Hypotonic resistance of boar spermatozoa: sperm subpopulations and relationship with epididymal maturation and fertility

Hypotonic resistance of boar spermatozoa was investigated by measuring the ratio of live/dead spermatozoa (SYBR-14/propidium iodide) by flow cytometry after hypotonic stress. The survival rate of ejaculated spermatozoa incubated in hypotonic solutions ranging from 3 to 330 mmol/kg followed a sigmoid curve that fitted a simple logistic model. The critical osmolality value (Osm(crit)) at which 50% of spermatozoa died was determined with this model. Hypotonic resistance of spermatozoa increased with temperature between 15 and 39 degrees C and decreased after hydrogen superoxide treatment, but was not modified during 8 days of preservation in Beltsville thawing solution. Hypotonic resistance markedly decreased during epididymal maturation and after ejaculation as OSMcrit at 15 degrees C was 54.7 +/- 3.2, 68.5 +/- 10.6, 116.7 +/- 2.1 and 194.3 +/- 3.7 mmol/kg for the caput, corpus, cauda and ejaculated spermatozoa respectively. Hypoosmotic stress of 100 mmol/kg revealed a sperm subpopulation exhibiting increased hypotonic resistance compared with the whole ejaculate (OSmcrit= 67.8 +/- 2.1 mmol/kg). Consistent differences were observed between lean and standard breeds (Pietrain versus Large White) and between boars within the same breed. According to data collected by artificial insemination centers during a large-scale field trial, hypotonic resistance of ejaculates was found to be positively correlated with in vivo fertility.