Phospholipid composition of isolated guinea pig sperm outer acrosomal membrane and plasma membrane during capacitation in vitro.

After capacitation of guinea pig spermatozoa in vitro, the plasma membrane was mechanically separated from the spermatozoa in the presence or absence of HgCl2 and subsequently isolated by density gradient centrifugation. Examination of the spermatozoa by electron microscopy after homogenization in the presence of HgCl2 revealed that plasma membrane was removed only from the acrosomal region and remained predominantly intact posterior to the equatorial segment of the sperm head, as well as the midpiece and tail. In comparison, spermatozoa homogenized under similar buffer conditions but in the absence of HgCl2 lose the large apical segment of the acrosome and the plasma membrane is removed essentially from the entire cell. If spermatozoa were homogenized in the absence of Hg2+, analysis of plasma membrane phospholipid composition revealed a complete loss of lysophosphatidylcholine (LPC) from the plasma membrane after incubation of spermatozoa in minimal capacitating medium (MCM-PL) for 2 hours. Under these culture conditions the addition of Ca2+ (5 mM) to the capacitated spermatozoa induced approximately 78 +/- 5% (n = 3) of the motile spermatozoa to undergo acrosome reactions while still maintaining sperm motility (80 +/- 5%) (n = 3). If the spermatozoa were homogenized in the presence of Hg2+, a time course study revealed that plasma membrane LPC loss occurred between 60 and 90 minutes of incubation. This complete loss of LPC was evident when approximately half of the capacitated spermatozoa had undergone acrosome reactions. Incubation of the spermatozoa with the metabolic and acrosome reaction inhibitor, 2-deoxyglucose (10 mM) for 2 hours, maintained the plasma membrane phospholipid composition similar to that in the noncapacitated state. These data provide evidence that changes in the plasma membrane phospholipid composition may be associated with guinea pig sperm capacitation.