Hyperpolarized [1-C-13]Acetyl-l-Carnitine Probes Tricarboxylic Acid Cycle Activity In Vivo

Mitochondrialoxidative phosphorylation (OXPHOS) is sensitive toa variety of biological factors, and dysregulated OXPHOS is observedduring the development of numerous pathological conditions. ATP productionvia OXPHOS is intrinsically dependent on the availability of acetyl-coenzymeA (CoA), which can enter the tricarboxylic acid (TCA) cycle to drivethe oxidative pathway. Acetyl-l-carnitine (ALCAR) is an interchangeableendogenous source of acetyl-CoA, and therefore, ALCAR-derived probesare uniquely positioned for the assessment of OXPHOS. In this report,we develop hyperpolarized (HP) [1-C-13]ALCAR as a noninvasiveprobe to investigate cardiac TCA cycle activity in vivo. We initially synthesized the isotopically labeled substrate anddemonstrated that the C-13 nucleus maintained a suitable T (1) value (50.1 & PLUSMN; 0.8 s at 3 T) and polarizationlevels (21.3 & PLUSMN; 5.3%) to execute in vivo metabolicmeasurements. HP [1-C-13]ALCAR was employed for cardiacanalyses of OXPHOS in rats under fed and fasted conditions. [5-C-13]Glutamate was successfully detected, and the metabolitewas used to analyze the TCA cycle activity in both nutritional states.These assessments were compared to analogous experiments with theHP [1-C-13]pyruvate. Our report represents the first studyto demonstrate that HP methods using [1-C-13]ALCAR enabledirect analyses of mitochondrial function and TCA cycle activity,which are fundamental to cardiac cell homeostasis.

Automated summary

Mitochondrial oxidative phosphorylation (OXPHOS) is sensitive to various biological factors and dysregulation of OXPHOS is observed in the development of many pathological conditions. Acetyl-l-carnitine (ALCAR) is an endogenous source of acetyl-coenzyme A (CoA) and can be used to assess OXPHOS. In this study, hyperpolarized (HP) [1-C-13]ALCAR was developed as a noninvasive probe for investigating cardiac tricarboxylic acid (TCA) cycle activity in vivo. This study demonstrates the first use of HP methods with [1-C-13]ALCAR to analyze mitochondrial function and TCA cycle activity in the heart.