Using microphysiometry to study the pharmacology of exogenously expressed m1 and m3 muscarinic receptors

The microphysiometer, an instrument that uses a semiconductor-based sensor to monitor cellular metabolic activity, has been shown to detect the activation of a variety of receptors in living cells, largely irrespective of the signal-transduction mechanism. Using the Cytosensor Microphysiometer, we have studied agonist concentration responses for the activation of CHO-K1 cell lines exogenously expressing rat m1 or m3 receptors. Three levels of receptor expression were investigated for each subtype. Carbachol is more potent for m3 than m1 receptors (0.5 to 1.0 log unit lower EC50); for both, potency correlates positively with receptor density. The results agree well with those obtained by measuring phosphoinositide hydrolysis and intracellular [CA++] in the same cells. We also determined that two subtype-selective antagonists, pirenzepine (for m1) and p-fluoro-hexahydrosila-difenidol (for m3) displayed appropriate differential ability to shift carbachol concentration-response curves in the microphysiometer. This study provides additional evidence that pharmacological results obtained by microphysiometry are consistent with those obtained by more conventional functional assays.