β-Adrenergic receptor subtypes that mediate ractopamine stimulation of lipolysis

Ractopamine HCl is an β-adrenergic receptor (ssAR) ligand that was recently approved for use in swine to enhance carcass leanness. The RR stereoisomer of ractopamine is the most active of the four stereoisomers exhibiting the highest affinity and signaling response. The RR isomer exhibits selective activation of the porcine β2AR, which might limit the lipolytic response to ractopamine because the β1AR is the predominant subtype in swine adipocytes and may mediate most of the lipolytic response. Therefore, we determined the βAR subtypes that mediate the lipolytic response to ractopamine in swine adipocytes. In order to confirm the predominant role of the β1AR in porcine adipocytes, isoproterenol-stimulated lipolysis was inhibited by increasing doses of subtype-selective antagonists. Inhibition curves were biphasic using β1AR antagonists (CGP 20712A and bisoprolol) and curve analysis indicated that both β1AR and β2AR contributed to lipolysis with 50 to 60% of the response coming from the β1AR. Inhibition with the β2AR antagonist clenbuterol revealed only one class of βAR that closely approximated the kinetics of the β1AR. When the RR isomer of ractopamine was the lipolytic agent, similar results to isoproterenol were observed, except that the estimated contribution of the β1AR was 38%. That β2AR antagonists did not detect a contribution of the β2AR to lipolysis may indicate that the β1AR masked the response to the β2AR. Dose titration with the RR isomer in the presence of a saturating concentration of β1AR or β2AR antagonists indicated that each subtype was present in sufficient quantities to stimulate lipolysis near maximally. Data indicate that both the β1AR and β2AR are functionally linked to lipolysis in swine adipocytes and that ractopamine activates each subtype. The RR isomer of ractopamine stimulated adenosine 3',5'-cyclic phosphate accumulation with equal efficacy to isoproterenol through the cloned porcine β2AR, but was only 35% as efficacious through the cloned porcine β1AR. These data confirm the β2AR selectivity of the RR stereoisomer, but suggest the partial agonism through the β1AR is sufficient to activate lipolysis through both subtypes in swine adipocytes.