Preservation of stallion sperm quality by native phosphocaseinate: a direct or indirect effect?

Milk-based diluents are generally considered efficient for survival of stallion spermatozoa in vitro. However, milk is a complex and variable medium and native phosphocaseinate is a milk component that is more efficient for preservation of sperm motility and fertility, although the mechanisms involved in this protection have not yet been elucidated. The aim of the present study was to characterize the interactions between native phosphocaseinate and equine spermatozoa. No binding between sperm membranes and native phosphocaseinate was observed using indirect immunofluorescent staining or electron microscopy and native phosphocaseinate showed no indirect protective effect on spermatozoa after incubation in two distinct storage chambers separated by a dialysis membrane. In addition, the intracellular Ca2+ concentrations in spermatozoa did not alter after incubation in native phosphocaseinate. The favourable influence of the native micelle structure of the casein was observed only for spermatozoa stored at 15 degrees C. In conclusion, the results of the present study indicate that native phosphocaseinate has a direct protective effect on equine spermatozoa, without any evidence of binding to sperm membranes.