1. Biochemical basis for the effects of the beta-adrenergic receptor agonist ractopamine hydrochloride in beef cattle

2. Ractopamine
Repartitioning agent: shifts energy use from fat to muscle deposition
Structurally similar to catecholamines and other compounds that bind to the beta-adrenergic receptor

3. Beta-Adrenergic Receptor Agonists
Epinephrine
Norepinephrine
Dopamine

4. Ractopamine Hydrochloride
Optaflexx (Elanco Animal Health) approved by FDA in 2003 to feed to cattle at a rate of mg/hd/d for last d of finishing period
Research suggests 200 mg Optaflexx/d for last 28 d in feedlot improve steer ADG and G:F by and 17-18% (Laudert et al., 2004;Gruber et al., 2007)

5. 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

6. 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

7. 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”

8. Biocarta

10. 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