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Effect of norethisterone and its A‐ring reduced metabolites on the acrosome reaction in porcine spermatozoa

Summary. The synthetic progestin, norethisterone (NET), has been reported as a contragestational postcoital agent in humans, rodents and rabbits. The effect and molecular mechanisms of NET and its A‐ring reduced metabolites, 5α‐NET and 3β5α‐NET, on the acrosome reaction (AR) are unknown. The aim of this study was to assess the effect of these compounds on an in vitro progesterone‐induced AR in porcine spermatozoa. The spermatozoa were obtained from semen ejaculated by proven fertile adult pigs. Seminal plasma removed and incubated under capacitating conditions was performed in TALP‐Hepes medium for 4 h. Progesterone (P4) and three different progestins: norethisterone (NET), 5α‐norethisterone (5α‐NET) and 3β5α‐NET were then added at equimolar doses, and the spermatozoa were incubated for 15 min. Double‐staining with PSA‐FITC and Hoechst‐33258 assessed the AR and sperm viability. Both P4 and NET induced the AR, while 5α‐NET not only did not induce this process, but was able to block the effect of P4 on the spermatozoa. 3β5α‐NET was not able to inhibit P4 action. These results suggest that NET and its A‐ring reduced metabolites act in different ways on the progesterone‐induced AR in porcine spermatozoa.