Calcium transients in isolated amphibian skeletal muscle fibres: detection with aequorin.

Abstract

Single twitch muscle fibers isolated from frogs and toads were microinjected with the Ca2+-sensitive bioluminescent protein aequorin. The fibers contracted normally and emitted flashes of light (aequorin responses) in reponse to stimulation for many hours thereafter. No luminescence was detected from healthy fibers at rest. Aequorin diffused from the injection site at a rate consistent with a diffusion coefficient of 5 .times. 10-8 cm2/s. During trains of isometric contractions there was a progressive reduction in amplitude and rate of decline of the aequorin response, an observation consistent with the theory that Ca was redistributed from sites of release to sites of sequestration under such circumstances. In isometric tetani light emission continued to rise long after the plateau of force was achieved. This and the fact that the amplitude of the tetanic aequorin response increased steeply with increasing stimulus frequency suggest that in tetani the sarcoplasmic Ca2+ concentration may normally be above the level required to saturate the contractile apparatus.