The effect of supra-physiological Ca²⁺ concentration on the activity of Ca²⁺- sensitive tricarboxylic cycle dehydrogenases was investigated with the aim to determine if a very fast decrease in mitochondrial NAD(P)H content induced by Ca²⁺ overload may be caused by changes in activity of these dehydrogenases. We showed that pyruvate and 2-oxoglutarate dehydrogenases in heart mitochondria were stimulated by Ca²⁺ in the physiological range of concentration but were not affected by Ca²⁺ overload. In complete contrast, Ca²⁺ affected isocitrate dehydrogenase (ICDH) only in the supra-physiological range of concentration. The effect of Ca²⁺ on ICDH depended on isoform and substrate concentration. We showed that in heart mitochondria NADP-ICDH could be from 100 to 350-fold more active than NAD-ICDH. The ratio of their activities decreased with an increase in Ca²⁺ concentration, because NADP-ICDH was inhibited by Ca²⁺, while NAD-ICDH was activated. We concluded that among the key mitochondrial dehydrogenases only ICDH isoforms might potentially contribute to Ca²⁺-overload-induced changes in NAD(P)H concentration.

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