CONTRACTION AND RELAXATION IN THE ABSENCE OF A SARCOPLASMIC-RETICULUM- MUSCLE-FIBERS IN THE SMALL PELAGIC TUNICATE DOLIOLUM

Previous ultrastructural observations suggested that Doliolum muscle f ibres apparently lacked both sarcoplasmic reticulum and transverse tub ular-system. External Ca2+ is required for contraction, caffeine does not evoke contraction, nor does it increase intracellular Ca2+ level. Ryanodine at 50 mu M has no effect on electrically-evoked contractions . Further, electrical stimulation in external solutions lacking Na+ le ads to sustained contracture. We conclude that intracellular Ca2+ stor es are absent in these rapid obliquely-striated fibres, and that reduc tion in internal Ca2+ levels following contraction depends upon Na+-Ca 2+ exchange across the sarcolemma.