Distribution of filipin-sterol complexes in sperm membranes from hypercholesterolaemic rabbits

The distribution of membrane filipin-sterol complexes (FSC) was examined ultrastructurally in cauda epididymal sperm from normal and hypercholesterolaemic rabbits. Membrane FSC were quantitatively analysed on replicas of filipin-treated cells. We determined a significant difference in FSC concentration in the plasma membrane of the acrosome region (PMAR) of hypercholesterolaemic animals compared to normal rabbits. Hypercholesterolaemic animals had 0.56 +/- 0.05 FSC complex per micron 2 (enriched Cholesterol diet: Diet 2) in the marginal segment of PMAR; 0.62 +/- 0.05 FSC complex per micron 2 (enriched Cholesterol and fish oil diet: Diet 3) and only 0.28 +/- 0.01 FSC complex per micron 2 for normal animals (Control Diet 1). In the principal (anterior) segment we found 0.54 +/- 0.10 FSC complex per micron 2 (Diet 2), 0.56 +/- 0.03 FSC complex per micron 2 (Diet 3) and 0.30 +/- 0.04 FSC complex per micron 2 (Control Diet 1). We also counted 0.47 +/- 0.1 FSC complex per micron 2 in the equatorial segment of PMAR for Diet 2, 0.27 +/- 0.05 and 0.28 +/- 0.04 FSC complex per micron 2 in Diet 1 and Diet 3 respectively. Diet 4 (fish oil) did not differ from the control. An increase in the Cholesterol (Chol) level in biological membranes or a difference in the Chol membrane domains could cause a variation in the membrane rigidity that could modify the sperm membrane fusion capacity and functionality. The results presented in this paper are in agreement and could explain the decrease in the kinetic of the sperm acrosome reaction that we have observed in experimentally hypercholesterolaemic rabbits (Díaz-Fontdevila & Bustos-Obregón, 1992).