Localisation of intracellular calcium stores in the striated muscles of the jellyfish Polyorchis penicillatus: possible involvement in excitation-contraction coupling

SUMMARY When jellyfish striated muscles were stimulated directly, the amplitude of contractile tension increased as the stimulation frequency increased. Application of 10 mmol l –1 caffeine reduced the amplitude of contractile tension and abolished this facilitatory relationship, indicating that calcium stores participate in excitation–contraction coupling. Calcium stores were identified ultrastructurally using enzymatic histochemistry to localize CaATPases, and potassium dichromate to precipitate calcium. Electron energy-loss spectroscopy was used to verify the presence of calcium in precipitates. Both CaATPase and calcium were localised in membrane-bound vesicles beneath the sarcolemma. We concluded that sub-sarcolemmal vesicles could act as calcium stores and participate in excitation–contraction coupling.