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ß-Adrenergic receptor subtypes that mediate ractopaminestimulation of lipolysis^1,2

S. E. Mills^*,3, M. E. Spurlock^* and D. J. Smith

^* Department of Animal Sciences, Purdue University, West Lafayette, IN 47907 and and USDA, ARS, Biosciences Research Laboratory, Fargo, ND 58105

³ Correspondence: Lilly Hall (phone: 765-494-4845; fax 765-494-9346; E-mail:smills{at}purdue.edu).

Ractopamine HCl is an ß-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 ß₂AR, which might limit the lipolytic response to ractopamine because the ß₁AR is the predominant subtype in swine adipocytes and may mediate most of the lipolytic response. Therefore, we determined the ßAR subtypes that mediate the lipolytic response to ractopamine in swine adipocytes. In order to confirm the predominant role of the ß₁AR in porcine adipocytes, isoproterenol-stimulated lipolysis was inhibited by increasing doses of subtype-selective antagonists. Inhibition curves were biphasic using ß₁AR antagonists (CGP 20712A and bisoprolol) and curve analysis indicated that both ß₁AR and ß₂AR contributed to lipolysis with 50 to 60% of the response coming from the ß₁AR. Inhibition with the ß₂AR antagonist clenbuterol revealed only one class of ßAR that closely approximated the kinetics of the ß₁AR. When the RR isomer of ractopamine was the lipolytic agent, similar results to isoproterenol were observed, except that the estimated contribution of the ß₁AR was 38%. That ß₂AR antagonists did not detect a contribution of the ß₂AR to lipolysis may indicate that the ß₁AR masked the response to the ß₂AR. Dose titration with the RR isomer in the presence of a saturating concentration of ß₁AR or ß₂AR antagonists indicated that each subtype was present in sufficient quantities to stimulate lipolysis near maximally. Data indicate that both the ß₁AR and ß₂AR 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 ß₂AR, but was only 35% as efficacious through the cloned porcine ß₁AR. These data confirm the ß₂AR selectivity of the RR stereoisomer, but suggest the partial agonism through the ß₁AR is sufficient to activate lipolysis through both subtypes in swine adipocytes.

Key Words: ß-Adrenergic Receptors • Lipolysis • Pigs

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