THE USE OF A23187 TO DEMONSTRATE THE ROLE OF INTRACELLULAR CALCIUM IN CAUSING ULTRASTRUCTURAL DAMAGE IN MAMMALIAN MUSCLE

Abstract

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 mm 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 places 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 them 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: 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 α-actinin from skeletal muscle (24). A Ca²⁺-activated protease with a pH optimum of 7.5 has also been