Autoradiographic Studies on the Binding Sites of ^<125>I-Adrenomedullin in Rat Tissues

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Adrenomedullin (AM) is a novel hypotensive peptide isolated from the human pheochromocytoma. In the present study, ^<125>I-AM was intravenously administered to rats and its distribution was examined by autoradiographic (ARG) techniques using wholebody, light microscopic (LM) and electron microscopic (EM) methods. Whole-body-ARG after administration of ^<125>I-AM showed the highest radioactivity in the lung (paren-chyma), followed by the renal cortex and the liver. When localization of ^<125>I-AM in the lung was examined by LM-ARG, silver grains were localized over the arteries, capillaries and venules. In the arteries, the endothelial cells of the small arteries (lumen diameter 20-50μm) were preferentially labelled and there was little labelling on the large arteries. There were few silver grains on the smooth muscle cells along the entire pulmonary vasculature system. EM-ARG showed many silver grains on the endothelial cells of capillaries and venules. Very few silver grains were observed on the lung when an excess amount of unlabelled AM was administered with ^<125>I-AM. These results indicated that ^<125>I-AM specific binding sites are most abundant in the lung, and the major binding sites are on the endothelial cells of small arteries, alveolar capillaries and venules.
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