Changes in structural and functional properties of skeletal muscle by L-carnitine supplementation to suckling piglets

To investigate, whether L-carnitine is effective in stimulating the early postnatal increase in the number of myofibres, 30 piglets of low (LW) and middle (MW) birth weight from 6 German Landrace sows were supplemented once daily with 400 mg L-carnitine (n=16) or a placebo (n=14) from days 7 to 28 of age (weaning and slaughtering). Carnitine tended to decrease the perirenal fat percentage, and supplemented females exhibited a significantly lower percentage of lipid in the carcass. Carnitine concentration in semitendinosus muscle (ST) was more than doubled in response to treatment. The total number of ST myofibres was increased by 19% in treated LW pigs thereby reaching the unchanged level of MW littermates. Myofibres tended to be smaller and protein concentration and protein/DNA ratio were significantly lower in treated pigs. Muscular isocitrate dehydrogenase activity as a marker of oxidative metabolism tended to be increased, whereas lactate dehydrogenase and creatine kinase activities, and fibre type composition were not influenced by carnitine. The results suggest that carnitine supplementation during the suckling period stimulates the early postnatal increase in myofibre number in piglets of low birth weight. This might compensate for the far-reaching consequences of low birth weight on growth, carcass and meat quality. Introduction Piglets of low birth weight exhibit a lower total number of skeletal muscle fibres at birth and throughout life compared with piglets of middle and heavy birth weight (Rehfeldt & Kuhn, 2006). These pigs have a limited potential for muscular lean accretion, and therefore deposit more fat resulting in a lower carcass quality at market weight. Moreover, due to the highly hypertrophied fibres, meat quality is poor, as indicated by higher drip loss, lower pH45, lower impedance values and higher content of heat-stable collagen (Gondret et al., 2006; Rehfeldt et al., 2008). Consequently, an increase in the number of muscle fibres could contribute to improve carcass and meat quality. The majority of muscle fibres is formed prenatally (e.g. Rehfeldt et al., 2000). However, some increase in the total fibre number has also been observed in pig semitendinosus muscle shortly after birth (Rehfeldt et al., 2000) and could be associated with the appearance of a third generation of small fibres during the first two postnatal weeks (Lefaucheur et al., 1995). L-carnitine has been shown to stimulate prenatal myofibre formation (Musser et al., 2001). In growing and finishing pigs, carnitine increased protein accretion and percentage of lean and muscle, but decreased fat deposition (Owen et al., 1996; 2001; Heo et al., 2000). Therefore, the objective of this study was to investigate whether carnitine has the potential to affect early postnatal muscle growth and body composition of suckling piglets. Material and methods Thirty piglets of low (LW) and middle (MW) birth weight (each within one third of frequency distribution of all piglets born) from 6 German Landrace sows were supplemented orally once daily with 400 mg carnitine (n=16) or a placebo (n=14) from days 7 to 28 (weaning) of age. At slaughter on d 28, blood samples were collected for the analysis of IGF-I, glucose, urea and non-esterified fatty acids. Samples from semitendinosus (ST) muscle were collected for analysis of DNA, protein, carnitine, activities of creatine kinase (CK), lactate dehydrogenase (LDH) and isocitrate dehydrogenase (ICDH). For histological examination, one sample each was collected from the dark (deep), bright (superficial) and central portion of the mid-belly. Serial transverse sections were stained with eosin, and for NADH-tetrazolium reductase which enables classification into red oxidative, intermediate oxidative and white glycolytic fibres. Fibre type distribution and fibre cross sectional area (FCSA) were determined on 900 muscle fibres (300 in each portion) by image analysis (TEMA v1.00, Scan Beam APS, Hadsund, Denmark). The number of fibres per unit area was used to estimate the total number of fibres by multiplication with the ST muscle cross-sectional area (MCSA). Body composition was determined by dissection and chemical analysis. For statistical analysis, all data were subjected to analysis of variance, using the Mixed Model procedure of SAS (Version 9.1, SAS Inst. Inc., Cary, NC, USA) with treatment, sex, birth weight group and