Abstract 19344: Involvement of Mitochondrial Permeability Transition Pore (mPTP) in Cardiac Arrhythmias: Evidence From Cyclophilin D Knockout Mice

Abstract

Our previous studies have demonstrated that mitochondrial membrane potential ( ΔΨ m ) dissipation leads to mitochondrial Ca efflux via the mitochondrial permeability transition pore (mPTP), and consequently promotes spontaneous Ca waves (CaW) in isolated cardiomyocytes. In the present study, we have used a genetic mouse model which lacks cyclophilin D (CypD KO), a necessary regulator for mPTP opening, to assess the cardioprotective effect of mPTP inhibition on cellular CaWs, Ca alternans, and whole-heart inducible arrhythmias. Ventricular myocytes were isolated from wild type (WT) and CypD KO mice. Tetramethylrhodamine (TMRM) was used as an indicator of ΔΨ m. Mitochondrial calcein release was used as the index of mPTP opening. Cytosolic Ca was imaged using Fluo-4-AM. Spontaneous CaWs were induced in the presence of 4mM external Ca. The protonophore FCCP was used to depolarize Δ Ψ m . FCCP caused Δ Ψ m depolarization to the same extent in both WT and CypD KO myocytes, however, CypD KO cells exhibited significantly less mPTP opening than WT cells (p < 0.05). Consistent with these results, treatment with FCCP caused significant increases in WT CaW rate but no increase in CypD KO CaW rate. These results were further confirmed by treating WT cells with the mPTP blocker cyclosporin A (CsA). Furthermore, occurrence of Ca alternans after treatment with FCCP and programmed stimulation was observed in a significantly higher number of WT cells (11 of 13), than in WT cells treated with CsA (2 of 8; p < 0.05) or CypD KO cells (2 of 10; p < 0.01). At the whole-heart level, pseudo-Lead II ECGs were recorded from ex vivo, Langendorff-perfused WT and CypD KO hearts, and the incidences of T-wave alternans (a precursor of lethal arrhythimas) and arrhythmias induced by programmed S 1 -S 2 stimulation were compared. We observed T-wave alternans in 5 of 7 WT hearts, but none in 5 CypD KO hearts (p < 0.05) and in only 1 of 6 WT hearts treated with CsA (p <0.05). Consistent with these results, WT hearts exhibited a significantly higher average arrhythmia score than CypD KO (p <0.01) or WT hearts treated with CsA (p < 0.01). In conclusion, CypD deficiency-induced mPTP inhibition attenuates CaWs and Ca alternans during mitochondrial depolarization, and thereby protects against arrhythmogenesis in the heart.