Endocytosis of Androgen‐Binding Protein, Cluster in, and Transferrin in the Efferent Ducts and Epididymis of the Ram

Abstract

ABSTRACT: The amount of androgen‐binding protein (ABP) In luminal fluid from the central caput epididymidis of the ram, on a per sperm basis, remains the same as that in rete testis fluid (RTF) entering the ductuli efferentes, although >85% of the testicular protein is absorbed in proximal sites. To determine if ABP is spared from endocytosis in proximal sites and if proteins are differentially and selectively absorbed at specific sites in the excurrent ducts, we studied the endocytosis of ABP, clusterin, transferrin, and a 26/35‐kd dimer isolated from ovine RTF. Each protein was labeled with colloidal gold and microinjected into the lumen of a ductulus efferens and five specific sites in the ductus epididymidis; uptake was quantified by electron microscopy. Endocytosis of each protein, including ABP, was substantially greater in the ductuli efferentes than in any site in the ductus epididymidis. More ABP was endocytosed in proximal regions of the epididymis than any other protein studied. Endocytosis of the 26/35‐kd dimer, like ABP, was greater in proximal sites of the epididymis, whereas endocytosis of clusterin and transferrin was greater in distal sites. Thus, there was a differential absorption, since proteins were endocytosed in one or another specific region of the epididymis, depending on the protein. ABP was endocytosed in the ductuli efferentes and caput epididymidis in amounts similar to or greater than other major testicular proteins, and was not spared from endocytosis in the proximal excurrent ducts. Previously reported maintenance of a similar amount of ABP, on a per sperm basis, throughout the ovine caput epididymidis probably results from recycling of endocytosed ABP or from epididymal secretion of ABP.