β-Adrenergic receptor subtypes that mediate ractopaminestimulation of lipolysis1,2

Abstract

Ractopamine HCl is an beta-adrenergic receptor (betaAR) 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 beta2AR, which might limit the lipolytic response to ractopamine because the betaAR is the predominant subtype in swine adipocytes and may mediate most of the lipolytic response. Therefore, we determined the betaAR subtypes that mediate the lipolytic response to ractopamine in swine adipocytes. In order to confirm the predominant role of the beta1AR in porcine adipocytes, isoproterenol-stimulated lipolysis was inhibited by increasing doses of subtype-selective antagonists. Inhibition curves were biphasic using beta1AR antagonists (CGP 20712A and bisoprolol) and curve analysis indicated that both beta1AR an beta2AR contributed to lipolysis with 50 to 60% of the response coming from the beta1AR. Inhibition with the beta2AR antagonist clenbuterol revealed only one class of betaAR that closely approximated the kinetics of the beta1AR. When the RR isomer of ractopamine was the lipolytic agent, similar results to isoproterenol were observed, except that the estimated contribution of the beta1AR was 38%. That beta2AR antagonists did not detect a contribution of the beta2AR to lipolysis may indicate that the beta1AR masked the response to the beta2AR. Dose titration with the RR isomer in the presence of a saturating concentration of beta1AR or beta2AR antagonists indicated that each subtype was present in sufficient quantities to stimulate lipolysis near maximally. Data indicate that both the beta1AR and beta2AR 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 beta2AR, but was only 35% as efficacious through the cloned porcine beta1AR. These data confirm the beta2AR selectivity of the RR stereoisomer, but suggest the partial agonism through the beta1AR is sufficient to activate lipolysis through both subtypes in swine adipocytes.

Ractopamine, a Livestock Feed Additive, Is a Full Agonist at Trace Amine–Associated Receptor 1

“…Contrary to expectations based on the literature (Mills et al, 1990(Mills et al, , 2003, our in vitro results failed to confirm RAC as an agonist of the hb 2 AR coexpressed with hCFTR in Xenopus oocytes. Importantly, whether the other documented target of RAC's actions, human b 1 AR, responds to RAC when coexpressed with hCFTR in Xenopus oocytes remains to be investigated.…”

“…Importantly, whether the other documented target of RAC's actions, human b 1 AR, responds to RAC when coexpressed with hCFTR in Xenopus oocytes remains to be investigated. Using an in vitro cAMP assay, Mills et al (2003) reported RAC-stimulated cAMP accumulation in Chinese hamster ovary cells expressing cloned porcine b 1 AR and b 2 AR. Consequently, the differences between their results and ours could be due to the fact that the two studies used receptors from different species, different expression systems, and different methods of monitoring changes in intracellular cAMP levels.…”

Physiopathological changes related to the use of ractopamine in swine: Clinical and pathological investigations

“…Ractopamine (RAC) is a ß-adrenergic agonist (ß-agonist) which at repartitioning doses has been found to increase muscle growth and decrease fat deposition in different animal species (Baker et al, 1984;Mersmann, 1998;Mills and Spurlock, 2003;Ricks et al, 1984;Sillence, 2004;Watkins et al, 1990). In pigs and cattle treated during the final period of the finishing improved feed efficiency, heavier carcass weights, and higher dressing percentages have been documented (Gu et al, 1991a,b; Laudert et al, 2005a,b; Poletto et al, 2009;See et al, 2004).…”

Effect of ractopamine on Nile tilapia in the end of grow-out period

“…Furthermore, according to recent works focusing on lipogenic gene expression in the adipose tissue of finishing pigs, ractopamine can reduce the RNA transcription of genes related to lipid synthesis, such as sterol regulatory binding protein-1 (SREBP-1), which is a transcriptional factor that drive genes involved in the synthesis of fatty acids (Horton et al, 2003) and fatty acid synthase (Reiter et al, 2007;Halsey et al, 2011), a key enzyme involved in the synthesis of fatty acids (Ferreira et al, 2013). Although no difference was found in the weight gain between control and treatment groups during the experimental period (average of 170 g) ( Table 3), considering that ractopamine may have more efficacy on blocking lipogenesis instead of stimulating lipolysis (Mills et al, 2003), the animals that received ractopamine may have had a decrease of lipogenesis rate, especially at 8 mg kg −1 of ractopamine. This could be supported by the fat percentage difference (P<0.05) verified in abdominal musculature at 8 mg kg −1 of ractopamine (Table 2).…”