Effect of Plasma Free Fatty Acid Concentration on the Content and Composition of the Free Fatty Acid Fraction in Rat Skeletal Muscles

Abstract

Skeletal muscles contain a fraction of free (unesterified) fatty acids. This fraction is very small, but important since it contributes to the creation of the plasma-myocyte free fatty acid concentration gradient. Maintenance of this gradient is necessary for blood-borne fatty acids to be transported into the cell. There are no data on the regulation of the content and composition of the free fatty acid fraction in the cell. The aim of the present study was to examine the effect of an elevation and a reduction in the plasma-borne free fatty acid concentration on the content and composition of the free fatty acid fraction in different skeletal muscle types. The experiments were carried out on male Wistar rats with 280 - 310 g body weight. They were divided into four groups - 1, control; 2, exercised 3 h on a treadmill moving with a speed of 1,200 m/h and set at + 10° incline; 3, treated with heparin; and 4, treated with nicotinic acid. Samples of the soleus as well as the red and white sections of the gastrocnemius muscles were taken. These muscles are composed mostly of slow-twitch oxidative, fast-twitch oxidative-glycolytic and fast-twitch glycolytic fibres, respectively. Lipids were extracted from the muscle samples and from the blood; the free fatty acid fraction was isolated by means of thin-layer chromatography. The individual free fatty acids were identified and quantified using gas-liquid chromatography. The plasma concentration of free fatty acids was as follows: control group, 236.1 ± 32.9; after exercise, 407.4 ± 117.5; after heparin, 400.8 ± 36.8; and after nicotinic acid, 102.5 ± 26.1 μmol/l (p < 0.01 vs. control values in each case). The total content of the free fatty acid fraction in the control group was as follows: white gastrocnemius, 27.6 ± 7.3; red gastrocnemius, 52.2 ± 13.9; soleus, 72.3 ± 10.2 nmol/g. Elevation in plasma free acid concentration during exercise increased the total content of free fatty acids in the white gastrocnemius (38.7 ± 13.9) and in the soleus (103.4 ± 15.9 nmol/g; rest-exercise: p < 0.05 and p < 0.01, respectively), but had no effect in the red gastrocnemius. Neither elevation in the plasma free fatty acid concentration with heparin nor reduction with nicotinic acid affected the total content of the free fatty acid fraction in the muscles examined. The ratio of plasma concentration of individual acid to muscle concentration for the same acid varied greatly, depending on acid, muscle type and experimental group. The ratio was positive (above unity) for each acid almost in all cases with the exception of certain acids in the nicotinic acid-treated group where it was below unity. We conclude that the skeletal myocytes maintain a stable level of free fatty acid fraction in the wide range of plasma free fatty acid concentrations.

Key words

Free fatty acids · Skeletal muscle · Plasma · Exercise · Heparin · Nicotinic acid

References

• 1 Hamilton J A, Kamp F. How are free fatty acids transported in membranes? Is it by proteins or by free diffusion through the lipids.  Diabetes. 1999;  48 2255-2269
• 2 Zakim D. Fatty acids enter cells by simple diffusion.  Proc Soc Exp Biol Med. 1996;  212 5-14
• 3 Van der Vusse G J, Reneman R S. Lipid metabolism in muscle. In: Rowell LB, Sheperd JT (eds.) Handbook of Physiology, Section 12-Exercise: Regulation and integration of multiple systems. New York; Oxford Press 1996: 952-994
• 4 Górski J, Nawrocki A, Murthy M. Characterization of free and glyceride-esterified long chain fatty acids in different skeletal muscle types of the rat.  Mol Cell Biochem. 1998;  178 113-118
• 5 Van der Vusse G J, Roemen T HM. Gradient of fatty acids from plasma to skeletal muscle in dog.  J Appl Physiol. 1995;  78 1839-1843
• 6 Kiens B, Roemen T HM, van der Vusse G J. Muscular long-chain fatty acid content during graded exercise in humans.  Am J Physiol: Endocrinol Metab. 1999;  276 E352-E357
• 7 Gorski J, Bonen A. Palmitate incorporation into lipids pools of contracting red and white muscles.  Mol Cell Biochem. 1997;  166 73-83
• 8 Nawrocki A, Gorska M, Z·endzian-Piotrowska M, Gorski J. Effect of acute streptozotocin diabetes on fatty acid content and composition in different lipid fractions of rat skeletal muscle.  Horm Metab Res. 1999;  31 252-256
• 9 Ariano M A, Armstrong R B, Edgerton V R. Hindlimb muscle fiber populations of five mammals.  J Histochem Cytochem. 1979;  21 51-55
• 10 Sullivan T E, Armstrong R B. Rat locomotory muscle fiber activity during trotting and galloping.  J Appl Physiol Respirat Environ Exercise Physiol. 1978;  44 358-363
• 11 Folch J, Lees M, Stanley G HS. A simple method for the isolation and purification of total lipids from animal tissues.  J Biol Chem. 1957;  226 497-509
• 12 Van der Vusse G J, Roemen T HM, Reneman R S. Assesment of fatty acids in dog left ventricle myocardium.  Biochim Biophys Acta. 1980;  617 347-352
• 13 Conquer J, Mahadevappa V G. Evidence for the possible involvement of protein kinase C in the activation of non-specific phospholipase A₂ in human neutrophils.  J Lipid Mediat. 1991;  3 113-123
• 14 Morrison W R, Smith L M. Preparation of fatty acid methyl esters and dimethylacetals from lipids with boron fluoride-methanol.  J Lipid Res. 1964;  5 600-608
• 15 Budohoski L, Gorski J, Nazar K, Kaciuba-UsŽciłko H, Terjung R L. Traicylglycerol synthesis in the different skeletal muscle fiber sections of the rat.  Am J Physiol Endocrinol Metab. 1996;  34 E574-E581
• 16 Dyck D J, Peters S J, Glatz J, Gorski J, Keizer H, Kiens B, Liu S, Richter E A, Spriet L L, van der Vusse G J, Bonen A. Functional Differences in lipid metabolism in resting skeletal muscle of various fiber types.  Am J Physiol: Endocrinol Metab. 1997;  35 E340-E351
• 17 Okano G, Shimojo T. Utilization of long-chain free fatty acids in white and red muscle of rats.  Biochim Biophys Acta. 1982;  710 122-127
• 18 Paul P. Uptake and oxidation of substrates in the intact animal during exercise. In: Pernow P, Saltin B (eds.) Muscle metabolism during exercise. New York; Plenum 1971: 225-247
• 19 McMurray R G, Hackney A C. Endocrine responses to exercise and training. In: Garrett WE, Kirkendall DT (eds.) Exercise and Sport Science. Philadelphia; Lippincott, Williams and Wilkins 2000: 135-161
• 20 Glatz J FC, Luiken J JFP, Bonen A. Involvement of membrane-associated proteins in the acute regulation of cellular fatty acid uptake.  J Mol Neurosci. 2001;  16 123-132
• 21 Turcotte L P. Muscle fatty acid uptake during exercise: possible mechanisms.  Exerc Sport Sci Rev. 2000;  28 4-9
• 22 Issekutz B, Borts W M, Miller H I, Paul P. Turnover rate of plasma FFA in humans and in dogs.  Metab Clin Exp. 1967;  16 1001-1009
• 23 Glatz J FC, Van der Vusse G J. Cellular fatty acid-binding proteins: their function and physiological significance.  Prog Lipid Res. 1996;  35 234-282
• 24 Hagenfeldt L, Wahren J, Pernow B, Räf L. Uptake of individual free fatty acids by skeletal muscle and liver in man.  J Clin Invest. 1972;  51 2324-2330
• 25 Carlson L A, Fröberg S O, Nye E R. Acute effects of nicotinic acid on plasma, liver, heart and muscle lipids.  Acta Med Scand. 1966;  180 571-579