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 volu me (solids and water)). Most evaluations of osmotic sperm cell behaviour ha ve been based on the mean volume of the cell population. In the present stu dy, both mean and modal volumes of samples of sperm were evaluated. Both me an and modal volumes responded to environmental osmolality via the Boyle-Va n't Hoff relationship; however the modal volume showed a more sensitive res ponse than the mean volume. This was confirmed for both ejaculated and epid idymal spermatozoa. After incubation under capacitating conditions, the dif ference in modal and mean volume response of ejaculated sperm was considera bly diminished and, in epididymal sperm, completely abolished. The sperm os motic behaviour was still consistent with the Boyle-Van't Hoff equation, bu t the apparent osmotically inactive modal cell volume decreased after expos ure to capacitating conditions in both ejaculated and epididymal sperm samp les. The changes in epididymal sperm were more intensive. Due to its enhanc ed sensitivity to environmental osmolality and incubation under capacitatin g conditions, the modal volume could be used as a parameter for evaluating sperm population response, such as for detecting environmentally or cryopre servation-induced membrane changes.