The impact of feeding β2‐adrenergic agonists on rainbow trout muscle β2‐adrenoceptors and protein synthesis
The β2‐adrenergic agonists (β2‐AAs) clenbuterol (CLEN) and ractopamine (RACT) were fed to rainbow trout Oncorhynchus mykiss for 30–37 days and red and white muscle β2‐adrenoceptor (β2‐AR) binding characteristics and mRNA expression were assessed in parallel with fractional protein synthetic rates. Feeding CLEN or RACT had no significant effect on any body or physiological variables measured. There were no significant differences in the number of binding sites (Bmax) while a significant increase in the Kd was observed for the β‐ARs of red and white muscle membranes isolated from β2‐AA‐fed rainbow trout. No change in β2‐AR mRNA levels was observed with β2‐AA feeding, implying that these β2‐AAs do not act at the transcriptional level at least for the β2‐AR. The β2‐AA treatments, however, did significantly increase red and white muscle fractional protein synthesis rates in whole protein, myofibrillar protein and sarcoplasmic soluble protein fractions. Although not conclusive, this study supports a direct link between rainbow trout muscle β2‐ARs and muscle protein synthesis.
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