Protective effects of iodixanol during bovine sperm cryopreservation

The aim of cryopreservation is to maintain cellular integrity, thereby enabling resumption of proper biological functioning after thawing. Here we propose OptiPrep (60% iodixanol in water) as a protectant during sperm cryopreservation using pooled bull semen as the model. We evaluated OptiPrep concentration effect and its relation to cryopreservation by comparing frozen-thawed and chilled samples. Semen, extended in Andromed with 0 (control), 1.25%, 2.5%, and 5% OptiPrep, was compared after either chilling or freezing in large volume by directional freezing. Sample evaluation included sperm motility upon thawing and after 3h incubation at 37 degrees C for frozen-thawed samples and after 3h and 6h of chilling for chilled samples; viability, acrosomal integrity, and hypoosmotic swelling were also tested for frozen-thawed and chilled samples. Chilled samples with 5% OptiPrep showed inferior viability (P=0.047) and 3h motility (P=0.017) relative to that for chilled samples with 2.5% OptiPrep and inferior viability (P=0.042), acrosomal integrity (P=0.045), and 0h motility (P=0.024) relative to that for chilled samples with 1.25% OptiPrep. The 1.25%, 2.5%, and control samples did not differ. In frozen-thawed samples, 2.5% OptiPrep was superior to all other concentrations for 3h motility (control, P=0.007; 5% OptiPrep, P=0.005; 1.25% OptiPrep, P=0.004) and to 1.25% OptiPrep for acrosomal integrity (P=0.001). In a search for a protection mechanism, we measured glass transition temperature (T(g)) of Andromed and of Andromed with 1.25%, 2.5%, and 5% OptiPrep. Andromed (-58.78 degrees C) and 1.25% OptiPrep (-58.75 degrees C) groups had lower mean T(g) than that of the 2.5% (-57.67 degrees C) and the 5% (-57.10 degrees C) groups. Directional cryomicroscopy revealed that the presence of iodixanol alters ice crystal formation into an intricate net of dendrites. Thus, iodixanol appears to possess cryoprotective properties by helping spermatozoa maintain motility and membrane integrity, possibly through altering ice crystals formation into a more hospitable environment and increasing the glass transition temperature.