Skeletal muscle mitochondrial inertia is associated with carnitine acetyltransferase activity and physical function in humans

Abstract

BACKGROUND

At the onset of exercise, the speed at which phosphocreatine (PCr) decreases toward a new steady state (PCr on-kinetics) reflects the readiness to activate mitochondrial ATP synthesis, which is secondary to Acetyl-CoA availability in skeletal muscle. We hypothesized that PCr on-kinetics are slower in metabolically compromised and older individuals and are associated with low carnitine acetyltransferase (CrAT) protein activity and compromised physical function.

METHODS

We applied ³¹P-magnetic resonance spectroscopy ( ³¹P-MRS) to assess PCr on-kinetics in 2 cohorts of volunteers. Cohort 1 included patients who had type 2 diabetes, were obese, were lean trained (VO ₂max > 55 mL/kg/min), and were lean untrained (VO ₂max < 45 mL/kg/min). Cohort 2 included young (20–30 years) and older (65–80 years) individuals with normal physical activity and older, trained individuals. Previous results of CrAT protein activity and acetylcarnitine content in muscle tissue were used to explore the underlying mechanisms of PCr on-kinetics, along with various markers of physical function.

RESULTS

PCr on-kinetics were significantly slower in metabolically compromised and older individuals (indicating mitochondrial inertia) as compared with young and older trained volunteers, regardless of in vivo skeletal muscle oxidative capacity ( P < 0.001). Mitochondrial inertia correlated with reduced CrAT protein activity, low acetylcarnitine content, and functional outcomes ( P < 0.001).

CONCLUSION

PCr on-kinetics are significantly slower in metabolically compromised and older individuals with normal physical activity compared with young and older trained individuals, regardless of in vivo skeletal muscle oxidative capacity, indicating greater mitochondrial inertia. Thus, PCr on-kinetics are a currently unexplored signature of skeletal muscle mitochondrial metabolism, tightly linked to functional outcomes. Skeletal muscle mitochondrial inertia might emerge as a target of intervention to improve physical function.

TRIAL REGISTRATION

NCT01298375 and NCT03666013 (clinicaltrials.gov).

FUNDING

RM and MH received an EFSD/Lilly grant from the European Foundation for the Study of Diabetes (EFSD). VS was supported by an ERC starting grant (grant 759161) “MRS in Diabetes.”