Figure 5. Model to describe the possible mechanisms of action of sex hormone-binding globulin/androgen-binding protein (SHBG/ABP) in the male reproductive tract. The rat Sertoli cell produces SHBG/ABP (1) (Hagenas et al, 1975; Oke and Suárez-Quian, 1993; Joseph, 1994), which, after binding to steroids or in its free form, interacts with specific plasma membrane receptors on germ cells (2) (Frairia et al, 1992; Porto et al, 1992). (The interaction depicted in the Figure is at a pachytene spermatocyte. However, the SHBG/ABP may also bind to round and elongated spermatids, although there is less evidence for this.) In turn, the receptor could activate a second messenger cascade (3), as was demonstrated in other tissues (Fortunati et al, 1996; Nackla et al, 1997), or, alternatively, the SHBG/ABP protein could be internalized (4) (Gerard et al, 1991, 1994). Once in the germ cell cytoplasm, SHBG/ABP could interact with a cytoplasmic cofactor (5?), although this mechanism of action is not yet demonstrated, and ultimately, the bound complex is delivered to the nucleus, where it interacts with nuclear proteins or DNA (6). It is also possible that the germ cell synthesizes its own SHBG/ABP (7) and in turn functions in an autocrine manner to modulate the intracellular effects of sex steroids or to be delivered into the nuclei (Joseph et al, 1996). The SHBG/ABP protein also migrates to the epididymis, where it is internalized by the principal cells (8) (Feldman et al, 1981; Pelliniemi et al, 1981; Gerard et al, 1998).