Spermatozoa in the sperm-peak-fraction of the boar ejaculate show a lower flow of Ca2+ under capacitation conditions post-thaw which might account for their higher membrane stability after cryopreservation

Boar spermatozoa collected in the ejaculate sperm peak-portion (P1, first 10 mL of the sperm-rich fraction, SRF), had shown a higher resilience to freezing and thawing compared to spermatozoa from the rest of the ejaculate (2nd portion of the SRF plus the post-sperm-rich fraction, PSRF), even when using a simplified freezing technique, as long as spermatozoa were incubated in their own seminal plasma (SP). This experiment studied the stability of P1- and SRF-P1 boar spermatozoa frozen in MiniFlatPacks (MFP), post-thaw, using flow cytometry. Since spermatozoa from either portion showed similar cryosurvival and low proportions of unstable membranes (<3%, annexin-V/propidium iodide staining), and only a tendency for SRF-P1 live spermatozoa to depict acrosome exocytosis (FITC-PNA/PI/H33342); they were explored for Ca2+ contents using a Fluo-4 probe under in vitro capacitating conditions (mBO+ medium), as well they were tested for their ability to sustain a short Ca2+-ionophore (A23187) in vitro challenge. The proportions of live spermatozoa depicting high Ca2+-levels were initially <2% but increased over incubation time, particularly in SRF-P1(P < 0.05), while proportions of live spermatozoa with low Ca2+-levels were basically constant over incubation time (∼11–14%), for either portion. Incubation in capacitation medium did not modify the proportions of low-Ca2+ but dramatically increased the proportions of high-Ca2+ spermatozoa (P < 0.001) already after 15 min exposure, highest for SRF-P1 spermatozoa. While the proportion of live spermatozoa with intact acrosome was significantly decreased among SRF-P1 (P < 0.001), that of P1-spermatozoa remained unchanged, probably owing to the lowest relative content of cytosolic Ca2+. The results suggest that spermatozoa in the P1-portion are more resilient to express acrosome exocytosis post-thaw compared to those bathing in the rest of the SRF-fraction when cryopreserved using a simplified technique, in MFPs.