Development of operational models of receptor activation including constitutive receptor activity and their use to determine the efficacy of the chemokine CCL17 at the CC chemokine receptor CCR4

BACKGROUND AND PURPOSE The operational model provides a key conceptual framework for the analysis of pharmacological data. However, this model does not include constitutive receptor activity, a frequent phenomenon in modern pharmacology, particularly in recombinant systems. Here, we developed extensions of the operational model which include constitutive activity and applied them to effects of agonists at the chemokine receptor CCR4. EXPERIMENTAL APPROACH The effects of agonists of CCR4 on [35S]GTP?S binding to recombinant cell membranes and on the filamentous (F-) actin content of human CD4+ CCR4+ T cells were determined. The basal [35S]GTP?S binding was changed by varying the GDP concentration whilst the basal F-actin contents of the higher expressing T cell populations were elevated, suggesting constitutive activity of CCR4. Both sets of data were analysed using the mathematical models. RESULTS The affinity of CCL17 (also known as TARC) derived from analysis of the T cell data (pKa= 9.61 +/- 0.17) was consistent with radioligand binding experiments (9.50 +/- 0.11) while that from the [35S]GTP?S binding experiments was lower (8.27 +/- 0.09). Its intrinsic efficacy differed between the two systems (110 in T cells vs. 11). CONCLUSIONS AND IMPLICATIONS The presence of constitutive receptor activity allows the absolute intrinsic efficacy of agonists to be determined without a contribution from the signal transduction system. Intrinsic efficacy estimated in this way is consistent with Furchgott's definition of this property. CCL17 may have a higher intrinsic efficacy at CCR4 in human T cells than that expressed recombinantly in CHO cells.