Separate binding sites for intact PTH 1-84 and synthetic PTH 1-34 in canine kidney

Summary

The present studies examine the characteristics of parathyroid hormone (PTH) receptor binding in canine basolateral renal cortical membranes using iodinated preparations of intact bovine PTH 1-84 and [Nle⁸, Nle¹⁸, Tyr³⁴] bPTH 1-34 amide. A solid phase lactoperoxidase technique was used to iodinate the bPTH 1-84. The PTH 1-34 analog was iodinated using chloramine T. Both radioligands were purified by reverse phase high pressure liquid chromatography (HPLC). Specific binding of¹²⁵I PTH 1-84 reached equilibrium at 3 hours whereas binding of the¹²⁵I PTH 1-84 analog reached equilibrium at 45 minutes. Excess bPTH 1-84 resulted in complete inhibition of binding of¹²⁵I bPTH 1-84, whereas 22±1.6% of the bound radioligand remained bound in the presence of excess synthetic bPTH 1-34. These data suggested the possibility of a binding site for the carboxy-terminal region of intact PTH, or binding sites selective for intact hormone. Therefore, additional studies were performed with PTH fragments, PTH 28-53, PTH 35-84, and PTH 53-84. In contrast to previous studies in other systems, these fragments did not result in significant displacement of¹²⁵I PTH 1-84. Analysis of binding of¹²⁵I PTH 1-84 and¹²⁵I [Nle⁸, Nle¹⁸, Tyr³⁴] PTH 1-34 amide, using LIGAND, both indicated a single site model with similar affinities. Thus, the data are consistent either with multiple receptors with similar affinities or a second binding site for bPTH 1-84 on the same receptor. In the canine kidney membranes, bPTH 1-84 and synthetic bPTH 1-34 were equipotent in activating adenylate, although the dose-response curve for bPTH 1-84 was shifted slightly to the right (Kact 2 nm for synthetic bPTH 1-34 versus 5 nm for bPTH 1-84). The present studies suggest that there are binding sites in canine kidney which are selective for the intact hormone and support the existence of more than one class of PTH receptors or a second binding site for intact PTH on a single PTH receptor.