The use of occupation isoboles for analysis of a response mediated by two receptors: M2 and M3 muscarinic receptor subtype-induced mouse stomach contractions.

Smooth muscle contains multiple muscarinic receptor subtypes, including M2 and M3. M2 receptors outnumber M3 receptors. Based on the potency of subtype selective anticholinergics, contraction is mediated by the M3 subtype. However, results from knockout (KO) mice show that the M2 receptor mediates approximately 45% of the contractile response produced by the M3 receptor. The traditional theory of one receptor mediating a response does not allow assessment of interactions between receptors when more than one receptor participates in a response. Our study was performed using a novel analysis method based on dual receptor occupancy to determine how M2 and M3 receptor subtypes interact to mediate contraction in mouse stomach. Cumulative carbachol concentration contractile responses were determined for wild-type, M2-KO, and M3-KO stomach body smooth muscle. Using affinity constants for carbachol at M2 and M3 cholinergic receptors, the concentration values were converted to fractional receptor occupation. The resulting occupation-effect relations showed maximum effects for the M2 and M3 subtypes, respectively. These occupation-effect relations allow determination of the additive (expected) isobole based on this dual occupancy, thereby providing a curve (mathematically derived) for comparison against the experimentally derived value in wild type. The actual values determined experimentally in the wild type were not statistically significantly different from that predicted by the isobole. This confirms that the interaction between these mutually occupied receptors is additive. The new method of analysis also expands the traditional Schild theory that was based on a single receptor type to which the agonist and antagonist bind.