Response of goat sperm to hypoosmotic steps modelled probit analysis

Hypoosmotic swelling test (HOS) has been proposed by many authors to evaluate the functional integrity of the sperm membrane. Our approach in this experiment has consisted in exposing spermatozoa to a wide range of osmotic pressures then evaluating the reacted sperm cells by flow cytometry and finally modelling the sperm cell responses. Semen samples were diluted in skim milk or NPPC (native phosphocaseinate) extenders, and stored at 4 degrees C for 3 days. At D0 and D3 aliquots from each ejaculate (n=12) were submitted to seven hypoosmotic solutions varying from 230 to 10mOsm/kg. Sperm samples were analyzed using flow cytometry to determine two populations of spermatozoa identified by propidium iodide (PI): PI+ (including PI, red fluorescence) and PI- (excluding PI, no fluorescence). Spermatozoa PI+ were considered as spermatozoa with membrane damages. PI+ exhibited a high variation from 230 to 10mOsm/kg which was considered as a dose-response curve. Data were modelled using Mixed procedure and probit analysis to a sigmoid curve. Each model curve characterized the profile of response of the variable PI+ to the range of osmotic pressure from 230 to 10mOsm/kg. The estimated parameters modelling the sigmoid curves are discussed in order to evaluate the effect of extender (skim milk versus NPPC) and duration of preservation (D0 versus D3). Such modelling could help to differentiate storage method ejaculates within males or between male, contributing therefore to improve semen technology.