Binding and Internalization of a Luteinizing Hormone-Releasing Hormone Agonist by Rat Gonadotrophic Cells. A Radioautographic Study

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Abstract
In order to localize the binding sites for LHRH and the possible internalization of the peptide in the rat pituitary gland, radioautography was performed as a function of time after iv injection of the iodinated stable LHRH agonist [DSer(tBu)6,des-Gly-NH210]LHRH ethylamide into intact (both sexes) and castrated female rats. At the light microscopic level, the silver grains were found in a small percentage of cells (gonadotrophs) in intact rats and were restricted to castration cells in gonadectomized rats. In control animals injected with both iodinated [D-Ser(tBu) 6,des-Gly-NH210]LHRH ethylamide and an excess of unlabeled peptide, no silver grains could be detected. At the electron microscopic level, only gonadotrophs appear to contain silver grains. The time-course study in intact animals showed that 3 min after injection most silver grains were associated with the plasma membrane. At longer time intervals after injection (10 and 30 min), label was mostly detected in the Golgi apparatus and, to a lesser extent, in lysosomes and secretory granules. One and three hours after injection, most silver grains were associated with the Golgi apparatus, lysosomes, and secretory granules. No sex differences could be observed. In castrated female rats, migration of label from the plasma membrane to the Golgi apparatus, secretory granules, and lysosomes was more rapid than in intact animals. These results indicate that, after binding to the plasma membrane, LHRH is rapidly internalized, accumulating first in the Golgi apparatus and later in secretory granules and lysosomes. The uptake of label by lysosomes is probably in relation to peptide and/or receptor degradation, whereas the significance of association of radioactivity with secretory granules remains to be elucidated.

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