Abstract Body: Background
Metabolic flexibility is decreased during cardiac stress. In heart failure, mitochondrial pyruvate carrier (MPC) expression is reduced, limiting glucose oxidation. Cardiomyocyte-specific MPC loss drives heart failure in mice; however, glucose derived carbons continue to enter the TCA cycle independent of MPC, suggesting an alternative transporter. Glucose derived lactate has been proposed to enter the mitochondria through monocarboxylate transporter 1 (MCT1), though its mitochondrial localization remains controversial.
Methods & Results
To determine whether MCT1 supports MPC-independent oxidative metabolism in vivo, we performed [U-¹³C]glucose tracing in MPC inducible cardiomyocyte knockout (iCKO) mice with or without pharmacologic MCT1 inhibition (AZD3965). Consistent with prior observations, MPC1^iCKO hearts maintained fractional labeling of TCA intermediates. However, combined MPC1 deletion and MCT1 inhibition significantly reduced TCA intermediate labeling while preserving pyruvate and lactate enrichment. We next tested MCT1-dependent lactate oxidation by Langendorff perfusion. MCT1^iCKO hearts exhibited marked reduction of TCA intermediate labeling from [U-¹³C]lactate, despite preserved lactate and pyruvate enrichment. MCT1^iCKO hearts increased TCA labeling from [U-¹³C]glucose, consistent with compensatory substrate utilization. To directly assess mitochondrial lactate import, we generated MCT1^iCKO mice expressing a mitochondrial tag for rapid isolation. During perfusion with [2-D]lactate, which loses its label upon conversion to pyruvate, mitochondrial M+1 lactate was enriched only in the control, supporting MCT1-dependent mitochondrial lactate uptake. Lactate dehydrogenase (LDH) activity was detected in isolated mitochondria (heart, liver, and kidney) by NAD⁺ to NADH fluorescence, increased with membrane permeabilization (alamethicin), preserved after pretreatment of proteinase K, and inhibited by oxamate. Under neurohormonal stress (phenylephrine and angiotensin II), MCT1^iCKO mice developed greater hypertrophy, progressed more rapidly to reduced ejection fraction, and had reduced survival. Mitochondrial MCT1 expression was increased in human failing hearts compared to donor controls, a potential compensatory metabolic adaptation to reduced MPC.
Conclusions
Mitochondrial MCT1 enables direct lactate utilization and provides functional redundancy for glucose and lactate oxidation, supporting metabolic flexibility and adaptation to cardiac stress.