Growth characteristics of rats receiving ractopamine hydrochloride and the metabolic disposition of ractopamine hydrochloride after oral or intraperitoneal 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 rats, 2) to determine the total absorption of [14C]ractopamine HCl after oral administration, and 3) to determine the disposition of radioactivity and the urinary elimination of unchanged [14C]ractopamine in rats after oral or i.p. administration of [14C]ractopamine. Twenty-seven 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 implanted i.p. with osmotic pumps primed to deliver 312 micrograms 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 had greater cumulative net weight gains and ADG on d 2, 6, 8, 10, and 12 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 radioactivity administered orally as [14C]ractopamine (2.9 mg) was 87.9% during a 24-h experimental period; biliary, urinary, and fecal excretion of radioactivity was 58.5%, 28.7%, and 1.4% of that administered, respectively. Urine from rats dosed orally with [14C]ractopamine contained 1.9% of the radioactivity as the parent compound, and urine from rats dosed i.p. contained 22.6% of the radioactivity as parent ractopamine. Ractopamine HCl increased weight gain and efficiency of feed utilization when administered i.p. to rats, but not when administered orally. The ineffectiveness of oral ractopamine for stimulating the growth of rats was probably due to extensive presystemic metabolism of ractopamine.