Osmotic properties of boar spermatozoa and their relevance to cryopreservation

A series of six experiments was conducted to determine the fundamental cryobiological properties of boar spermatozoa to develop optimal approaches for cryopreserving this important cell type. In the first experiments, boar spermatozoa samples were diluted in various osmolalities of experimental solutions (185-900 mOsmol kg-1) to provide hypo-, iso-, and hyperosmotic conditions. Equilibrium cell volumes (Expts 1 and 2) were measured after exposure for 3 min and the change in cell volume was measured over time using an electronic particle counter (Expt 3). The isosmotic cell volume was found to be 26.3 +/- 0.39 microns 3 (mean +/- SEM; n = 5). Over this range of osmolalities, boar spermatozoa behaved as linear osmometers (a linear volume versus 1/osm plot, r2 = 0.99) with an osmotically inactive cell fraction of 67.4 +/- 4.5%. The rate of water permeability (Lp) was determined to be 1.03 +/- 0.05 microns min-1 atm-1, which was consistent within and among donors (P > 0.130). A second series of experiments was performed to determine the effect of temperature and osmolality on boar sperm motility (Expt 4), and the effect of osmolality on the integrity of the sperm plasma membrane and its temperature dependence. Plasma membrane integrity was measured before and after boar spermatozoa were returned to an isosmolality (Expt 6). Motility was not affected at 30 degrees C, relative to that at room temperature, but was significantly decreased (P < 0.05) at 8 degrees C and 0 degree C (yielding a relative reduction to 85% and 35% of original motility, respectively; n = 6). Sperm motility was not significantly decreased (P > 0.05) until the osmolality reached 210 mOsmol kg-1, at which time motility began to decrease from 95% to 10% of the original value at 90 mOsmol kg-1. The integrity of the plasma membrane of boar spermatozoa was found to be dependent on temperature, donor and osmolality, decreasing significantly (P < 0.05) below room temperature, and below 185 mOsmol kg-1 (P < 0.05). There was no significant difference (P > 0.10) in the integrity of the plasma membrane of the samples before and after returning to 290 mOsmol kg-1, indicating that osmotic damage occurs during the initial change from isosmotic to hyposmotic media. These osmotic characteristics could be used to determine optimal conditions for cryopreservation of boar spermatozoa.