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Gq-coupled receptor assays: a comparison of FLEXstation and FLIPR





Activation of G q-coupled receptors induces the flow of calcium ions out of intracellular compartments into the cytosol via a signaling cascade. Cell signaling through calcium mobilization is responsible for a broad range of cell functions including cell death, proliferation, adhesion and motility. When these processes malfunction, it can lead to disease. Consequently, the functions of Gqcoupled receptors are widely studied, and these proteins are popular drug targets for biotechnology and pharmaceutical companies.

The cell line used in these studies, M1 CHO, is stably transfected with the M1 muscarinic receptor. This transmembrane protein is a Gqcoupled acetylcholine receptor. It is stimulated with carbachol, which leads to the release of calcium into the cytosol, and is inhibited by pirenzepine. Thesefindings demonstrate that a similar range of EC50 and IC50 values for carbachol and pirenzepine, respectively, are obtained with two different fluorescence microplate readers, FlexStation or FLIPR.

Cells were seeded the night before the experiment at a concentration of 30,000 cells/well in a volume of 100 l per well of black walled, clear bottomed, 96-well microplates (E&K distributors, cat# 655090). The next day, cells were incubated for one-half of an hour with 1X Loading Buffer from the FLIPR Calcium Assay Kit. This buffer contained a final concentration of 2.5 mM probenecid to inhibit endogenous efflux pumps present in M1 CHO cells. FlexStation and FLIPR were used to dispense agonist and antago nist into the microplate containing cells and then monitor the mobilization of calcium into the cell cytosol.

Half-log dilutions of activator or inhibitor were used. Specifically, the range of 0 to 1 M for carbachol was used for agonist studies. For antagonist studies, cells were first treated with a range (0 to 10 M) of pirenzepine for a total of two minutes, then a dose of 0.5 M carbachol was added to all wells of the microplate. Results are representative of at least two independent experiments performed over a period of at least two different days. Each point on the dose response curves (Figures 5 and 6) is an average of eight replicates. Each replicate value is the difference between the maximum response to agonist and the minimum or background signal (Max-Min). The curve fit for the dose response curve is 4-parameter, for which the C value is the concentration of drug that causes 50% of maximal response (EC50 or IC50 value, see Figures 5 and 6). The EC50 values for carbachol using FlexStation ranged from 1230 nM, whereas it ranged from 1316 nM using FLIPR. The average IC50 value for pirenzepine was 458 nM using FlexStation and 523 nM using FLIPR. The FlexStation data in Figures 1 and 3 were smoothed using a moving average of three and four points, respectively.

As described above, the EC50 and IC50 values obtained for carbachol and pirenzepine, respectively, were consistent between FlexStation and FLIPR. Since cell preparation and treatment are the same when using either instrument, this allows for assay development on the FlexStation and easy assay transfer to the FLIPR for screening purposes. Other benefits are that SoftMax Pro software makes it simple to set up the instrument and analyz e the data. In conclusion, FlexStation provides an easy-to-use, versatile tool to facilitate assay development.

1 Buck, M. A., and C. M. Fraser. 1990. Biochem. Biophys. Res. Commun. 173: 666-672.

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