Quantification of circulating beta-endorphin and white blood cells in heifers after intrauterine administration of Escherichia coli endotoxin

Previous work established that repeated intrauterine infusions of Escherichia coli (E. coli) endotoxin during the follicular phase in heifers abolished or delayed the preovulatory surge of luteinizing hormone (LH). To test the hypothesis that endotoxin-induced increases in blood-borne beta-endorphin (B-EP) might contribute to the observed suppression of LH, serum concentrations of B-EP were quantified in five treated and three control heifers over the 60-hour period of 10 intrauterine infusions. Serum concentrations of beta-endorphin/beta-lipoprotein (B-EP/B-LPH) were quantified in unextracted serum by radioimmunoassay validated for that purpose. Concentrations ranged from 0.1 to 0.8 ng B-EP equivalents per ml, with an overall mean of 0.23 +/- 0.12 ng/ml. There were no differences between treated and control heifers despite the fact that treated heifers displayed elevated serum cortisol. The number of total and specific types of white blood cells did not differ between treatment and control groups or over time. In summary, the lack of correlation between the suppression of the LH surge and elevation of serum B-EP/B-LPH does not support the hypothesis that blood-borne B-EP may be involved in the suppression of the LH surge after intrauterine infusion of endotoxin.