The relationship between apparent potentiation and the magnitude of the control response

The effect of a potentiating drug on ion channel function is typically evaluated by comparing current responses to the control agonist in the presence and absence of the potentiator. Differences in ratios of responses are then taken as proof of distinct potentiation properties when comparing modulation by different compounds. In these experiments, the concentration of the agonist is typically kept low to generate a small fractional control response. The precise relative magnitude of the control response is, however, not standardized among labs and can range from a concentration producing a response equal to just 2% of maximal (EC2) to over EC25 in different studies. Here, we have investigated the relationship between the magnitude of the control response and the expected response ratio. As the EC value of the control response increases, the ratio of responses to agonist in the presence and absence of the potentiator decreases. We provide equations to calculate the expected response ratios at different levels of control responses and free energy changes at different response ratios. Lastly, we discuss the effect of the value of EC of the control response on the efficacy of negative allosteric inhibitors.