Effect of the addition of exogenous progesterone and the progesterone receptor inhibitor (RU 486) on boar cryopreservation semen extenders

Cryopreservation of porcine spermatozoa is detrimental due to their high sensitivity to cold shock, leading to changes akin to capacitation, known as cryocapacitation. These changes, including the acrosomal reaction, hypermotility induction, and protein phosphorylation, might be influenced by the presence of progesterone in seminal plasma and egg yolk, used in most freezing extenders. We tested the effect of various progesterone concentrations added to the freezing extenders (1, 10, and 100 μg/mL). At 100 μg/mL, progesterone decreased the proportion of straightness and tended to reduce viability and the proportion of progressive motility (p < 0.1). At 10 μg/mL, it increased reacted acrosomes in dead sperm (p < 0.05), protein phosphorylation rate (p < 0.05), and tended (p < 0.1) to enhance linear movement compared to the control. To counteract the capacitating effect of progesterone, we examined the effect of antiprogesterone mifepristone (RU 486) at concentrations of 5, 10, 20, 50, 100, and 200 μM, and co-incubated 10 μM of RU 486 with 10 μg/mL of progesterone. RU 486 maintained capacitation levels and motility parameters similar to the control, although high concentrations (100 μM) tended (p = 0.152) to increase protein phosphorylation. Co-incubation reduced the acrosome reaction in dead sperm, and RU 486 appeared to prevent hypermotility stabilizing motility and viability parameters compared to samples with progesterone alone. Protein phosphorylation increased and RU 486 could not restore capacitation to control levels due to its competitive antagonism for progesterone receptors, having less affinity than progesterone, which displaces RU 486 at high concentrations, allowing normal sperm capacitation.