Exposure of progesterone receptors on the plasma membranes of stallion spermatozoa as a parameter for prediction of fertility.

Subfertility in stallions is attributed to the inability of spermatozoa to undergo the acrosome reaction in response to progesterone. In the present study, it was assessed whether there is a correlation between stallion fertility, defined on the basis of first cycle foaling rate and first cycle 'non-return rate', and the proportion of spermatozoa with exposed progesterone receptors on their plasma membranes. Semen from Dutch Warmblood (n=10) and Friesian (n=4) stallions was analysed. Progesterone 3-(o-carboxymethyl) oxime-BSA coupled with fluorescein isothiocyanate was used as a progesterone receptor probe and ethidium homodimer was used as a supravital stain. A high correlation was observed between the proportion of spermatozoa with exposed progesterone receptors and stallion fertility (r > 0.70; P < 0.01). This result indicates that exposure of progesterone receptors is a potential parameter for predicting stallion fertility.