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 (beta-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 beta-AR (beta(1)- and beta(2)-subtypes) and MyHC isoforms (I beta,IIa, IIx/d, and Ilb) 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 beta(1)-AR mRNA, but resulted in a tendency (P=0.073) towards a linear decrease in beta(2)-AR mRNA abundance. Even though MyHC I beta 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 Ilb mRNA abundance as RAC feeding duration was increased. In conclusion, RAC feeding alters gene expression of beta(2)-AR and MyHC isoforms in the LD muscle of pigs not carrying the RyR1 mutation in a time-dependent manner. (C) 2014 Elsevier B.V. All rights reserved.