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.