Autoradiographic localization of serum retinol-binding protein in rat testis.

An interstitial target site in the rat testis for vitamin A has been detected using radioactively labeled rretinol-binding protein (RBP) in autoradiography at the light microscope level. RBP purified from rat serum was iodinated by the lactoperoxidase procedure and was also complexes with [3H]-retinol. The radioactively labeled RBP was administered to rat testis tissue by a direct intratesticular injection, an iv injection, and through an in vitro incubation of decapsulated tissue in medium containing the labeled protein. Localization on the autoradiograms of both [125I]RBP and [3H]retinol-RBP was interstitial; there was no interaction of RBP with cells in or on the seminiferous tubules. The patterns of localization were identical in normal rats and vitamin A-deficient rats. [3H]etinol uncomplexed with RBP (administered with rat serum albumin) was distributed evenly throughout testicular tissue, with no specific localization. Although the labeling of the cells on the autoradiograms was not visible diminished by the presence of excess unlabeled RBP, an in vitro binding assay of a crude interstitial cell preparation using [125I]RBP displayed saturable binding, indicative of receptors for RBP. An in vitro binding assay of Sertoli cells in culture with [125I]RBP demonstrated no specific binding, nor did [125I]RBP in autoradiography of Sertoli cell monolayers display interaction with these cells. Results from this investigation show that the plasma transport protein for vitamin A, RBP, interacts initially with cells in the interstitium of the testis. (Endocrinology 108: 658, 1981)