Surface transformation of ram spermatozoa in uterine, oviduct and cauda epididymal fluids in vitro

Genital tract fluids were collected continuously from conscious ewes through catheters inserted surgically into the uterus and oviducts. Cauda epididymal spermatozoa and fluid were obtained through catheters inserted into the transected vas deferens. The washed spermatozoa were labelled using the surface-specific chloroglycoluril-Na125I procedure. High-resolution electrophoretic analysis of sperm plasma membrane preparations revealed a partial loss of a major surface component (i.e. Mr 97,000) during incubation in uterine and oviduct fluids. This specific loss resulted in a shift in radioactivity distribution toward an Mr 24,000 component which had been previously identified as a sialoglycoprotein. No significant changes in the distribution of radiolabelled surface components were detectable when the spermatozoa were incubated in synthetic medium. Incubation of unlabelled spermatozoa in 125I-labelled uterine fluid showed that adsorption of exogenous fluid components was highly selective; an Mr 16,000 polypeptide was greatly enriched on the sperm surface although it was only a minor component in the incubation fluid. Adsorption of labelled oviduct fluid components was also selective and involved predominantly high molecular weight components (i.e. Mr 140,000, 95,000, 78,000, 53,000). When spermatozoa were incubated in labelled cauda epididymal fluid after exposure to unlabelled uterine and oviduct fluids, several fluid components were incorporated by the plasma membrane, indicating that surface renovation of 'capacitated' spermatozoa may be a more general process rather than a specific event. These results suggest that capacitation of ram spermatozoa involves loss of specific surface proteins as well as selective adsorption of exogenous fluid components and point to a polypeptide in uterine fluid as an active constituent.