Submitted to: Journal of Animal Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/2/2002
Publication Date: 9/7/2002
Citation: Mills, S.E., Kissel, J., Bidwell, C.A., Smith, D.J. 2002. Stereoselectivity of porcine beta-adrenergic receptors for ractopamine stereoisomers. Journal of Animal Science 81:122-129. Interpretive Summary: Beta adrenergic agents have been known for over 20 years to increase lean muscle deposition and increase the efficiency of feed use in livestock species. The mechanisms through which beta-agonists work to enhance carcass composition are still unknown, but it is thought that beta-agonists are working by direct action on adrenergic receptors present in muscle and fat. This study was conducted to determine which chemical form of ractopamine, a beta-agonist approved for use in swine, interacts most efficiently with swine adrenergic receptors. Two cellular models were used, fat cells isolated from swine and Chinese hamster ovary cells expressing cloned beta-1 and beta-2 adrenergic receptors. It was found that the RR form of ractopamine (one geometric configuration of the four geometric configurations present in ractopamine) bound to beta-1 and beta-2 adrenergic receptors. Activation of adenylate cyclase occurred only through the beta-2 adrenergic receptor in Chinese hamster ovary cells and the RR stereoisomer was most efficient at stimulating receptors in hog adipocytes. These results suggest that ractopamine is not selective for the hog beta-1 and beta-2 adrenergic receptor in swine and that the mechanism of ractopamine's action is likely similar to that of other beta-agonists.
Technical Abstract: Ractopamine HCl is a beta-adrenergic receptor (beta AR) ligand that was recently approved for use in swine to enhance carcass leanness. Ractopamine is a mixture of four stereoisomers (RR, RS, SR, SS). In order to determine which stereoisomers are active in the pig and whether they exhibit beta AR subtype selectivity, receptor affinity and adenylyl cyclase activation were determined using cloned porcine beta-1 and beta-2 AR expressed in Chinese hamster ovary cells. Dissociation constants were determined by competitive displacement of [125I]iodocyanopindolol binding by ractopamine stereoisomers. The RR isomer had the highest affinity for both beta-1 and beta-2 AR. Dissociation constants for the other stereoisomers were higher relative to the RR stereoisomer. Isoproterenol stimulated adenylyl cyclase activity 600% relative to basal rates in CHO cells, regardless of beta-AR subtype. Ractopamine stereoisomers did not significantly (P>0.05) stimulate adenylyl cyclase through the beta-1 AR at moderate or high concentrations. In contrast, the RR isomer increased adenylyl cyclase activity 200 to 300%, relative to basal rates, through the beta-2 AR at moderate and high concentrations; the SR stereoisomer increased adenylyl cyclase activity nearly 100%. Neither the RS nor SS stereoisomers were effective at activating adenylyl cyclase activity through the beta-2 AR. A similar pattern of stereoselective activation of adenylyl cyclase was exhibited using porcine adipocytes. The porcine beta-AR exhibited stereoselectivity towards ractopamine stereoisomers with the RR isomer exhibiting the highest affinity for the beta-1 and beta-2 AR. In contrast, ractopamine stereoisomers seemed to be more effective at eliciting cAMP responses from beta-2 AR than beta-1 AR. The RR isomer is likely the functional stereoisomer of ractopamine.