In Vivo Demonstration of Receptors in Rat Liver to the Amino-Terminal Region of Parathyroid Hormone*

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Abstract
The specific binding of the amino-terminal region of parathyroid hormone (PTH) to liver has been assessed by in vivo radioautography employing a biologically active, l25I-labeled synthetic bPTH analog ([Nle-8,18, Tyr-34]bPTH-(1–34) amide) as a probe. Two minutes after intrajugular injection of the labeled analog into rats, free hormone was separated from that bound to cells by intracardiac perfusion with buffer (30 sec), followed by perfusion-fixation with glutaraldehyde. By light and electron microscope radioautography, the distribution of silver grains over the liver sections revealed localization over the periphery of hepatocytes as well as over endothelial cells. In simultaneous control experiments, the unlabeled synthetic active fragment bPTH-(1–34) and unlabeled intact native bPTH-(1– 84) significantly inhibited binding of the labeled analog to liver hepatocytes but not to endothelial cells. Greater uptake of the 125I-labeled bPTH analog was found in rat liver (28% of the injected dose) than in kidney, bone, or other organs examined. In parallel experiments using purified plasmalemma fractions from hepatocytes, both bPTH-(l–34) and intact bPTH-(1–84) demonstrated a dose-dependent activation of adenylate cyclase which was less than that of glucagon but greater than that of epinephrine. The combined results support the concept of the liver as a target organ for the amino-terminal biologically active region of PTH.

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