Effect of human plasma on the potency of antagonist ligands for inhibition of PTH(1-34)-stimulated cAMP accumulation at the human PTH1 receptor
| Antagonist | No Plasma | 20% Plasma | ||
|---|---|---|---|---|
| pKB (KB, nM) | % Maximal rPTH Response | pKB (KB, nM) | % Maximal rPTH Response | |
| TIP(7-39) | 7.13 ± 0.27 | 86 ± 2 | 7.67 ± 0.06 | 98 ± 1 |
| (74) | (21) | |||
| [d-Trp12,Tyr34]PTH(7-34) | 6.45 ± 0.01 | 113 ± 13 | 6.90 ± 0.00 | 122 ± 3 |
| (350) | (130) | |||
| PTHrP(7-34) | 6.00 ± 0.06 | 103 ± 3 | 6.36 ± 0.40 | 102 ± 7 |
| (1000) | (440) | |||
The human PTH1 receptor was transiently expressed in COS-7 cells and cAMP determined as described under Materials and Methods. Plasma, antagonist, and varying concentrations of agonist were added to the cells in rapid succession and the cells incubated for 40 min at 37°C. The effect of the antagonist on the rPTH(1-34) EC50 andEmax was determined using 1 μM TIP(7-39), 1 μM [d-Trp12,Tyr34]PTH(7-34), and 3.2 μM PTHrP(7-34). The values are mean ± range from two experiments. Plasma produced little effect on cAMP accumulation in the absence of agonist (1.6 ± 0.02 and 1.92 ± 0.08 pmol/well in the absence and presence of plasma, respectively). The −logEC50for rPTH(1-34) was 9.34 ± 0.04 and 9.63 ± 0.13 in the absence and presence of 20% plasma, respectively. The rPTH(1-34)Emax in the presence of 20% plasma was 88 ± 21% of the Emax in the absence of plasma.