Use of a spreadsheet to quantitate the equilibrium binding of an allosteric modulator.

Using the program Microsoft EXCEL, a spreadsheet was developed for constrained, simultaneous analysis of multiple datasets obtained from equilibrium binding experiments, according to an allosteric model of interaction. This approach was used to quantitate the interaction between the modulator (heptane-1,7-bis (dimethyl 3'-phthalimidopropyl) ammonium bromide) (C7/3-phth) and the radioligands [3H]N-methylscopolamine and [3H]quinuclidinyl benzilate at cortical and atrial muscarinic receptors. The interaction between various concentrations of the radioligands and C7/3-phth, in the guinea pig atrium and in the rat cerebral cortex, could be well described by the allosteric model. The affinity of C7/3-phth for unoccupied atrial receptors was significantly higher than for cortical receptors. The negative cooperativity between [3H]quinuclidinyl benzilate and the modulator was higher in cortex than that between the modulator and [3H]N-methylscopolamine. It is suggested that the described method has wide applicability because of the extensive availability of spreadsheet programs, the analytical advantages offered by constrained, simultaneous nonlinear regression and the ability to adapt the spreadsheet to almost any model of ligand-receptor interaction.