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

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 20 mM 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 20 mM 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.