Carnitine palmitoyltransferase I (CPTI) activity regulates the transport of long chain fatty acids into the mitochondria and is inhibited by malonyl-CoA (M-CoA) in vivo. Little is known regarding CPTI regulation in human skeletal muscle during exercise.
To investigate the effects of physiological concentrations of calcium and energy charge metabolites on CPTI activity in intermyofibrillar (IMF) and subsarcolemmal (SS) populations of mitochondria.
Nine recreationally active males volunteered for this study. IMF and SS mitochondria were extracted from the vastus lateralis muscle by homogenization and differential centrifugation. Muscle concentrations of M-CoA, calcium, and free AMP, ADP and inorganic phosphate (Pi) representative of 65% VO2max were added to IMF and SS mitochondrial preparations. CPTI activity was measured.
Maximal CPTI activity was significantly higher in IMF vs. SS mitochondria (325 ± vs. 233 ± 25, P = 0.018 nmol/min/g wm). Citrate synthase was similar in both mitochondrial fractions (2.0 ± 0.3 vs. 2.1 ± 0.3 nmol/min/g in IMF vs. SS). Addition of a physiological concentration of M-CoA (0.7 μ M) resulted in a similar inhibition of CPTI activity in IMF vs. SS mitochondria (29.9 ± 5.8% vs. 34.4 ± 5.3%). The addition of calcium, and free AMP, ADP and Pi in the presence of 0.7 μ M M-CoA had no significant effect on CPTI activity when compared to 0.7 μ M M-CoA alone in both fractions. However there was a trend (P = 0.098) for ADP to release the M-CoA inhibition in the IMF fraction.
Maximal CPTI activity is higher in IMF than SS mitochondria, however M-CoA sensitivity between the fractions is similar. The results suggest that calcium and energy charge metabolites do not increase CPTI activity during exercise in human skeletal muscle. Supported by NSERC & CIHR, Canada.