Changes in oestrone sulphate concentrations in peripheral plasma of Pony mares associated with follicular growth, ovulation and early pregnancy

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Abstract
A simple and rapid (< 2 h) immunoassay method was developed based upon a novel separation technique called LIDIA (Ligand Differentiation Immunoassay), enabling direct estimation of the concentration of estrone sulfate in ethanolic extracts of blood plasma. An antiserum raised against estrone-3-glucuronyl-BSA [bovine serum albumin] was used which showed a higher cross-reaction with the sulfate than the glucuronide metabolite. The assay had a sensitivity of 5.2 pg/tube and acceptable inter-(< 18%) and intra-(< 8.5) assay precision. Analysis of samples of peripheral venous plasma obtained daily from Pony mares showed that the mean concentration of estrone sulfate started to rise from a baseline value (< 300 pg/ml) at 6 days and reached a peak (> 850 pg/ml) at 2 days before follicular rupture as determined by rectal palpation. Progesterone concentrations only started to rise above baseline (< 0.5 ng/ml) on the day of ovulation and reached a peak 8 days later. Analysis of samples obtained during the first 30 days of pregnancy showed that there was no increase in estrone sulfate at the time estrus would have been expected had the mares not conceived.