Heterogeneous osmotic behaviour in boar sperm populations and its relevance for detection of changes in plasma membrane.

The spermatozoa of most mammals behave as 'perfect osmometers'. The volume response to osmolality obeys the Boyle-Van't Hoff relationship (i.e. volume changes are determined by the osmotically active fraction of the cell volume (solids and water)). Most evaluations of osmotic sperm cell behaviour have been based on the mean volume of the cell population. In the present study, both mean and modal volumes of samples of sperm were evaluated. Both mean and modal volumes responded to environmental osmolality via the Boyle-Van't Hoff relationship; however, the modal volume showed a more sensitive response than the mean volume. This was confirmed for both ejaculated and epididymal spermatozoa. After incubation under capacitating conditions, the difference in modal and mean volume response of ejaculated sperm was considerably diminished and, in epididymal sperm, completely abolished. The sperm osmotic behaviour was still consistent with the Boyle-Van't Hoff equation, but the apparent osmotically inactive modal cell volume decreased after exposure to capacitating conditions in both ejaculated and epididymal sperm samples. The changes in epididymal sperm were more intensive. Due to its enhanced sensitivity to environmental osmolality and incubation under capacitating conditions, the modal volume could be used as a parameter for evaluating sperm population response, such as for detecting environmentally or cryopreservation-induced membrane changes.