Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2002
Publication Date: 12/12/2002
Citation: Mills, S.E., Spurlock, M.E., Smith, D.J. 2002. Beta-adrenergic receptor subtypes that mediate ractopamine stimulation of lipolysis. Journal of Animal Science 81:662-668. Interpretive Summary: Ractopamine is a beta-adrenergic agonist feed-additive that enhances the leanness of market hogs. The mechanisms of ractopamine's leanness-enhancing actions are unknown, but are believed to be mediated through beta-adrenergic receptors located on muscle and adipose tissue cells. The objective of this study was to determine what type of adrenergic receptor subtype the active stereoisomer (RR) of ractopamine activates in hog adipose tissue. It was found that ractopamine may stimulate adipose tissue lipolysis through either the beta-1 or beta-2 adrenergic receptor. Because the beta-1 adrenergic receptor appears to be the most common adrenergic receptor type in swine adipose tissue, ractopamine may mediate lipolytic effects through this receptor in swine. The Beta-1 AR may be the preferred target receptor because it is the most abundant subtype in swine adipocytes, but targeting the beta-2 AR should also result in reduced fat accretion in swine. These results suggest that ractopamine has the capacity to mediate leanness through a receptor subtype hypothesized to mediate the leanness-enhancing effects of other beta-agonists.
Technical Abstract: The RR stereoisomer of ractopamine exhibits the highest beta-AR affinity and signaling response of the four ractopamine stereoisomers. The RR isomer exhibits selective activation of the porcine beta-2 AR. Our objective was to determine the beta-AR subtypes that mediate the lipolytic response to ractopamine in swine adipocytes. Isoproterenol-stimulated lipolysis was inhibited in a dose-response by subtype-selective antagonists. Inhibition curves were biphasic using beta-1 AR antagonists (CGP 20712A and bisoprolol; both the beta-1 AR and beta-2 AR contributed to lipolysis with 50 to 60% of the response coming from the beta-1 AR. Inhibition with the beta-2 AR antagonist clenbuterol revealed only one class of beta-AR that closely approximated the kinetics of the beta-1 AR. Similar results were found when the RR isomer of ractopamine replaced isoproterenol as the lipolytic agent, except the estimated contribution of the beta-1 AR was 38%. That beta-2 AR antagonists did not detect a contribution of the beta-2 AR to lipolysis may indicate that the beta-1 AR masked the response to the beta-2 AR. Dose titration with the RR isomer in the presence of a saturating concentration of beta-1 AR or beta-2 AR antagonist indicated that each subtype was present in sufficient quantities to stimulate lipolysis near maximally. Both the beta-1 AR and beta-2 AR are functionally linked to lipolysis in swine adipocytes and ractopamine activates each subtype. The RR isomer of ractopamine stimulated cAMP accumulation with equal efficacy to isoproterenol through the cloned porcine beta-2 AR but was only 35% as efficacious through the cloned porcine beta-1 AR. These data confirm the beta-2 AR selectivity of the RR stereoisomer, but suggest the partial agonism through the beta-1 AR is sufficient to activate lipolysis through both subtypes in swine adipocytes.