Biochemical basis for the effects of the betaadrenergic receptor agonist ractopamine
hydrochloride in beef cattle
Ractopamine
• Repartitioning agent: shifts energy use from
fat to muscle deposition
• Structurally similar to catecholamines and
other compounds that bind to the betaadrenergic receptor
Beta-Adrenergic Receptor
Agonists
Epinephrine
Norepinephrine
Dopamine
Ractopamine Hydrochloride
• Optaflexx (Elanco Animal Health)
approved by FDA in 2003 to feed to cattle
at a rate of 70-439 mg/hd/d for last 28-42 d
of finishing period
• Research suggests 200 mg Optaflexx/d for
last 28 d in feedlot improve steer ADG and
G:F by 15-17 and 17-18% (Laudert et al.,
2004;Gruber et al., 2007)
Agonists and Antagonists
• Agonist: Able to bind to receptor and elicit
response
• Partial agonist: Binds to receptor, but produces
only a partial response
• Antagonist: Binds to receptor, but does not
produce any response, thus blocking signal
Beta-Adrenergic Receptors
•
•
•
•
Member of G-protein coupled receptor superfamily
7 transmembrane receptor linked to G-protein
G stimulatory protein (Gs)
Active Gs can bind to adenylate cyclase to activate it
Epinephrine
Beta-Adrenergic Receptors Con’t
• Adenylate cyclase converts ATP to cAMP
• cAMP can activate protein kinase A (PKA)
• PKA can phosphorylate proteins in the cell
– Phosphorylation of enzymes may cause them to be
turned “on” or “off”
Biocarta
http://web.indstate.edu/thcme/mwking
Summary
• Ractopamine hydrochloride is a beta-agonist that works via
the β-AR
• Activation of the β-AR results in production of PKA and
phosphorylation of HSL stimulating lipolysis
• “Repartitioning” effects of ractopamine due to stimulation
of lipolysis and changes in muscle protein metabolism