In humans, skeletal muscle carnitine metabolism changes with exercise in healthy subjects.1 High-intensity exercise is characterized by an increase in acylcarnitines and a decrease in free and total carnitine content in the vastus lateralis muscle.1 In addition, exercise in humans is associated with increases in plasma acylcarnitine concentrations and excretion of free carnitine in urine.2,3
L-carnitine, a betaine derivative of BHBA, is found in virtually all cells of higher animals as well as in some microorganisms and plants. In animals, it is synthesized almost exclusively in the liver. Carnitine is released into the circulation by the liver primarily as C2-carnitine, and the actual ester pattern is a result of the uptake and release action of the peripheral tissues. Two essential amino acids (ie, lysine and methionine) serve as primary substrates for the biosynthesis of C2-carnitine. The primary biochemical function of carnitine is related to its ester-forming capability with reference to its involvement in β-oxidation of the long-chain fatty acids. For the transfer of activated long-chain fatty acids across the inner mitochondrial membrane, acyl-CoA esters are transesterified to form acylcarnitines. Shortchain acylcarnitines (acyl groups with < 10 carbon atoms) are formed from, and are in equilibrium with, the corresponding intracellular short-chain acyl-CoA esters. Thus, changes in the distribution of total carnitine between acylcarnitines and free carnitine reflect similar changes in the acyl-CoA pool.4–7 In addition to the involvement of carnitine in β-oxidation of the longchain fatty acids within the mitochondrion, carnitine can form an ester with several short- and mediumchain endogenous or exogenous fatty acids.7–9 Under some metabolic conditions, carnitine serves as a buffer of the metabolically critical mitochondrial acyl-CoA pool. Carnitine is used as a therapeutic agent in people to improve exercise performance.10,11
Exercise in Thoroughbreds results in a marked decrease in free carnitine and an almost equivalent increase in C2-carnitine and acetyl coenzyme A in working muscle.12–14 However, to our knowledge, plasma concentrations of various acylcarnitines following exercise in horses have not been assessed. The purpose of the study reported here was to evaluate alterations in skeletal muscle carnitine metabolism during exercise and training by measuring changes in plasma acylcarnitine concentrations in Standardbreds. Changes in plasma acylcarnitine concentrations occur in humans under similar conditions.2,3
Acyl coenzyme A
Butyryl- and isobutyryl-carnitine
Maximal heart rate
High-intensity exercise trained
Standardized exercise test
Pavo, Boxmeer, The Netherlands.
Boehringer Ingelheim, Alkmaar, The Netherlands.
Kagra, Graber A, Fahrwangen, Switzerland.
Cardio Perfect Inc, Atlanta, Ga.
Polar 610i, Polar Electro Oy, Kempele, Finland.
Hettich Zentrifugen, Tuttlingen, Germany.
HP 5890 Series II GC, Palo Alto, Calif.
Hewlett Packard 5989 B, MS Engine, Palo Alto, Calif.
CP Sil 19CB, Varian/Chrompack, Middelburg, The Netherlands.
Micromass Quattro Ultima LC/MS/MS, Waters Corp, Milford, Mass.
Alliance HPLC system, Waters Corp, Milford, Mass.
Randox kit Nonesterified fatty acid, FA115, Randox Laboratories Ltd, Antrim, England.
Randox kit Ranbut, RB 1007, Randox Laboratories Ltd, Antrim, England.
ABL, Radiometer, Rijswijk, The Netherlands.
SPSS, version 12 for Windows, SPSS Inc, Chicago, Ill.
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Composition of the concentrate feed for the Standadbred used in the study.
|Vitamin A||10,100 IU*|
|Vitamin D3||2,000 IU*|
|Vitamin E (DL-α-tocopherol acetate)||101 IU*|
|Copper (II) sulphate pentahydrate||12 mg*|
Per kilogram of feed.