Effects of sex and increased dietary SAA on growth performance and carcass traits
In the present study, barrows showed higher body weight from the beginning until the end of the experiment, which might in part explain the lower growth rate and feed intake in gilts than in barrows during the growing-finishing period. Meanwhile, feed efficiency as indicated by G:F was higher in gilts than in barrows during the 45–70 kg, 70–95 kg, and 11–110 kg periods. These results were in agreement with the previous reports that barrows had higher ADFI and ADG, but lower feed efficiency than gilts[16]. The higher feed effeciency of gilts could be explained by the greater protein deposition. In support of this, carcass analysis indicated that barrows had lower lean meat percentage and produced fatter carcass compared with gilts. Consistently, it was reported that improvement of feed efficiency in pigs could be achieved by increasing lean growth rate, which resulted in lower feed intake[17]. Increasing dietary SAA showed no effect on carcass traits, which agreed well with previous studies[18].
Effects of sex and increased dietary SAA on meat quality
In the present study, barrows showed lower shear force (a mechanical indicator of tenderness) than gilts, which agreed well with previous studies [19]. Meanwhile, it was also reported that tenderness scores were positively correlative with IMF content (P = 0.008)[19]. Consistent with these results, barrows also showed higher IMF in LM than gilts. Besides, it has been suggested that an increase in IMF leading to a decrease in the shear force could potentially be due to a decrease in the density of muscle which is positively correlated with protein content[20]. Consistent with this, barrows showed a lower crude protein content in LM compared with gilts, which also suggested the potential changes in the myofibrillar and cytoskeletal proteins correlating with shear force[21]. Mechanistically, IGF-1 has been shown to stimulate muscle protein synthesis and inhibit protein degradation via the ubiquitin-proteasome and autophagy-lysosome pathways[22]. Thus, the lower plasma IGF-1 concentration in barrows than in gilts provided physiological explanation for the reduced protein deposition and shear force.
The increase in dietary SAA as DL-Met or OH-Met tended to (P = 0.06) decrease the shear force of LM, which might also be explained by a tendency of a lower crude protein content in the LM of pigs fed increased SAA diets. As addressed above, the numerically lower plasma IGF-1 concentrations and, moreover, the down-regulation of IGF-1 expression in LM might in part account for the lower protein deposition in DL-Met- and OH-Met-fed pigs than in CON-fed pigs. Corresponding to the relatively lower crude protein deposition in LM, there was a reduction in crude protein of liver in pigs fed increased SAA diets, which might be associated with the limitation of circulating essential amino acids levels. In support of this, pigs fed DL-Met and OH-Met showed lower plasma histidine levels at 95–110 kg period compared with the CON (Additional file 3–5), but the mechanism for increased SAA-induced change of amino acids profile remains to be elucidated.
Remarkably, in comparison to the CON-fed pigs, the DL-Met- and OH-Met-fed pigs had increased glycolytic potential in LM. Generally, the higher level of glycolytic potential would increase drip loss and lead to pale, soft and exudative (PSE) pork. However, the decreased drip loss, in comparison to the control, excluded the occurrence of PSE pork in the DL-Met and OH-Met treatment. A moderately increased level of glycolytic potential was even considered to be beneficial for the development of tenderness[23]. Based on these results, we speculated that dietary DL-Met or OH-Met supplementation above growth requirements improved meat tenderness through changing protein and energy metabolism. Consistent with this assumption, the mRNA level of GLUT-4 and AMPKα2 in LM significantly increased in the DL-Met or OH-Met groups. AMPK can activate phosphorylase kinase, which then activates glycogen phosphorylase and promotes glycogenolysis[24]. GLUT-4, as the primary transporter of glucose uptake in the muscle, is up-regulated in the muscle upon activation of AMPK[25]. We also found the up-regulated expression of FATP-1 whereas the down-regulated expression of HSL in the DL-Met and OH-Met treatments compared with the CON treatment, further suggesting the activation of AMPK pathway[26]. In contrast, it has been reported that pigs (initial weight ~ 105 kg) fed diets supplemented with Met at 5 times the level of the control diet, for the last 14 days before slaughter, showed a tendency of reduced lipid content and glycolytic potential in the LM[27]. The long-term (20 weeks) supplementation and the lower increase (25%) of Met applied in the current study might have led to different mechanisms of response. In support of this, following increased SAA consumption, plasma taurine levels were elevated from 11–70 kg period whereas it remained unchanged from 70–110 kg period, indicating the different responses of pigs varying in body weight to supplemental Met sources.
A decreased drip loss was observed in DL-Met- and OH-Met-fed pigs compared to the CON-fed pigs. As indicated by previous studies[27], Met supplementation in pig diets improved the water-holding capacity (WHC) of LM, which was associated with the increased antioxidant capacity as manifested by the lower level of MDA in LM. However, the content of MDA in LM showed no difference among treatments in the present study. This might be associated with that Met supplementation had no significant effect on glutathione synthesis, which agreed well with previous studies[28]. It was even observed that reduced liver glutathione concentration following dietary supplementation with excessive cysteine[29]. These results suggested that the antioxidant capacity in response to SAA supplementation varied with the supplemental levels. Physically, drip loss originates from the spaces between muscle fiber bundles and the perimysial network, and the spaces between muscle fibers and the endomysial network[30]. Myofiber is constituted by four different fiber types including slow-oxidative or type I, fast oxido-glycolytic or type IIa, and fast glycolytic IIx or IIb. Glycolytic muscles had larger extra-myofibrillar fluid spaces and higher CSA than oxidative muscles[31–32]. Moreover, drip loss was negatively related to fiber area percentages of type I and IIa, while positively related to type IIx and IIb percentage[33]. Interestingly, we found that the increase in dietary SAA decreased the CSA of myofibers and down-regulated mRNA levels of MyHC IIx. Mechanistically, AMPK can directly regulate the transcriptional activity of PPARδ in skeletal muscles and thus increase the proportion of type I fibers (by 38%)[34]. Consistently, AMPKα2 in LM was up-regulated in the DL-Met and OH-Met groups in comparison to the CON group. Thus, it would appear that the decreased drip loss following increased SAA consumption was associated with the change of fiber types. In addition, we also found that drip loss was significantly affected by the interaction of diet and sex. Specifically, for the CON-fed pigs, drip loss of LM was higher in gilts than in barrows. As mentioned above, both IMF and marbling score were higher in barrows than in gilts. It was reported that marbling score was negatively correlated with drip loss (r = -0.459; P = 0.001)[35]. In the present study, the drip loss tended (P = 0.08) to be negatively correlated (r = -0.52) with the IMF content. Thus, the higher IMF and marbling score in LM might account for the lower drip loss in the CON-fed barrows than in the CON-fed gilts.
Interestingly, the lightness of fresh muscle as indicated by L45min value was higher in the OH-Met-fed barrows than in the DL-Met- and CON-fed barrows. It has been reported that muscle lightness and plasma lactate and glucose reflect increased muscle glycogenolysis which, in turn, can explain the change of muscle lightness and plasma lactate and glucose levels[36]. Consistent with the higher lightness value, the OH-Met treatment had higher degree of muscle glycogenolysis as suggested by the higher glycolytic potential. Moreover, the increase in the glycolytic potential in the OH-Met treatment was mainly associated with the increased content of lactate, the end-product of glycolysis. In contrast, the increase in the glycolytic potential in the DL-Met treatment was mainly ascribed to the increase in free glucose and glucose-6-P content, the initial product of glycolysis. These results indicate that a higher degree of complete glycolysis process did occur in muscle of the OH-Met treatment rather than in that of the DL-Met treatment. The highest lightness value, L45min, observed in the barrows of the OH-Met treatment was accompanied with the highest muscle lactate level, further suggesting the contribution of lactate to lightness value. Pigs fed OH-Met had higher serum acetate and propionate, both of which can stimulate GLP-1 secretion via activating the GPR43 receptors in intestinal L-cells[37]. Consistently, GPR43 expression was upregulated in LM and plasma GLP-1 concentration was increased in the OH-Met treatment. GLP-1 can increase glycogen synthase activity and stimulate both glucose oxidation and lactate formation[38]. Taken together, SCFA-induced GPR43/GLP1 signaling might account for the higher level of lactate in the OH-Met treatment.