Calcitriol deficiency reduces myocardial ATP production with disturbed mitochondrial dynamics and increased uncoupling protein 2 expression

Vitamin D deficiency and myocardial ATP depletion are associated with heart failure. Although vitamin D may regulate mitochondrial function, its effect on cardiac ATP synthesis remains unclear. This study investigated whether calcitriol deficiency directly modulates ATP production in cardiomyocytes and explored the underlying mechanisms. Seahorse Extracellular Flux Analyzer, PCR, enzyme activity microplate assay, fluorescence staining, and immunoblotting were used to study mitochondrial bioenergetics, mitochondrial DNA copy number, citrate synthase activity, mitochondrial dynamics and morphology, mitochondrial membrane potential, and protein expressions of uncoupling protein 2 (UCP2) and Akt in HL-1 cardiomyocytes without (calcitriol deficiency) and with calcitriol (0.1 nM for 24 h). Calcitriol-deficient cardiomyocytes exhibited lower ATP production and suppressed mitochondrial respiration than calcitriol-treated cells. Calcitriol-deficient cells had fewer mitochondrial DNA copies, diminished citrate synthase activity, smaller and fragmented mitochondria, a more positive mitochondrial membrane potential as well as lower expression levels of mitofusin 1, optic atrophy 1, and phosphorylated Akt proteins, alongside higher expression levels of mitochondrial fission 1 and UCP2 proteins than calcitriol-treated cells. Akt inhibition significantly attenuated the effects of calcitriol on increasing cardiac ATP production, enhancing mitochondrial respiration, upregulating mitofusin 1 and optic atrophy 1, and downregulating mitochondrial fission 1. In conclusion, calcitriol deficiency suppresses Akt activation and subsequently alters mitochondrial dynamics. Moreover, it upregulates UCP2 expression. These effects reduce ATP production in cardiomyocytes, thus increasing the risk of heart failure.