Thermodynamic analysis does not allow discrimination of agonists and antagonists at human CCK2S-receptors

Studies have shown that measurement of thermodynamic parameters (enthalpy, ΔH° and entropy, ΔS°) can allow discrimination of agonists and antagonists (e.g. Weiland, G.A., Minneman, K.P., Molinoff, P.B., 1979. Fundamental difference between the molecular interactions of agonists and antagonists with the β-adrenergic receptor. Nature, 281, 114.). Recently, we found that agonists and antagonists were not thermodynamically-distinguished at cholecystokinin (CCK)2-receptors in rat cerebral cortex. However, in this study, the possibility that thermodynamic discrimination at CCK2-receptors exists but that it was not detected, could not be excluded because radioligand binding studies and functional assays were performed in different rat tissues. Therefore, we have repeated these studies using the recombinant CCK2 short isoform (CCK2S)-receptor expressed in NIH3T3 cells, so that ligand affinity (pKI) and intrinsic activity (α) measurements could be made in exactly the same receptor system. CCK-8S but not R-L-365,260, S-L-365,260, JB95008, JB93242 or PD134,308 expressed intrinsic activity in an IP assay. The pKD of [3H]-JB93182 decreased with increasing temperature. pKI values for antagonists (R-L-365,260, S-L-365,260, JB95008) and agonists (pentagastrin, CCK-8S) were higher at 4 than at 30 °C. There was no effect of temperature on pKI values for the antagonists, PD134,308 and JB93242. Therefore, CCK2-receptor agonists and antagonists at human CCK2S-receptors cannot be discriminated by thermodynamic analysis.