Cyclosporin A Increases Mitochondrial Buffering of Calcium: An Additional Mechanism in Delaying Mitochondrial Permeability Transition Pore Opening.

Regulation of mitochondrial free Ca is critically important for cellular homeostasis. An increase in mitochondrial matrix free Ca concentration ([Ca]) predisposes mitochondria to opening of the permeability transition pore (mPTP). Opening of the pore can be delayed by cyclosporin A (CsA), possibly by inhibiting cyclophilin D (Cyp D), a key regulator of mPTP. Here, we report on a novel mechanism by which CsA delays mPTP opening by enhanced sequestration of matrix free Ca. Cardiac-isolated mitochondria were challenged with repetitive CaCl boluses under Na-free buffer conditions with and without CsA. CsA significantly delayed mPTP opening primarily by promoting matrix Ca sequestration, leading to sustained basal [Ca] levels for an extended period. The preservation of basal [Ca] during the CaCl pulse challenge was associated with normalized NADH, matrix pH (pH), and mitochondrial membrane potential (ΔΨ). Notably, we found that in PO (P)-free buffer condition, the CsA-mediated buffering of [Ca] was abrogated, and mitochondrial bioenergetics variables were concurrently compromised. In the presence of CsA, addition of P just before pore opening in the P-depleted condition reinstated the Ca buffering system and rescued mitochondria from mPTP opening. This study shows that CsA promotes P-dependent mitochondrial Ca sequestration to delay mPTP opening and, concomitantly, maintains mitochondrial function.