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Regulation of CPT I activity in intermyofibrillar and subsarcolemmal mitochondria from human and rat skeletal muscle

¹Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph N1G 2W1; and ²Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5

Submitted 23 May 2003 ; accepted in final form 23 August 2003

Carnitine palmitoyltransferase I (CPT I) is considered the rate-limiting enzyme in the transfer of long-chain fatty acids (LCFA) into the mitochondria and is reversibly inhibited by malonyl-CoA (M-CoA) in vitro. In rat skeletal muscle, M-CoA levels decrease during exercise, releasing the inhibition of CPT I and increasing LCFA oxidation. However, in human skeletal muscle, M-CoA levels do not change during moderate-intensity exercise despite large increases in fat oxidation, suggesting that M-CoA is not the sole regulator of increased CPT I activity during exercise. In the present study, we measured CPT I activity in intermyofibrillar (IMF) and subsarcolemmal (SS) mitochondria isolated from human vastus lateralis (VL), rat soleus (Sol), and red gastrocnemius (RG) muscles. We tested whether exercise-related levels (65% maximal O₂ uptake) of calcium and adenylate charge metabolites (free AMP, ADP, and P_i) could override the M-CoA-induced inhibition of CPT I activity and explain the increased CPT I flux during exercise. Protein content was 25-40% higher in IMF than in SS mitochondria in all muscles. Maximal CPT I activity was similar in IMF and SS mitochondria in all muscles (VL: 282 ± 46 vs. 280 ± 51; Sol: 390 ± 81 vs. 368 ± 82; RG: 252 ± 71 vs. 278 ± 44 nmol·min^-1·mg protein^-1). Sensitivity to M-CoA did not differ between IMF and SS mitochondria in all muscles (25-31% inhibition in VL, 52-70% in Sol and RG). Calcium and adenylate charge metabolites did not override the M-CoA-induced inhibition of CPT I activity in mitochondria isolated from VL, Sol, and RG muscles. Decreasing pH from 7.1 to 6.8 reduced CPT I activity by 34-40% in both VL mitochondrial fractions. In summary, this study reports no differences in CPT I activity or sensitivity to M-CoA between IMF and SS mitochondria isolated from human and rat skeletal muscles. Exercise-induced increases in calcium and adenylate charge metabolites do not appear responsible for upregulating CPT I activity in human or rat skeletal muscle during moderate aerobic exercise.

carnitine palmitoyltransferase I; malonyl-coenzyme A; fat oxidation

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