Receptor-mediated binding and uptake of GnRH agonist and antagonist by pituitary cells

The intracellular pathway of an enzyme resistant GnRH agonist (D-Lys6-GnRH) conjugated to ferritin or to colloidal gold was followed in cultured pituitary cells. After an initial uniform distribution over the cell surface of gonadotropes, the electrondense marker was internalized, either individually or in small groups. Some, but not all marker was associated with invaginated membrane specializations showing a proteineous coat at their cytoplasmic site. After longer incubation times, the marker appeared in the lysosomal compartment and the Golgi apparatus, where it could be found in the vesicular as well as cisternal portion. In addition, the receptor-mediated endocytosis of the GnRH antagonist D-p-Glu1-D-Phe2-D-Trp3-D-Lys6-GnRH was studied by light and electron microscopic autoradiography after 30 and 60 min of incubation to ensure uptake. At both time points, in in vitro as well as in vivo studies, silver grains were localized over cytoplasmic organelles of castration cells, including dilated endoplasmic reticulum, lysosomes, and clear vesicles. No consistent association with cell nuclei, mitochondria, or secretory vesicles could be observed. The results suggest that both agonist and antagonist are binding selectively to the plasma membrane of gonadotropes and subsequently are taken up via receptor-mediated endocytosis for degradation or possible action on synthetic processes.