Transbilayer motion of spin-labelled phospholipids in the plasma membrane of epididymal and ejaculated ram spermatozoa

The transbilayer movement and distribution of spin-labelled phospholipid analogues were studied in the plasma membrane of ram sperm cells isolated from functionally different regions of the epididymis (caput, cauda) and from the ejaculate. As already shown for ejaculated cells, (i) a rapid movement of aminophospholipid analogues phosphatidylserine and phosphatidylethanolamine from the exoplasmic to the cytoplasmic leaflet, and (ii) a slow transbilayer movement of phosphatidylcholine were observed in the plasma membrane of maturating ram sperm cells. This suggests an asymmetric steady state transbilayer distribution of phospholipids, with a preferential enrichment of aminophospholipids on the cytoplasmic leaflet of those cells. The fast inward redistribution of the aminophospholipid analogues is consistent with the presence of an aminophospholipid translocase activity in all three sperm cell preparations. The translocase activity was enhanced slightly during the epididymal transit of spermatozoa. However, compared with epididymal sperm cells, a marked increase in the aminophospholipid translocase activity and a more pronounced phospholipid asymmetry for phosphatidylethanolamine was established for ejaculated spermatozoa. The physiological relevance of the rapid removal of aminophospholipids from the exoplasmic leaflet of the plasma membrane of ram sperm cells is discussed. The quality of sperm cell fractions was characterized by the cell morphology, membrane integrity and the cellular ATP concentration.