Abstract

Current operational models of agonism and allosterism quantify ligand actions at receptors where agonist concentration-response relationships are non-hyperbolic by introduction of a transducer slope that relates receptor occupancy to response. However, for some receptors, non-hyperbolic concentration-response relationships arise from multiple endogenous agonist molecules binding to a receptor in a cooperative manner. Thus, we developed operational models of agonism in systems with cooperative agonist binding, and evaluated the models by simulating data describing agonist effects. The models were validated by analyzing experimental data demonstrating the effects of agonists and allosteric modulators at receptors where agonist binding follows hyperbolic (M4 muscarinic acetylcholine receptors) or non-hyperbolic relationships (metabotropic glutamate receptor 5 and calcium-sensing receptor). For hyperbolic agonist-concentration response relationships, no difference in estimates of ligand affinity, efficacy or cooperativity were observed when the slope was assigned to either a transducer slope or to an agonist binding slope. In contrast, for receptors with non-hyperbolic agonist concentration-response relationships, estimates of ligand affinity, efficacy or cooperativity varied depending on the assignment of the slope. The extent of this variation depended upon the magnitude of the slope value, agonist efficacy, and, for allosteric modulators, on the magnitude of cooperativity. The modified operational models described herein are well suited to analyzing agonist and modulator interactions at receptors that bind multiple orthosteric agonists in a cooperative manner. Accounting for cooperative agonist binding is essential to accurately quantify agonist and drug actions.