The role of membrane-active lipids in the protection of ram spermatozoa during cooling and storage.

The interactions of ram spermatozoa with exogenous liposomes of varying composition were studied, with the aim of examining the mechanisms by which some lipids protect against cold-induced damage during cryostorage. Liposomes containing various preparations of phosphatidylcholine and cholesterol enhanced sperm survival during storage at 5 degrees C, both in terms of motility and acrosomal integrity. A membrane-fluidizing agent, A2C, was slightly deleterious in this respect. Cholesterol-containing liposomes were not superior in their effects to those prepared without cholesterol. Thus stabilization of the plasma membrane by cholesterol loading may be unimportant. When sodium vanadate was used as a functional probe of membrane integrity, the cryoprotective effects of lipids were apparent despite increased plasma membrane permeability. Incubation of spermatozoa with positively charged liposomes, containing stearylamine, caused considerable loss of motility and acrosomal damage, coupled with cellular aggregation. There was also some evidence that the presence of calcium lessened the effectiveness of liposomes in protecting spermatozoa against damage during cooling.