Abstract
Anatomical site and divergent functionalities of muscles can be related to differences in IMF content, metabolism and adipogenic gene expression. Then, potential differences in different muscles in beef cattle were studied. As a second objective, the main sources of experimental variability associated to RT-qPCR results were analyzed following a nested design in order to implement appropriate experimental designs minimizing gene expression variability. To perform the study Longissimus thoracis (LT), Semitendinosus (SM), Masseter (MS), Sternomandibularis (ST) and subcutaneous adipose tissue (SAT) samples of Pirenaica young bulls (n = 4) were collected for IMF, collagen and protein quantification, analysis of adipocyte size distribution and gene expression (PPARG, CEBPA, FAPB4 and WNT10B). A greater IMF content was observed in MS and SM muscles, which had a bimodal adipocyte size distribution while it was unimodal in the muscles LT and ST. This suggest that the different IMF accretion in the muscles studied might be related to different rates of hyperplasia and hypertrophy and that IMF might develop later in LT and ST muscles. The former differences were not mirrored by the expression of the genes analyzed, which might be related to the different contribution of mature and non-mature adipocytes to the total gene expression. When comparing IMF and SAT gene expression, late and early developing tissues respectively, expression of PPARG, CEBPA and FABP4 was higher in the SAT, in agreement with bigger cell size and numbers. The variability study indicates that the analytical factors that add higher variability to the gene expression are the sampling and RT and therefore, it would be appropriate to include those replicates in the design of future experiments. Based on the results, the use of MS and SM muscles could allow less expensive experimental designs and bigger sample size that could permit the detection of lower relevant differences in gene expression.
Highlights
It is well established that muscles display differences in muscle fibre type, proteolytic activity, connective tissue and intramuscular fat (IMF) percentage, attributes that are related to the molecular, metabolic, structural, and contractile properties of the muscle [1, 2]
Chemical fat content was higher in MS and SM compared to Longissimus thoracis (LT) and ST muscles (P < 0.05) but no statistically significant differences were found in the protein content among muscles (Table 1)
It was observed that the minimum, mean, mode (P < 0.05) and median (P 0.001) of the adipocytes were significantly smaller in LT than in ST muscles (Table 2), both with unimodal adipocyte size distribution
Summary
It is well established that muscles display differences in muscle fibre type, proteolytic activity, connective tissue and intramuscular fat (IMF) percentage, attributes that are related to the molecular, metabolic, structural, and contractile properties of the muscle [1, 2]. Intramuscular fat content, adipocytes and adipogenic genes in muscles characteristics have a deep influence on meat quality traits as well [3]. Quality of meat can be compromised by the low tendency of some cattle populations to accumulate IMF. This is the case of Pirenaica breed for example, which is widely used in northern Spain and is highly appreciated for its value as a genetic resource and its contribution to the maintenance of local beef production systems [6]. Deepening the knowledge of IMF accretion mechanisms would be important to find appropriate strategies to increase the value of the meat produced in these systems
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