Regulatory mechanism of contraction in the proboscis retractor muscle of a sipunculid worm, Phascolosoma scolops.

Regulatory mechanism of contraction in the proboscis retractor muscle of Phascolosoma scolops was studied by physiological measurements and cytochemical electron microscopy. The magnitude of K+-contracture was dependent on external Ca2+ concentration and the contracture disappeared in Ca2+-free solution. The K+-contracture was suppressed by application of procaine and Mn2+. Caffeine induced contracture even when external Ca2+ was absent. Ultrastructural observations of the retractor muscle cells showed the presence of a large number of vesicles (subsarcolemmal vesicles), corresponding to the sarcoplasmic reticulum in vertebrate skeletal muscle, underneath the plasma membrane. For the cytochemical electron microscopy, the muscle fibers were fixed with 1% OsO4 solution containing 2% K-pyroantimonate. In the relaxed fibers, pyroantimonate precipitates were localized along the inner surface of plasma membrane and in the subsarcolemmal vesicles. In the contracting fibers, the precipitates were uniformly distributed in the myoplasm. The X-ray microanalysis revealed that the precipitates contained Ca. These results suggest that the contractile system is activated by the influx of extracellular Ca2+ as well as by the release of Ca2+ from the intracellular structures such as the inner surface of the plasma membrane and subsarcolemmal vesicles.