Comparative Freezing Response of Ejaculated and Epididymal Bovine Spermatozoa

In the present study, we report the effects of cooling ejaculated and epididymal bovine sperm from the same animals with and without a cryoprotective agent, glycerol. Water transport data during freezing of ejaculated and epididymal bovine sperm cell suspensions were obtained at a cooling rate of 20 °C/min under two different conditions: i) in the absence of cryoprotective agents, CPAs; and ii) in the presence of 0.7 M glycerol. Using previously published values, the bovine sperm cell was modeled as a cylinder of length 39.8 μm and a radius of 0.4 μm with an osmotically inactive cell volume, V_b, of 0.61V_o, where V_o is the isotonic cell volume. The subzero water transport response is analyzed to determine the variables governing the rate of water loss during cooling of bovine spermatozoa, i.e. the membrane permeability parameters (reference membrane permeability, L_pg and activation energy, E_Lp). The predicted best-fit permeability parameters ranged from, L_pg = 0.021 to 0.038 μm/min-atm and E_Lp = 27.8 to 41.1 kcal/mol. The subzero water transport response and consequently the subzero water transport parameters are not significantly different between the ejaculated and epididymal bovine spermatozoa under corresponding cooling conditions. If this observation is found to be more generally valid for other mammalian species as well, then the sperm extracted from the testicles of an animal during post-mortem can also be optimally cryopreseved using procedures similar to those derived for ejaculated sperm.