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

The study explores how various cryoprotective agents react and modulate the stress tolerance of stallion sperm during cryopreservation. It particularly focuses on hydroxyl ethylene starch (HES), with findings suggesting that optimal concentrations of glycerol, sucrose, and HES provide similar motility of sperm post-thaw. Other factors examined include the effectiveness of cryoprotectants in reducing membrane dehydration and adjusting water transport during the freezing process.

Research Objectives

• This research aims to understand how different cryoprotective agents, both membrane-permeable and -impermeable, influence stallion sperm’s tolerance to osmotic stress and stabilize membranes during cryopreservation.
• Among these agents, the study particularly focuses on hydroxyl ethylene starch (HES) due to its unique property of causing minimal osmotic stress to cells, thanks to its large molecular weight.

Key Findings

• Post-thaw, the motility of sperm was found to be similar when glycerol, sucrose, and HES were used at their optimal concentrations for cryoprotection.
• When stallion sperm were returned to isotonic medium post-thaw, the highest percentages of intact plasma membranes were observed with HES.

Understanding Membrane Phase and Permeability Behavior Using Fourier Transform Infrared Spectroscopy

• This spectroscopic technique allowed the researchers to study the behavior of membranes and their permeability at subzero temperatures.
• Cryoprotectants were found to reduce the initial rate of membrane dehydration during the freezing process.
• These agents also decreased the activation energy for water transport, causing an increase in the total extent of freezing-induced dehydration.

Area of Cryoprotectant Effectiveness

• Studies using liposomes as a model system demonstrated that only the membrane-permeable cryoprotective agents glycerol and ethylene glycol could safeguard membranes against leakage. In contrast, egg yolk, sucrose, and HES could not provide this protection.
• However, using differential scanning calorimetry studies, it was found that sucrose and HES were able to raise the glass transition temperature of the freezing extender, indicating their effectiveness in forming a more stable glassy matrix at higher subzero temperatures.

Conclusion

• Combining different cryoprotective agents with different protective functions can increase sperm cryosurvival rates. The study suggests that membrane-permeable cryoprotective agents stabilize membranes and modulate the rate of cellular dehydration, while di- and polysaccharides increase the glass transition temperature, facilitating storage and handling at higher subzero temperatures.