Membrane fluidity and lipid content of human spermatozoa selected by swim-up method

In this work, we examined whether spermatozoa (spz) from normospermic ferti le patients and selected by a swim-up (S-U) procedure had a particular memb rane fluidity related to their maturity and their lipid content as compared with the sperm cells from the whole ejaculate (total sperm). Swim-up selec ted sperm had a reduced cytoplasmic space as revealed by a lower creatine k inase (CK) activity compared with total sperm (2 +/- 1 vs. 12 +/- 5 mUl/10( 7) spz p < 0.05). The cholesterol (Chol) and total phospholipid (PL) conten ts were significantly lower in S-U selected sperm than in total sperm (0.72 <plus/minus> 0.08 vs. 1.20 +/- 0.30 nmol/10(6) spz for Chol and 1.77 +/- 0 .17 vs. 2.78 +/- 0.50 nmol/10(6) spz for PL, p < 0.05) and such a decrease was observed for the three major membrane PL: phosphatidylethanolamine (PE) , phosphatidylcholine (PC) and sphingomyelin (SM). However, these decreases were not associated with a change in either Chol/PL or PC/(PC + PE) molar ratios. Membrane fluidity estimated by fluorescence polarization remained c omparable between the S-U sperm fraction and total sperm (fluorescence pola rization anisotropy, r, which is inversely proportional to the fluidity: 0. 235 <plus/minus> 0.006 vs. 0.230 +/- 0.005). The sperm membrane fluidity ob tained in normospermic patients was compared with abnormospermic ones (olig oasthenoteratospermia). In abnormospermic patients, the membrane fluidity w as decreased in migrated spermatozoa compared with total sperm (anisotropy: 0.210 +/- 0.010 vs. 0.250 +/- 0.013, p < 0.01). Our data suggest that the S-U method selected a subpopulation of mature spermatozoa characterised by a low content of Chol and PL, likely related to a reduced membrane area. Th e fact that Chol/PL and PC/ (PC + PE) molar ratios were unchanged shows a m aintenance of the membrane quality. This was confirmed by the fluorescence anisotropy measurement showing no difference in plasma membrane fluidity be tween S-U selected sperm and total sperm. In abnormal semen the migrated sp ermatozoa had a lower fluidity compared with total sperm suggesting a defec tive sperm function. These results bring new elements characterizing the S- U selected spermatozoa.