Recovery of ultrastructural changes accompanying caffeine contractures in isolated muscle fibres of the crayfish.

Reversible caffeine contractures in isolated muscle fibres of the crayfish (Astacus fluviatilis; m. extensor carpopoditi) are accompanied by swelling of the diadic cisternae of the sarcoplasmic reticulum and by constriction of the T-system tubules. The membranes of the tubules may coalesce and form pentalaminar structures. These changes in the diadic region can be found already in fibres fixed 10 s after the sudden introduction of 4 mM or 6 mM caffeine solution. The changes are completely reversible. The recovery period after a 6 mM caffeine contracture lasts 20 min. The restitution of ultrastructural changes induced by caffeine correlates well with the recovery of responsiveness to a repeated application of caffeine. The pretreatment of fibres with procaine prevents both the generation of caffeine contractures and the occurrence of changes in the ultrastructure. No changes or a very slight enlargement of diadic cisternae were found during potassium contractures of comparable size and duration to caffeine contractures. The effect of procaine as well as the restitution of ultrastructural changes after the recovery of responsiveness to caffeine support the view that the cisternal swelling is a direct consequence of the calcium releasing action of caffeine. Isolated swelling of diadic cisternae and the absence of any changes in the remaining parts of the sarcoplasmic reticulum during reversible caffeine contractures could point to the existence of functional differentiation of the respective parts of the sarcoplasmic reticulum. The absence of ultrastructural changes during potassium contractures might indicate a different source of the activating calcium for contraction and/or a different mechanism of its release.