The osmotic reactivity of boar spermatozoa during incubation in vitro was studied using a hypo-osmotic swelling test in conjunction with electronic measurement of cell volume. Sperm populations showed fluctuations in both iso-osmotic cell volume and hypo-osmotic volume response that fitted mathematical models for periodicity. Significant differences of frequency and amplitude were observed during sperm incubation under capacitating conditions as compared with those under non-capacitating conditions. In addition, different boars showed specific differences in their fluctuation characteristics under capacitating conditions. During incubation under capacitating conditions, a decrease in osmotic reactivity was observed that correlated with a decrease in motility, while the absolute value of the earliest maximum of the osmotic-induced response correlated with an increase in the proportion of discharged acrosomes. The time course of the cyclical behaviour of osmotic reactivity may be a useful parameter for assessing boar sperm response to capacitating conditions.
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