La. Krummen et al., LOCALIZATION OF INHIBIN AND ACTIVIN-BINDING SITES IN THE TESTIS DURING DEVELOPMENT BY IN-SITU LIGAND-BINDING, Biology of reproduction, 50(4), 1994, pp. 734-744
Inhibin and activin are related proteins thought to be potential parac
rine regulators of testicular development and maintenance of spermatog
enesis. Messenger RNA and proteins immunologically related to both fac
tors have been identified in the adult testis. However, the role(s) of
these factors in paracrine regulation of testicular function is poorl
y understood. To identify potential targets for inhibin and activin in
immature and adult testis, we used in situ binding of [I-125]-labeled
ligands to localize and describe the distribution of binding sites fo
r inhibin and activin in testes of 15-, 18-, 21-, 30-, 45-, and 60-day
-old rats. Nonspecific binding was defined as that occurring in the pr
esence of a 1000-fold excess of unlabeled recombinant human (rh) inhib
in or activin. [I-125]-Inhibin was found to bind to interstitial cells
throughout development. Inhibin binding was shown to co-localize with
cells that showed positive staining for 3beta-hydroxysteroid dehydrog
enase (3beta-HSD). Competition studies demonstrated that this binding
was indeed specific for inhibin. In contrast, [I-125]-activin showed t
wo distinct patterns of binding. First, [I-125]-activin was shown to b
ind in a non-stage-dependent manner to cells located in the basal comp
artment of the seminiferous tubules in testis obtained from animals of
all ages studied. Binding of [I-125]-activin in the periphery of the
tubule could be inhibited entirely by coincubation with excess unlabel
ed activin and partially with excess unlabeled inhibin. The ability of
inhibin to compete with activin for binding appeared to be more prono
unced in younger animals. In 45- and 60-day-old animals, a second stag
e-dependent component of [I-125]-activin binding was also apparent. Th
is binding was localized to spermatids found in stage VII-VIII tubules
and was inhibited by the presence of excess activin, but not inhibin.
These results indicate that inhibin can bind specifically to testicul
ar interstitial cells throughout development and may be an important r
egulator of Leydig cell testosterone production or interstitial cell f
unction. In contrast, activin appears to bind in a specific and stage-
dependent manner to receptors or high-affinity binding proteins on spe
rmatids as well as to sites on the periphery of all seminiferous tubul
es. These results support the hypothesis that both activin and inhibin
may act at several levels to regulate proliferation or differentiatio
n of germ and Sertoli cell function as well as to modulate interstitia
l cell activity.