Parathyroid hormone binding in vivo to renal, hepatic, and skeletal tissues of the rat using a radioautographic approach.
We have examined in vivo binding of bovine (b) PTH-(1-84) and the analog [Nle8,18, Tyr34]bPTH-(1-34) amide to hepatic, skeletal, and renal rat tissues. Bioactive 125I-labeled bPTH-(1-84) or the 125I-labeled bPTH-(1-34) analog was injected alone into experimental animals or with either unlabeled PTH or unlabeled unrelated hormone into control animals. Corresponding tissues from experimental and control animals were then processed for light and electron microscope radioautography and analyzed quantitatively and qualitatively. Binding of both PTH forms occurred on hepatocytes and sinusoidal cells in liver, and hepatocyte binding was clearly specific and competitive. Intact PTH-(1-84), but not the amino-terminal fragment, bound to Kupffer cells, indicating a sequence-specific interaction. In bone, specific competitive PTH binding was seen over osteoblasts, but not osteoclasts. Skeletal PTH binding was also seen over connective tissue mononuclear cells, and sinusoidal endothelial cells. In kidney, specific competitive PTH binding was seen over glomerular podocytes, and over the antiluminal surface of cells of the proximal tubule, the thick ascending limb of Henle's loop, and the distal tubule. Noncompetitive binding was seen on the luminal surface of proximal convoluted tubule cells. We have, therefore, distinguished specific competitive binding sites, probably related to hormone action, from noncompetitive binding sites, presumably associated with hormone metabolism, on discrete cell types or cell regions within several tissues. Our approach provides morphological correlates to the biochemical interaction of PTH with its target tissues and should enhance our understanding of the relationship of target cell structure and function.