Cryoprotective agent and temperature effects on human sperm membrane permeabilities: convergence of theoretical and empirical approaches for optimal cryopreservation methods.

Previous reports have left unresolved discrepancies between human sperm cryopreservation methods developed using theoretical optimization approaches and those developed empirically. This study was designed to investigate possible reasons for the discrepancies. Human spermatozoa were exposed to 1 mol/l glycerol, 1 mol/l dimethyl sulphoxide (DMSO), 1 mol/l propylene glycol (PG) or 2 mol/l ethylene glycol (EG) at 22, 11 and 0 degrees C, then returned to isosmotic media while changes in cell volume were monitored. Activation energies (E(a)) of the hydraulic conductivity (L(p)) in the presence of cryoprotective agents (CPA) (L(p)(CPA)) were 22.2 (DMSO), 11.9 (glycerol), 15.8 (PG), and 7.8 (EG) kcal/mol. The E(a) values of the membrane permeability to CPA (P(CPA)) were 12.1 (DMSO), 10.4 (glycerol), 8.6 (PG) and 8.0 (EG) kcal/mol. These data indicated that even at low temperatures, EG permeates fastest. The high L(p)(CPA) in the presence of EG and low associated E(a) would allow spermatozoa to remain closer to equilibrium with the extracellular solution during slow cooling in the presence of ice. Collectively, these data suggest that the increase of the E(a) of L(p) in the presence of CPA at low temperature is the likely reason for the observed discrepancy between theoretical predictions of spermatozoa freezing response and empirical data.