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Effects of Source and Level of Added Zinc on Muscle Fiber Diameter and Skeletal Muscle Protein Turnover of Finishing Pigs Fed Ractopamine HCl
A total of 320 pigs (PIC 327 × 1050, 98 kg BW) were used in a 35-d study to determine the effects of supplemental Zn on muscle fiber cross-sectional area (CSA) and muscle specific relative gene expression of finishing pigs fed ractopamine-HCl (RAC; Elanco Animal Health, Greenfield, IN). Pens were randomly allotted to diets with 2 pigs per pen. Treatments (n=8) were a corn-soybean meal control diet (0.66% SID Lys), a diet (0.92% SID Lys) with 10 ppm RAC, the RAC diet plus 75, 150, and 225 ppm added Zn from ZnO or Availa-Zn (Zinpro, Eden Prairie, MN). All diets contained 55 ppm Zn from ZnSO4. One pig was randomly selected from 16 pens per treatment for collection of Longissimus lumborum muscle biopsies on d 0, 8, 18, and 32. Pigs fed the RAC diets had increased (P=0.05) CSA of type IIA fibers and a trend for increased (P=0.10) type IIX CSA. Pigs fed RAC diets with added Zn tended to have increased (quadratic, P=0.10) type IIB fiber CSA. There were no treatment effects on number of fiber associated nuclei (P>0.10). Pigs fed RAC diets had increased (P=0.06) relative expression of IGF-1 on d 8 compared to those fed the control diet, but there were no differences on d 18 and 32 (P>0.10). Relative expression of IGF-1 decreased (quadratic, P<0.04) with increasing levels of Zn on d 8 and 18. There was no difference in IGF-1 receptor (IGF-1R) expression (P>0.10) between pigs fed the control and RAC diets. Pigs fed diets with increasing levels of Zn had a tendency for decreased (quadratic, P<0.08) relative expression of IGF-1R on d 18. Pigs fed RAC diets had increased relative expression of Pax7 on d18 (P<0.04) and a tendency for increased (P<0.08) expression on d 8 and 32; however, there was no response in the relative expression of Myf5, MyoD, and Myogenin. Pigs fed increasing levels of added Zn had a tendency for increased (linear, P<0.10) relative expression of Pax7 on d 8 and 18; however, there was no response in the relative expression of Myf5, MyoD, and Myogenin. Therefore, RAC-stimulated hypertrophy occurred independent of satellite differentiation. Additional Zn did not dramatically increase muscle fiber CSA, possibly due to a reduction of muscle IGF-1 and IGF-1R mRNA expression caused by the Zn.
Keywords: IGF-1, ractopamine-HCl, zinc