Influence of intracellular pH on mitochondrial calcium during ischaemia of the isolated rat heart

Summary Under physiological conditions cardiac mitochondria seem to play a minor role in maintaining intracellular Ca²⁺ homoeostasis. However, under conditions of cellular Ca²⁺ overload, mitochondria may accumulate large amounts of Ca²⁺. Using transmission and analytical electron microscopy we investigated, in globally ischaemic rat heart preparations, the influence of intracellular pH on the development of Ca²⁺-containing intramitochondrial inclusions. We confirmed that under these experimental conditions Ca²⁺ was a major element of mitochondrial inclusions. The size of these inclusions increased with external Ca²⁺ concentration. An intracellular alkalinization, produced by addition of 20mm NH₄Cl to the perfusate prior to ischaemia, inhibited the formation of such inclusions. On the other hand, a pre-ischaemic intracellular acidification, produced by the addition and subsequent withdrawal of the 20mm NH₄Cl, increased the number of inclusions present at the end of an ischaemic episode. The presence of amiloride (10^–3 m), prior to and during ischaemia, increased the number of inclusions. These data suggest that cytoplasmic pH may be an important factor in mitochondrial Ca²⁺ accumulation in pathological conditions.