Muscarinic M3 receptor inactivation reveals a pertussis toxin-sensitive contractile response in the guinea pig colon: evidence for M2/M3 receptor interactions. 
The role of M2 and M3 receptors in the contractile and phosphoinositide responses elicited to oxotremorine-M was investigated in the guinea pig colon.  Under standard conditions, both the contractile and phosphoinositide responses were insensitive to pertussis toxin and irreversibly antagonized by alkylation of M3 receptors with N-(2-chloroethyl)-4-piperidinyl diphenylacetate.  After treatment with N-(2-chloroethyl)-4-piperidinyl diphenylacetate, the remaining contractile response was sensitive to pertussis toxin and weakly antagonized by the M2- and M4-selective antagonist AF-DX 116.  In contrast, the residual phosphoinositide response was unaffected by pertussis toxin.  The pertussis toxin sensitivity of the remaining contractile response suggests that the M2 receptor is mediating the contraction, whereas its weak antagonism by AF-DX 116 suggests that an alternate muscarinic subtype mediates the response.  To explain this enigma, we investigated a mathematical model for receptor action based on an interaction between two receptor subtypes (M2 and M3).  This model predicts that a response mediated by both the M2 and M3 receptor can be pertussis toxin sensitive yet exhibit an antagonistic profile indicative of an M3 response.