Postcontraction [acetylcarnitine] reflects interindividual variation in skeletal muscle ATP production patterns in vivo

Skeletal muscle inosine monophosphate formation preserves ΔGATP during incremental step contractions in vivo

The cause and consequences of inosine monophosphate (IMP) formation when adenosine triphosphate (ATP) declines during muscular contractions in vivo are not fully understood. The purpose of this study was to examine the role of IMP formation in the maintenance of the Gibbs free energy for ATP hydrolysis (ΔGATP) during dynamic contractions of increasing workload and the implications of ATP loss in vivo. Eight males (median 27.5, 25-35 yr range) completed an 8-min incremental protocol [2-min stages of isotonic knee extensions (0.5 Hz)] in a 3-T magnetic resonance (MR) system. Phosphorus MR spectra were obtained from the knee extensor muscles at rest and during contractions and recovery. Although the ATP demand during contractions was met primarily by oxidative phosphorylation, [ATP] decreased from 8.2 mM to 7.5 (range 6.4-8.0) mM and [IMP] increased from 0 mM to 0.6 (0.1-1.7) mM. Modeling showed that, in the absence of IMP formation, excess adenosine diphosphate (ADP) would result in a less favorable ΔGATP (P < 0.001). Neither [ATP] nor [IMP] had returned to baseline following 10 min of recovery (P < 0.001). Notably, Δ[ATP] was linearly related to the post-contraction reduction in muscle oxidative capacity (r = 0.74, P = 0.037). Our results highlight the importance of IMP formation in preserving cellular energy status by avoiding increases in ADP above that necessary to stimulate energy production pathways. However, the consequence of IMP formation was an incomplete recovery of [ATP], which in turn was related to decreased muscle oxidative capacity following contractions. These results likely have implications for the capacity to generate adequate energy during repeated bouts of muscular work.NEW & NOTEWORTHY An ∼9% decline in [ATP] led to the formation of inosine monophosphate (IMP) during submaximal muscular contractions. Modeling revealed IMP formed to preserve a favorable energy state (ΔGATP) by minimizing large increases in [ADP], whereas the loss of [ATP] did not alter ΔGATP. [ATP] did not recover by 10 min, and the loss of [ATP] was associated with a reduced oxidative capacity, providing a new link between [ATP] loss and an impaired energetic capacity in vivo.

Correlation between skeletal muscle acetylcarnitine and phosphocreatine metabolism during submaximal exercise and recovery: interleaved 1H/31P MRS 7 T study

Acetylcarnitine is an essential metabolite for maintaining metabolic flexibility and glucose homeostasis. The in vivo behavior of muscle acetylcarnitine content during exercise has not been shown with magnetic resonance spectroscopy. Therefore, this study aimed to explore the behavior of skeletal muscle acetylcarnitine during rest, plantar flexion exercise, and recovery in the human gastrocnemius muscle under aerobic conditions. Ten lean volunteers and nine overweight volunteers participated in the study. A 7 T whole-body MR system with a double-tuned surface coil was used to acquire spectra from the gastrocnemius medialis. An MR-compatible ergometer was used for the plantar flexion exercise. Semi-LASER-localized 1H MR spectra and slab-localized 31P MR spectra were acquired simultaneously in one interleaved exercise/recovery session. The time-resolved interleaved 1H/31P MRS acquisition yielded excellent data quality. A between-group difference in acetylcarnitine metabolism over time was detected. Significantly slower τPCr recovery, τPCr on-kinetics, and lower Qmax in the overweight group, compared to the lean group was found. Linear relations between τPCr on-kinetics, τPCr recovery, VO2max and acetylcarnitine content were identified. In conclusion, we are the first to show in vivo changes of skeletal muscle acetylcarnitine during acute exercise and immediate exercise recovery with a submaximal aerobic workload using interleaved 1H/31P MRS at 7 T.