The use of A23187 to demonstrate the role of intracellular calcium in causing ultrastructural damage in mammalian muscle.

Electron micrographs show that treatment of mouse diaphragm with the divalent cation ionophore A23187 causes major ultrastructural damage in the muscle. During the first 30 min of exposure to A23187, the mitochondria swell markedly but after 40 min they undergo further ultrastructural changes and a rapid dissolution of the myofilaments is also seen at this time. In place only remnants of the filaments remain. It is suggested that the ionophore causes the release of Ca2+ from the sarcoplasmic reticulum which, initially, is taken up by the mitochondria, causing then to swell; after 40 min the mitochondria release the accumulated Ca2+. It is argued that the rise in [ca2+]i stimulates neutral proteases in the myoplasm and that the sequence of events following ionophore treatment may act as a model for the involvement of Ca2+ in various myopathies. We have shown previously (29) that treatment of the cutaneous pectoris muscle of the frog with the divalent cation ionophore A23187 has three major effects: (i) The membrane potential (Em) is depolarized, an action that is found only when the Ca2+ -concentration of the bathing saline is very low. (ii) It causes an increase in resting tension and the development of contraction. This effect is found at both normal (1.8 mM) and low values of [Ca2+]o and is, therefore, independent of Ca2+ entry and of changes in Em. The ionophore is believed to act primarily by releasing Ca2+ from intracellular stores. (iii) It causes major ultrastructural damage to the muscle filaments. The evidence suggests that A23187 acts at the sarcoplasmic reticulum of frog muscle, causing the release of stored Ca2+ , and the consequent rise in [Ca2+]i stimulates a Ca2+ -activated protease which is responsible for the myofilament degradation. A calcium-activated factor has been isolated from rabbit skeletal muscle; this enzyme operates at neutral pH, hydrolyses denatured casein, and specifically removes the Z-lines and alpha-actinin from skeletal muscle (24). A Ca2+ -activated protease with a pH optimum of 7.5 has also been purified from porcine skeletal muscle; it removes Z-discs, degrades troponin and tropomyosin and partly degrades M lines, and it has been suggested that it may have a physiological role in the disassembly of intact myofibrils during the metabolic turnover of myofibrillar proteins (6, 7). This protases is not localized in membrane-bounded sub-cellular particles, but is believed to be in direct contact with the cytoplasm (26). These findings are of particular interest in thelight of recent suggestions that the cellular necrosis observed in various muscle diseases is a consequence of an increased net influx of calcium into cells which causes, in turn, calcium overloading of the muscle mitochondria (34, 35). We have, therefore, extended our previous studies with frog muscle by the examination of the action of A23187 on the mammalian skeletal muscle of mouse diaphragm...