Effects of ractopamine HCl stereoisomers on growth, nitrogen retention, and carcass composition in rats

The objectives of this study were to determine the effects of ractopamine HCl (RAC) stereoisomers (RR, RS, SR, and SS) on performance, carcass composition, and nitrogen retention in growing female rats. Forty-eight rats (eight rats/treatment) were treated with 0 or 320 microg/d of RAC or with 80 microg/d of the RR, RS, SR, or SS stereoisomers of ractopamine. Rats had free access to feed and water before and during the experiment. Ractopamine and stereoisomers were delivered via i.p. implanted osmotic pumps for 14 d, and rats were then slaughtered. Control rats were fitted with osmotic pumps containing saline. Ractopamine increased (P < .05) feed intake (d 1 to 6); body weight; carcass CP; and intake, apparent absorption, retention, and retained:intake ratio of CP on d 1 to 6 of the study. Ractopamine decreased (P < .05) carcass lipid and visceral lipid. Rats dosed with the RR stereoisomer responded similarly to rats dosed with RAC, except for carcass lipid. Carcass lipid was decreased (P < .01) by RAC relative to controls, but it was not different from controls in rats treated with the RR isomer. Compared with controls, BW, carcass CP, and CP retention were increased by the RR stereoisomer, and visceral lipid was decreased. The RS isomer also decreased visceral lipid (P < .10), but variables measured in rats dosed with the RS, SR, and SS isomers generally did not differ from controls. Results of this study indicate that the RR isomer of RAC is responsible for a majority of the leanness-enhancing effects of RAC in rats.