GROWTH-CHARACTERISTICS OF RATS RECEIVING RACTOPAMINE HYDROCHLORIDE AND THE METABOLIC DISPOSITION OF RACTOPAMINE HYDROCHLORIDE AFTER ORAL ORINTRAPERITONEAL ADMINISTRATION

Objectives of this study were 1) to measure the effect of oral or i.p. administration of ractopamine HCl on growth and feed utilization in r ats, 2) to determine the total adsorption of [C-14]ractopamine HCl aft er oral administration, and 3) to determine the disposition of radioac tivity and the urinary elimination of unchanged [C-14]ractopamine in r ats after oral or i.p. administration of [C-14]ractopamine. Twenty-sev en female Sprague-Dawley rats (164.6 +/- 5.7 g) were randomly assigned to control (CONT), oral (ORAL), and i.p. (IP) treatments. Control and ORAL rats were implanted i.p with sham pumps, and IP rats were implan ted i.p. with osmotic pumps primed to deliver 312 mu g of ractopamine HCl per 24 h. Control and IP rats received no dietary ractopamine, but ORAL rats received 20 mg of ractopamine HCl/kg of diet. The IP rats h ad greater cumulative net weight gains and ADG on d 2, 6, 8, 10, and 1 2 than CONT rats. The ADFI was greater for ORAL rats on d 2 and 4 than for CONT rats, and the gain:feed ratio was greater on d 2, 6, 8, 10, and 12 for IP rats than for CONT rats. Net weight gain, ADG, and gain: feed ratio did not differ between ORAL and CONT rats. Absorption of ra dioactivity administered orally as [C-14]ractopamine (2.9 mg) was 87.9 % during a 24-h experimental period; biliary, urinary, and fecal excre tion of radioactivity was 58.5%, 28.7%, and 1.4% of that administered, respectively. Urine from rats dosed orally with [C-14]ractopamine con tained 1.9% of the radioactivity as the parent compound, and urine fro m rats dosed i.p. contained 22.6% of the radioactivity as parent racto pamine. Ractopamine HCl increased weight gain and efficiency of feed u tilization when administered i.p. to rats, but not when administered o rally. The ineffectiveness of oral ractopamine for stimulating the gro wth of rats was probably due to extensive presystemic metabolism of ra ctopamine.