M, and maximum tension was observed at about 10−3 M. In the absence of Ca2+ (pCa>9) the preparations did not appreciably develop tension on addition of Mg·ATP. Similar experiments were performed using fiber preparations in which myosin was replaced with rabbit myosin. After this treatment, the preparations developed tension even in the absence of Ca2+ and the threshold concentration of Mg·ATP for contraction became lower than 10−7 M. Thus, the dependence of the tension development on Mg·ATP in the ABRM fibers was quite similar to that observed in muscles where the contraction is controlled by a myosin-linked regulatory system. However, fibers in which myosin had been replaced with rabbit myosin responded in a manner typical of an actin-linked regulatory system.