NEW INSIGHTS INTO INTERACTIONS BETWEEN THE HUMAN PTH PTHRP RECEPTOR AND AGONIST/ANTAGONIST BINDING/

We prepared a polyclonal antiserum [Ab-(88-97)] against residues 88-97 of the NH2-terminal tail of the human (h) parathyroid hormone (PTH)/P TH-related protein (PTHrP) receptor. Ab-(88-97) bound specifically to the receptor, as assessed by fluorescence-activated cell sorter analys is of HEK C21 cells, which stably express similar to 400,000 hPTH/PTHr P receptors per cell. Unlike PTH, Ab-(88-97) binding did not elicit ei ther adenosine 3',5'-cyclic monophosphate or intracellular calcium con centration signaling responses in these cells. Incubation of C21 cells for 90 min at 4 degrees C with hPTH-(1-34) plus antiserum reduced the Ab-(88-97) binding to the cells by up to 40-50% of control values in a PTH concentration-dependent fashion with a half-maximal effective co ncentration of similar to 5 nM. The decrease in Ab-(88-97) binding cau sed by hPTH-(1-34) was completely reversed by coincubation with hPTHrP -(7-34). We conclude that residues 88-97 of the hPTH/PTHrPR are involv ed, either directly or indirectly, in agonist but not antagonist bindi ng to the receptor.