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Tacrolimus and sirolimus are potent immunosuppressors used in transplantation. Tacrolimus has been suspected to alter mitochondrial respiration of different tissues but sirolimus has not been evaluated.

We evaluated the in vitro effect of tacrolimus and sirolimus on oxidative phosphorylation of isolated rat kidney mitochondria.

Oxygen consumption was measured with a Clark‐type electrode. Tacrolimus and sirolimus increased the resting rate (state 4) and had no significant effect on ADP‐stimulated respiration (state 3). The decrease of respiratory control ratio was concentration‐dependent with a biphasic curve for tacrolimus. The EC₅₀s were 3.4×10⁻¹¹ M and 2.3×10⁻⁸ M for tacrolimus and 4.4×10⁻¹⁰ M for sirolimus. The maximal inhibition was 20 and 14% for tacrolimus and sirolimus, respectively.

Tacrolimus and sirolimus had an uncoupling effect on oxidative phosphorylation related to a decrease of the …