title: Regulation of CPT I activity in intermyofibrillar and subsarcolemmal mitochondria from human and rat skeletal muscle.
abstract: 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 ( approximately 65% maximal O2 uptake) of calcium and adenylate charge metabolites (free AMP, ADP, and Pi) could override the M-CoA-induced inhibition of CPT I activity and explain the increased CPT I flux during exercise. Protein content was approximately 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 approximately 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.
authors.affiliation: Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada N1G 2W1. vbezaire@uoguelph.ca
authors.forename: Veronic
authors.initials: V
authors.lastname: Bezaire
authors.forename: George J F
authors.initials: GJ
authors.lastname: Heigenhauser
authors.forename: Lawrence L
authors.initials: LL
authors.lastname: Spriet
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000249:Adenosine Monophosphate / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002333:Carnitine Acyltransferases / Q000378:metabolism; D002334:Carnitine O-Palmitoyltransferase / Q000378:metabolism*; D015444:Exercise / Q000502:physiology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D008297:Male; D008316:Malonyl Coenzyme A / Q000378:metabolism*; D008931:Mitochondria, Muscle / Q000145:classification / Q000201:enzymology*; D018482:Muscle, Skeletal / Q000201:enzymology*; D009210:Myofibrils; D010805:Physical Conditioning, Animal / Q000502:physiology; D051381:Rats; D012016:Reference Values; D012508:Sarcolemma; D013347:Subcellular Fractions / Q000201:enzymology
chemical_list: D000249:Adenosine Monophosphate; D008316:Malonyl Coenzyme A; D000244:Adenosine Diphosphate; D002333:Carnitine Acyltransferases; D002334:Carnitine O-Palmitoyltransferase; D002118:Calcium