The effect of the ionophore A23187 on the ultrastructure and electrophysiological properties of frog skeletal muscle

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Abstract
The divalent cation ionophore A23187 has three major effects on the thin cutaneous pectoris muscle of frog: (1) The membrane potential is depolarized, an action that is found only when the [Ca2+] of the bathing saline is very low. (2) It causes an increase in resting tension and the development of contraction. This action is produced at both normal 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. (3) It causes major ultrastructural damage to the muscle filaments. It is believed that this damage is the result of the action of A23187 on the sarcoplasmic reticulum and the elevation of [Ca2+]i and we suggest that the action of this ionophore may serve as a useful model for the study of certain myopathies.