Contraction and relaxation in the absence of a sarcoplasmic reticulum: muscle fibres in the small pelagic tunicate Doliolum.

Previous ultrastructural observations suggested that Doliolum muscle fibres apparently lacked both sarcoplasmic reticulum and transverse tubular-system. External Ca2+ is required for contraction, caffeine does not evoke contraction, nor does it increase intracellular Ca2+ level. Ryanodine at 50 microM has no effect on electrically-evoked contractions. Further, electrical stimulation in external solutions lacking Na+ leads to sustained contracture. We conclude that intracellular Ca2+ stores are absent in these rapid obliquely-striated fibres, and that reduction in internal Ca2+ levels following contraction depends upon Na(+)-Ca2+ exchange across the sarcolemma.