LATERAL MOBILITY OF PLASMA-MEMBRANE LIPIDS IN BULL SPERMATOZOA - HETEROGENEITY BETWEEN SURFACE DOMAINS AND RIGIDIFICATION FOLLOWING CELL-DEATH

Compartmentalization of surface membrane antigens into discrete region s or domains is a characteristic feature of differentiated cells. In m ammalian spermatozoa at least 5 surface domains are known, implying th e presence of barriers or boundaries within the plasma membrane. Using the technique of fluorescence recovery after photobleaching (FRAP) to measure diffusibility of fluorescent lipid analogues '-dihexadecyl-3, 3,3'3'-tetramethylindocarbocyanine (DiIC(16)) and 5-(N-octa-decanoyl) aminofluorescein (ODAF), we have investigated lipid topology and dynam ics in the plasma membrane of ejaculated bull spermatozoa. Contrary to reports in the literature, we have found that DiIC(16) stains only de ad or damaged spermatozoa whereas ODAF intercalates into the plasma me mbrane of both live and dead cells, each type showing a distinctive st aining pattern. FRAP analysis with ODAF revealed that diffusion coeffi cients on live spermatozoa are significantly faster on the acrosome an d postacrosome (29.3x10(-9) cm(2)/second) than on the midpiece and pri ncipal piece (11.8x 10(-9) cm(2)/second). Recovery (R) is >90% in all domains. ODAF diffusion also shows regionalized temperature-sensitivit y with a 4-fold increase over the sperm head and a 1.8-fold increase o n the tail between 20 degrees C and 37 degrees C. Remarkably, dead or permeabilized spermatozoa rapidly develop a large immobile phase (R<25 %) over the whole plasma membrane. This rigidification is temperature insensitive and irreversible suggesting major changes in the physical state of membrane lipids. It is concluded that lipid diffusion in the plasma membrane of live bull spermatozoa is rapid and varies significa ntly between surface domains. Following permeabilization or cell death , however, a large immobile phase develops indicating substantial chan ges in membrane lipid disposition.