Urinary excretion of immunoreactive sporidesmin metabolites in sheep in relation to factors influencing susceptibility to sporidesmin intoxication

Aim. To study the urinary disposition of orally administered sporidesmins A and D in sheep and identify factors influencing their kinetics, particularly the influence of breeding for resistance and susceptibility to sporidesmin, the mycotoxin responsible for the hepatogenous photosensitisation, facial eczema. Methods. A competitive ELISA was used to monitor urinary output of immunoreactive metabolites after the intraruminal administration, to female Romney sheep, of either sporidesmin A or sporidesmin D, the nontoxic analogue. Preliminary characterisation of metabolites was carried out using HPLC with fractions monitored by ELISA. Results. Maximum urinary excretion rates of immunoreactive metabolites occurred 2-8 h after dosing with sporidesmin D and 15-30 h after dosing with sporidesmin A. Sporidesmin D caused no liver injury, as detected by changes in serum enzyme activity, while the liver injury caused by sporidesmin A was greatest for the sheep with the highest cumulative output of metabolite. When sporidesmin D was administered in two separate doses to sheep bred for either resistance or susceptibility to facial eczema, the variability of metabolic output between sheep within groups was much less after the second dose. The mean urinary metabolite excretion was greater for the susceptible than the resistant sheep but the difference was not significant. Potentiation (caused by pre-administration of small doses of sporidesmin A) resulted in a more severe reaction to the dosed sporidesmin A. Urinary output of metabolite was less in the potentiated than in the unpotentiated sheep. When resistant and susceptible sheep were dosed with sporidesmin A after potentiation there was no difference between them in their cumulative totals or excretion rates of immunoreactive metabolites. However, the volume of urine produced by the susceptible sheep was lower and less variable than the resistant sheep and consequently the concentration of their urinary metabolites was higher. Preliminary ELISA examination of HPLC-fractionated urine from a sheep dosed with sporidesmin A indicated the presence of several metabolites of sporidesmin. Conclusion. Sporidesmin A and metabolites are rapidly excreted in urine but not as rapidly as sporidesmin D and its metabolites. Only minor differences between sheep bred for resistance and susceptibility were seen. Potentiation caused a more severe reaction to sporidesmin A and less urinary excretion of the sporidesmin and its metabolites. Clinical relevance. This work is part of a programme with the aim of identifying FE-resistant animals without the need for sporidesmin dosing.