Equation: EC50 shift

Introduction

An competitive inhibitor competes for agonist binding to a receptor, and shifts the dose-response curve to the right without changing the maximum response. This model fits the two dose response curves and determines the fold shift.

Step by step

Create an XY data table. Enter the logarithm of the concentration of the agonist ligand into X. Enter response into Y in any convenient units. Enter data with no inhibitor into column A. Enter data collected with a constant concentration of inhibitor into column B.

From the data table, click Analyze, choose nonlinear regression, and choose the panel of equations:  Dose-Response -- Special. Then choose Dose shift.

If you have subtracted off any basal signal, constrain the parameter Bottom to a constant value of zero.

Model

<A>LogEC=LogEC50Control

<~A>LogEC=LogEC50Control + log(EC50Ratio)

Y=Bottom + (Top-Bottom)/(1+10^((LogEC-X)*HillSlope))

EC50Control is the concentration of agonist that gives half maximal response in the absence of modulator.

Top and Bottom are plateaus in the units of the Y axis (shared).

EC50Ratio is the ratio of EC50 in presence of inhibitor divided by EC50 of agonist alone.

HillSlope is the slope factor (shared)

Notes