Mimosine, administered orally, and two related compounds of chemical defleecing agents for sheep

Abstract

Oral doses of mimosine (ranging from 400 to 800 mg/kg body weight) were given to 71 sheep, either as two successive daily doses (10 sheep) or as a single dose (61 sheep). The effectiveness of these treatments for defleecing was assessed and in some sheep measurements were made of the concentration of mimosine in plasma after dosing, and of the rate of wool growth before and after dosing. In addition, the effectiveness as defleecing agents of two related compounds (an analogue of mimosine, 'isomimosine', and a metabolite, 3,4-dihydroxypyridine (DHP)) was assessed. Two successive daily doses of mimosine (300 mg/kg/day) allowed all sheep to be defleeced. With sheep consuming a daily ration of 600 g, a single oral dose of mimosine (400 mg/kg) was sufficient to allow defleecing of most sheep; when the daily ration was 1200 g, a dose of 600 mg/kg was required to allow defleecing of most sheep. The effectiveness of oral doses for defleecing could be explained by the absorption of mimosine over a period in excess of 24 hr. Concentrations of mimosine in plasma were usually above 100 µmoles/l 24 hr after an effective dose. Failure to achieve complete cessation of fibre growth occurred occasionally, irrespective of dose level, and two sheep died after dosing. Fasting prior to dosing appeared to obviate the effects of previous high nutrition, but increased the risk of toxic effects. Fasting resulted in exceptionally high concentrations of mimosine in plasma (200–300 µmoles/l) 1 and 2 days after dosing. Both fibre diameter and mass of wool grown were increased in the early regrowth after dosing with mimosine. 'Isomimosine' had a similar potency to mimosine for stopping fibre growth in sheep, whether given as an intravenous infusion or as a single oral dose. DHP, given as an intravenous infusion, was ineffective as a defleecing agent.