• Sperm function;
  • oviduct;
  • plasma membrane;
  • cell volume;
  • fertility

ABSTRACT: The plasma membrane is a key organelle with respect to sperm fertilizing ability. A sensitive way of testing plasma membrane functionality is to examine the sperm ability to moderate its swelling in response to hypo-osmotic stress (volume regulatory ability) using an electronic cell counter to assess cell volume changes. In this study of frozen-thawed bull sperm, we examined the relationship among sperm-oviductal epithelium binding capacity, osmotically induced swelling response, volume regulatory ability, and standard spermatologic parameters. Sperm cell volume distributions were measured under iso-osmotic conditions and after hypo-osmotic stress. The relative volume shift was calculated by comparing modal values of the cell volume distributions during transition from iso-osmotic to hypo-osmotic conditions. Significant correlations were found between volumetric parameters and sperm-oviduct binding capacity. Both the relative volume shift and regulative volume decrease correlated positively and significantly with the sperm-oviduct binding capacity. No significant correlations were found between sperm volumetric parameters and any of the standard sperm parameters with the exception of forward motility of Percoll-washed sperm. However, the use of multiple regression models improved the prediction level for binding capacity when motility parameters were combined with membrane integrity and volumetric parameters (R2 = .84). Spermatozoa of bulls with high nonreturn rates responded to hypotonicity as “perfect osmometers.” Subfertile bulls had lower binding indices and deficiencies in volume recovery after hypotonic challenge, indicating that intact volume regulatory ability is a necessary prerequisite for binding to oviductal epithelium and is related to fertility. Volumetric parameters therefore could be used as tools in semen evaluation programs.