Abstract

In this study the similarities and differences between the M2 and M4 subtypes in their recognition of agonists were explored. A CHO-K1 cell line transfected with the human m2 receptor was used as a homogeneous M2 tissue for comparison with two putative M4 systems (rat striatum and the N1E-115 mouse neuroblastoma cell line). The equilibrium binding dissociation constants and intrinsic efficacies for seven muscarinic agonists were determined for their stimulation of cyclic AMP inhibition via the M2 and M4 receptors. Partial receptor occlusion with propylbenzilylcholine mustard was used to determine binding constants for the more efficacious drugs and the reference agonist oxotremorine-M. The binding dissociation constants and relative efficacies for other agonists were then determined in reference to oxotremorine-M by a null method. For the M2 receptor the agonist binding dissociation constants ranged in potency from oxotremorine (1.5 microM) to bethanechol (171 microM), whereas relative efficacies varied from that of muscarine (relative efficacy = 0.9) to the value for McN-A343 (relative efficacy = 0.04). In general, most agonists bound with similar potencies to M2 and M4 receptors (Kd values within a factor of 2-3). However, oxotremorine bound to the N1E-115 and striatal M4 receptors about 3-fold and 10-fold less potently, respectively, than it did to the M2 receptor. Another exception was pilocarpine, which bound to the N1E-115 receptor (1.9 microM) with 8-fold and 12-fold higher potency than to the CHO-K1 M2 receptor and the striatal M4 receptor, respectively. Despite the low affinity of bethanechol for the M2 receptor, it was an efficacious agonist (maximal response equivalent to that of oxotremorine-M; relative efficacy = 0.6) at this subtype, whereas it was a partial agonist (60%) with lesser efficacy in the clonal M4 system. In contrast, McN-A343 and arecoline were significantly more efficacious at the two M4 receptors than they were at the M2 receptor. The M4 system in the rat striatum displayed some similarity to the N1E-115 M4 system, with regard to the efficacy ranking for certain agonists (arecoline greater than bethanechol greater than McN-A343 greater than or equal to pilocarpine). This rank order was different from the ranking of these four agonists in the M2 system, indicating that these two M4 receptors are more similar to each other in efficacy ranking than they are to the M2 receptor. However, the rat striatal and N1E-115 M4 receptor differed in their binding of oxotremorine and pilocarpine, indicating that these two M4 systems were not pharmacologically identical.(ABSTRACT TRUNCATED AT 400 WORDS)