Gene expression of beta-adrenergic receptors and myosin heavy chain isoforms induced by ractopamine feeding duration in pigs not carrying the ryanodine receptor mutation

To evaluate the effects of ractopamine hydrochloride (RAC) feeding duration on mRNA abundance of beta-adrenergic receptors (β-AR) and myosin heavy chain (MyHC) isoforms in the Longissimus dorsi (LD) muscle of pigs not carrying the ryanodine receptor type I (RyR1) mutation, 80 finishing barrows (initial body weight=69.4±7.87 kg) were randomly assigned to 1 of 5 treatments with 8 replicate pens per treatment and 2 pigs per pen. Pigs were fed a corn-soybean meal-based diet with no added RAC (control) or with 10 mg/kg RAC fed for 7, 14, 21, or 28 d before slaughter. On d 28, hair root samples were collected for RyR1 genotyping and LD muscle samples were collected to measure β-AR (β1- and β2-subtypes) and MyHC isoforms (Iβ, IIa, IIx/d, and IIb) gene expression using the quantitative real-time PCR method. Six heterozygous negative pigs for the RyR1 mutation were detected, and therefore not considered for statistical analysis. Increasing RAC feeding duration did not affect the abundance of β1-AR mRNA, but resulted in a tendency (P=0.073) towards a linear decrease in β2-AR mRNA abundance. Even though MyHC Iβ mRNA abundance was linearly decreased (P=0.002) with increasing RAC feeding duration, both MyHC IIa and IIx/d gene expressions were not affected by the RAC treatment. There was a linear increase (P=0.029) in MyHC IIb mRNA abundance as RAC feeding duration was increased. In conclusion, RAC feeding alters gene expression of β2-AR and MyHC isoforms in the LD muscle of pigs not carrying the RyR1 mutation in a time-dependent manner.