Activation of locomotor and grasping spine muscle fibres in chaetognaths: a curious paradox

Chaetognath muscle fibres resemble vertebrate muscle fibres in having an ab undant sarcoplasmic reticulum (SR) and analogues of the transverse (T) tubu lar system, but contraction is regulated differently. In intact chaetognath s electrically-evoked contractions of the striated locomotor muscles were l argely or totally blocked by d-tubocurarine, by surgical removal of the ven tral ganglion and by Co2+. Contractions of single cells enzymatically disso ciated from locomotor muscles were likewise blocked by Co2+, they twitched once only after calciseptine, showed neither contractures nor elevated intr acellular Ca2+ with caffeine, and ryanodine did not block contractions. Who le cell voltage-clamped locomotor muscle cells displayed a typical inward r ectified Ca2+ current that was sensitive to the Ca2+ channel blockers nifed ipine and calciseptine and showed voltage-dependent activation with a thres hold at similar to-25 mV and a peak inward current at similar to + 10 mV. I n contrast, whole cell voltage-clamped cells from the muscles operating the grasping spines of the head showed an initial very rapid and rapidly-inact ivating inward current abolished by tetrodotoxin (TTX), followed by a slowe r and slowly-inactivating inward current blocked by calciseptine. The relat ion between these observations and the unusual 'vertebrate-like' structure of the muscle cells is discussed.