Ca-induced Ca release from the sarcoplasmic reticulum of isolated myofibrillar bundles of barnacle muscle fibres

Isometric tension and aequorin light were recorded from isolated myofibrillar bundles (diameter 0.2 mm) of barnacle muscle fibres to examine Ca release from the sareoplasmic reticulum (SR). Transfer of a bundle from a pCa 6.7 solution to a pCa 5.8 solution, both buffered with 0.1mM EGTA, resulted in a phasic increase in myofibrillar free Ca²⁺ which was superimposed on a slow rise to a steady level and a fast rise in tension. The peak of the free Ca²⁺ response was higher than the free Ca²⁺ in the bulk solution. Treatment of the bundle with the detergent Brij to destroy the SR membranes abolished the phasic rise in Ca²⁺ and considerably reduced the amplitude of contraction. A second challenge of a bundle to the pCa 5.8 solution without prior reloading of the SR Ca store gave a much reduced phasic component. When a pCa 5.8 solution with 1.0 mM EGTA buffering was used, the phasic rise in myofibrillar free Ca²⁺ could not be detected and the rise in tension was four times slower than with 0.1 mM EGTA. The results are consistent with the operation of a Ca-induced Ca release mechanism in the SR membrane of this crustacean muscle.