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.
Bone
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Marine Laboratory, Citadel Hill, Plymouth, UK.
Inoue
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Tsutsui
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D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009126:Muscle Relaxation / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D012433:Ryanodine / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000502:physiology; D012964:Sodium / Q000502:physiology; D014561:Urochordata / Q000502:physiology*
D012433:Ryanodine; D003035:Cobalt; D002110:Caffeine; D012964:Sodium; D009543:Nifedipine; D002118:Calcium