Calcium ion accumulation and volume changes of isolated liver mitochondria. Calcium ion-induced swelling

1. Liver mitochondria suspended in an iso-osmotic buffered potassium chloride medium containing an oxidizable substrate and phosphate accumulated added Ca²⁺. During this process H⁺ appeared in the medium and the mitochondrial suspension showed increased light-scattering. Respiration was markedly stimulated. 2. The addition of excess of Ca²⁺, respiratory inhibitors or uncoupling agents caused extensive mitochondrial swelling associated with release of Ca²⁺ into the suspending medium. When the suspension became anaerobic extensive swelling also occurred. Only under conditions when the addition of uncoupling agents would have produced high rates of electron transport, e.g. in the presence of succinate, was the structural integrity of the mitochondrion maintained after Ca²⁺ accumulation. 3. Conditions that prevented respiration-dependent Ca²⁺ accumulation also prevented Ca²⁺-induced swelling. Bovine plasma albumin was without effect, indicating that U-factor was not involved. Oligomycin together with ADP or ATP partially stabilized the mitochondria against Ca²⁺-induced swelling. 4. It is suggested that a `high-energy' intermediate generated by coupled electron transport is required to prevent the mitochondrial swelling that results as a consequence of Ca²⁺ accumulation.