The effect of Ractopamine stereoisomers on in vitro adipose tissue accretion in bovine subcutaneous and intramuscular adipose tissue depots

Recent National Beef Quality Audits have stated a need for a consistent and uniform eating experience with beef. Among the improvements suggested by the audits is increased intramuscular (IM) fat and decreased subcutaneous (SQ) fat. Beta-agonists are nutrient repartitioning agents, specifically directing energy from adipose tissue into muscle. Approval of Ractopamine Hydrochloride (Optaflexx(TM); June 2003) by the FDA for use in beef cattle should provide a tool to reduce SQ fat and thus, increase carcass yield in beef cattle. However, there is little data to evaluate the affect of Ractopamine on the IM adipose tissue depot and its effect on beef quality. To address this question, samples from SQ and IM adipose tissue depots were incubated with various substrates to evaluate the effect of Ractopamine on adipose tissue metabolism and substrate utilization. A 10-5 M concentration of the active (RR) stereoisomer of Ractopamine HCl increased (P < 0.05) the release of non-esterified fatty acids (2,536 vs. 5,984 nMol/g/h) in subcutaneous adipose tissue without affecting (P > 0.05) the intramuscular depot. The inactive (SS) stereoisomer of Ractopamine did not affect lipolytic rates in either depot. Lipogenic rates in SQ adipose tissue were not affected ( P > 0.05) by any of the treatments. Lipogenic rates were decreased (P < 0.05) below basal in IM adipose tissue by 10 -8 M insulin, 10-5 M RR or 10-5 M SS. Adipose tissue from the SQ depot preferred acetate almost 4-fold over glucose (39.24 vs. 10.16 nMol/mg/h) as a substrate for fatty acid synthesis. There were no differences in substrate utilization by IM adipose tissue. These data suggest that bovine SQ adipose tissue exhibits similar responses to the RR stereoisomer of Ractopamine as other species. A similar effect in vivo would improve feed efficiency, cost per pound of gain and decrease labor costs associated with trimming the excess fat off of the carcass. Intramuscular adipose tissue is not affected by addition of the RR stereoisomer to incubations. The addition of the RR stereoisomer to lipolytic incubations with IM adipose tissue did not affect free fatty acid release over basal levels. If this is replicated in vivo, then animals fed Ractopamine might improve overall carcass leanness while not adversely affecting abundance of IM fat or eating quality.