Androgen binding protein secretion and endocytosis by principal cells in the adult rat epididymis and during postnatal development.

Androgen binding protein (ABP) has been shown to be secreted by Sertoli cells and to be actively taken up by the efferent ducts and proximal caput epididymidis and, yet, to be present at high concentrations in epididymal fluids. In the present study, ABP was immunolocalized by light microscopy in epithelial cells of the efferent ducts and epididymis of adult rats and during postnatal development and by electron microscopy in specific organelles within these cells. In adults, the efferent ducts actively endocytosed Sertoli cell-derived ABP. In the epididymis, principal cells displayed a variable staining reminiscent of a checkerboardlike pattern, with cells being intensely, moderately, or weakly reactive throughout their cytoplasm or unreactive. In the electron microscope, reactive cells displayed a labeling of their Golgi apparatus and secretory vesicles indicative of an epididymal-secreted form of ABP. However, labeling was also noted over endosomes of principal cells, but only of the initial segment and intermediate zone, which, along with labeling of coated pits and vesicles, indicated that ABP was also endocytosed by principal cells of these regions. The postnatal study revealed that principal cells attained an adultlike staining pattern indicative of secretion in a region-specific manner at different ages, suggesting that ABP secretion is regulated by different factors. Ligation of the efferent ducts of 15-day-old animals revealed no reaction along the entire epididymis in animals sacrificed at later ages, suggesting the importance of luminal testicular factors in its regulation during development. In addition, as in the adult, ABP was also endocytosed by principal cells, but only in the initial segment and intermediate zone. Taken together, the present results indicate that secretion of ABP occurs along the entire epididymis, whereas endocytosis is region specific. The functional role of ABP in the epididymis in relation to sperm maturation is discussed.