Ractopamine induces differential gene expression in porcine skeletal muscles

¹ Department of Animal Sciences, Purdue University, W. Lafayette, IN 47907

^* To whom correspondence should be addressed. E-mail: dgerrard{at}purdue.edu.

	Abstract

Ractopamine (RAC) improves growth by increasing lean accretion and decreasing fat deposition through repartitioning nutrients from adipose tissue to skeletal muscle. Although not completely understood, RAC alters the proportion of muscle fiber type composition toward a "faster-contracting" phenotype. Because one of the primary determinants of contractile speed is the relative abundance of myosin heavy chain (MyHC) isoform and because the genes encoding these isoforms are transcriptionally regulated, RAC likely alters MyHC gene expression. Using real time PCR, relative transcript abundance of individual type I, IIA, IIX and IIB and total MyHC, as well as glycogen synthase (GS), citrate synthase (CS), lactate dehydrogenase (LDH), peroxisome proliferator activated receptor (PPAR), ₁-adrenergic receptor (AR), and ₂-AR were determined in the LM of 44 pigs fed RAC (20 mg/kg) for 0, 1, 2, or 4 wk. In addition, MyHC isoform expression was determined in the LM and red (RST) and white (WST) semitendinosus muscles of 48 pigs fed RAC (20 mg/kg) for shorter periods of 12, 24, 48 or 96 h. Type I MyHC expression was unaffected (P > 0.73) by RAC administration. Type IIA MyHC expression decreased (P < 0.0001) by 96 h, was lower (P < 0.0001) by 1 wk and returned to normal by 4 wk. Type IIX MyHC mRNA decreased (P < 0.001) by 2 wk and continued to decrease (P < 0.0001) by 4 wk. Most interesting was an increase (P < 0.0001) in type IIB MyHC by 12 h which was maintained at an elevated level throughout the 4 wk feeding period. Abundance of GS transcript was increased (P < 0.05) similarly by 12 h, but was not different from controls at 2 wk and lower (P < 0.01) at 4 wk. Gene expression of ₁-AR was not affected by feeding RAC, whereas ₂-AR gene expression was decreased (P < 0.05) by 2 wk. These data show MyHC genes are differentially regulated by RAC and suggest that the beta adrenergic agonists-induced repartitioning effect is, in part, mediated by changing muscle fiber type-specific gene expression, perhaps through the ₂-AR.

Key Words: Fiber type, myosin heavy chain, ractopamine

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