Osmotic stress and membrane phase changes during freezing of stallion sperm: mode of action of cryoprotective agents.

The aim of this study was to determine how different membrane-permeable and -impermeable cryoprotective agents modulate tolerance of stallion sperm to osmotic stress and stabilize membranes during cryopreservation. Special emphasis was on hydroxyl ethylene starch (HES), which exposes cells to minimal osmotic stress due to its large molecular weight. Percentages of motile sperm post-thaw were found to be similar when glycerol, sucrose, and HES were used at their optimal concentrations. Percentages of plasma membrane intact sperm after return to isotonic medium were highest for HES. Fourier transform infrared spectroscopy studies were carried out to study subzero membrane phase and permeability behavior. Cryoprotectants were shown to decrease the initial rate of membrane dehydration during freezing, decrease the activation energy for water transport, and increase the total extent of freezing-induced dehydration. Freezing studies with liposomes as a model system showed that only the membrane-permeable cryoprotective agents glycerol and ethylene glycol protected membranes against leakage, whereas egg yolk, sucrose, and HES did not. Differential scanning calorimetry studies showed that sucrose and HES raise the glass transition temperature of the freezing extender and the difference in heat capacity associated with the glass transition. This indicates that these compounds enable formation of a stable glassy matrix at higher subzero temperatures. Sperm cryosurvival rates can be increased by combining different cryoprotectants with different protective functions; membrane permeable cryoprotective agents stabilize membranes and modulate the rate of cellular dehydration, whereas di- and polysaccharides increase the glass transition temperature and facilitate storage and handling at higher subzero temperatures.