Ultrastructural Autoradiographic Localization of Somatostatin-28 in the Rat Pituitary Gland

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Abstract
To identify the anterior pituitary cell type(s) containing somatostatin-28 (SS-28)-binding sites and to study the internalization processes of this peptide by the target cells, autoradiography was performed on rat anterior pituitaries removed at specific intervals (2-60 min) after i.v. injection of the [125I] ido-SS-28 agonist [Leu8,D-Trp22,Tyr25]SS-28 into intact, adrenalectomized, or castrated male rats. At the light microcopic level, the silver grains were found in 75% of cells. Concomitant injection of an excess of unlabeled peptide prevented the binding of label, verifying the specificity of binding. No specific labeling could be detected in the adrenocorticotrophs of adrenalectomized rats or gonadotrophs (castration cells) of castrated rats. At the EM level, 3 cell types (somatotrophs, thyrotrophs, and mammotrophs) appear to contain radiolabeled SS-28. The time-course study in somatotrophs of intact anaimals showed that 2 min after injection, most silver grains were associated with the plasma membrane. Five to 15 min after injection, label was found over both the plasma membrane and cytoplasmic organelles, especially the Golgi apparatus, lysosomes, and secretory granules. At the longest time interval (60 min), labeling was mostly associated with the cytoplasmic organelles. These results indicate that SS-28 binding sites are present only in those cell types in which somatostatin is known to regulate secretory functions. The present data also show that a rapid internalization of the radiolabeled peptide occurs.

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