Alterations in Lectin Binding to Guinea Pig Spermatozoa Accompanying in vitro Capacitation and the Acrosome Reaction

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Abstract
Surface carbohydrate distribution during in vitro capacitation of guinea pig (Cavia porcella) spermatozoa was probed by labeling membrane polysaccharides with fluorescent conjugates of the plant lectins concanavalin A (Con A), wheat germ agglutinin (WGA) and soybean agglutinin (SBA). Lectin distributions over the surface of cauda epididymal cells were characteristic of each lectin studied. Labeling of live and prefixed cells during various temperature regimens indicated that no significant redistribution of lectin binding sites occurs over the sperm head due either to lectin-induced or intrinsic membrane mobility changes. Spermatozoa exposed to lectins at prescribed intervals during capacitating incubations revealed few alterations in Con A and/or WGA labeling patterns. An increase in SBA binding over the surface of the flagellum was observed. Some cells exhibited a diminished fluorescence over the equatorial region with Con A or WGA. After Ca2+ stimulation of the acrosome reaction, fluorescence previously observed with Con A and WGA (but not SBA) was no longer present over the plasma membrane surrounding the equatorial segment and medial region of the acrosome. In reacted cells, the putative inner acrosomal membrane displayed some labeling with all 3 lectins, whereas the postacrosomal region bound only Con A and WGA. The high degree of structural regionalization and mosaicism present within the surface membrane of mammalian spermatozoa is demonstrated. A well defined sequence of changes in lectin binding appears to accompany both in vitro capacitation and the acrosome reaction. These changes are consistent with the notion of membrane fusion requiring a prior clearance of nonlipid components from the presumptive fusion sites.