Lower mitochondrial calcium uptake capacity than calcium retention capacity in the presence and absence of cyclosporin A

Version 2

Abstract

Mitochondrial calcium homeostasis impacts on calcium-dependent cellular bioenergetics and health. Mitochondria contribute to the buffering of cytosolic calcium levels by uptake and release, or storage of osmotically inactive calcium phosphates in the mitochondrial matrix. Defective calcium homeostasis causes bioenergetic stress, cell death, and diseases. Since the interplay between Ca²⁺ dynamics and oxidative phosphorylation is well recognized, mitochondrial calcium retention capacity is frequently measured as the amount of calcium titrated up to a threshold, when calcium uptake is reversed and permeability transition triggers the release of calcium, disruption of ion balance, dyscoupling, and inhibition of respiration. This is accompanied by leakage into the cytosolic space of signaling molecules such as cytochrome c. We measured extramitochondrial [Ca²⁺] using Calcium Green™-5N (CaG) simultaneously with high-resolution respirometry. The respiration medium MiRCa containing CaG did not impair respiration of isolated mouse liver mitochondria. Ca²⁺ uptake was incomplete as shown by the merely partial decline of [Ca²⁺] in the medium after initial [Ca²⁺] titrations and final titrations prior to permeability transition. Consequently, Ca²⁺ uptake capacity, corrected for the increase of [Ca²⁺] in the medium, was 45 % of the conventionally defined Ca²⁺ retention capacity in controls and 55 % in the presence of cyclosporin A. Our results suggest that Ca²⁺ retention capacities depend on experimental mitochondrial concentration, require critical evaluation, and should be replaced by Ca²⁺ uptake capacities properly corrected for increased extramitochondrial calcium concentrations.

Cite:

Cecatto C, Cardoso LHD, Grings M, Gnaiger E (2026) Lower mitochondrial calcium uptake capacity than calcium retention capacity in the presence and absence of cyclosporin A. BEC preprints 2026.3.  https://doi.org/10.26124/becprep.2025-0007.v2