the effects of continuous and intermittent feeding of β-agonist zilpaterol hydrochloride on muscle β-adrenergic receptors gene expression in feedlot male lambs

introduction the compounds known as β-adrenergic agonists (β-aa) are organic molecules that have the ability to bind to β-adrenergic receptors (β-ar) and start biochemical reactions that will result in increase of accretion of skeletal muscle and reduction in accretion of fat. the anabolic responses to β-aa are temporary, with a peak time occurring during the first 14 days, after which there is a linear decline in growth response, due to either down-regulation or desensitization of the β-ar. based on down-regulation of β-ar research, the hypothesis that intermittent feeding of β-aa could enhance response on growth performance was created. the objective of this study was therefore to ascertain the effects of continuous and intermittent use of zh for a period of 42 d on β-ar gene expression in lori-bakhtiari feedlot lambs. materials and methods the continuous feeding of zh (daily regimen), intermittent 1 d feeding zh followed by 1 d of withdrawal (1 on 1 off regimen), and intermittent 2 d feeding zh followed by 2 d of withdrawal (2 on 2 off regimen) were employed as the different feeding methods. thirty two lori-bakhtiari male lambs (initial bw=44±4.7 kg) were assigned to one of four treatments (8 lambs/treatment) based on initial bw and were fed with a diet content of 14% protein and supplemented with 0.2 mg/kg of live weight d-1zh. the basal diet without zh was the added control group. for evaluating gene expression, biopsy samples of the semimembranosus muscle were collected from 3 lambs per treatment before zh supplementation on d 0, and subsequently at d 21, and d 42. samples were rapidly frozen in liquid n2. in laboratory after total rna isolation from muscle, the rna was then reverse-transcribed into complementary dna (cdna). real time pcr for cdna samples was performed using an iq5 biorad instrument. results and discussion the results of this study showed that the main effects and period × regimen interactions effect was not significant on β1 and β2-adrenergic receptors gene expression. also there were no significant differences between treatments and control group. most of the physiological effects of catecholamines are due to specific interactions with β-adrenergic receptors (β -ar). the growth response to β-aa administration commonly is often reduced over time which is possibly a result of desensitization of the β-ar after chronic continuous exposure to the β-aa. the modification in the dosage of β-aa during the feeding period was previously proven to be a useful approach for compensating the reduced response in rats and lambs. likewise, hossner (2005) suggested that the intermittent treatment, e.g. treating for 2 days and resting for 2 days is one of the solutions for avoiding the desensitization and down-regulation of receptors. in studies evaluating mrna responses to β-aa, no change occurred in β1-ar mrna abundance due to β-aa treatment. baxa et al. (2010) explained that no detectable changes of β1-ar could be attributed to generally low abundance, which results in differences hidden by sample-to-sample variation. we were unable to detect differences in the mrna expression of β2-ar mrna in muscle samples. the decrease in β2-ar protein expression due to zh treatment may be in response to a posttranscriptional event. conclusion according to our results, it can be concluded that we could use zh intermittently (1 on 1 off-42 d) in feedlot lambs to decrease fattening costs versus of daily administration of zh. further investigation is suggested to detect the alteration of β-adrenergic receptors in continuous and intermittent feeding regimens for clarifying possibly down-regulation or desensitization of them.