A MUSCARINIC RECEPTOR DIFFERENT FROM THE M1, M2, M3 AND M4 SUBTYPES MEDIATES THE CONTRACTION OF THE RABBIT IRIS SPHINCTER

In order to analyse the subtype of muscarinic receptors involved in the methacholine-induced contraction of the rabbit iris sphincter we have determined equilibrium dissociation constants (K(B)) of various antagonists in the sphincter muscle. The values were compared with those observed at M1 (rabbit vas deferens), M2 (heteroreceptors in rat iris) and M3 receptors (guinea-pig ileum), or at the muscarinic receptors in the guinea-pig uterus. The methacholine-induced contraction of the uterus from immature guinea-pigs was competitively antagonized by pirenzepine (6.64, -log K(B)), 4-DAMP (8.39), hexahydrodifenidol (HHD; 7.00 for the (R)- and 5.40 for the (S)-enantiomer), p-fluoro-hexahydrosiladifenidol (p-F-HHSiD; 6.25) and valethamate bromide (8.04). The affinity of the antagonists is consistent with the presence of an M2 receptor. The -log K(B) values of the antagonists in the rabbit iris sphincter (6.43, p-F-HHSiD; 6.22, AQ-RA 741; 7.23 and 5.34, (R)- and (S)-trihexyphenidyl) were lower than, or within the lowest range of, estimates in the other experimental models, irrespective of the subtype selectivity of the antagonist. This excludes the presence of an M1, M2, M3 or M4 receptor in this smooth muscle. The affinity of UH-AH 37 in the iris was intermediate between that for M1 or M3, and M2 receptors. The low affinity of AQ-RA 741 and the low enantiomeric ratio of trihexyphenidyl (THP) in the iris (77.6) would be compatible with a presumed M5 receptor. Valethamate bromide and clozapine did not differentiate between M1, M2 and M3 receptors in the present functional studies; the pA2 values at the M1-M3 sites (8.46-8.57 and 7.26-7.58, respectively) were about 10 and 100 times higher, respectively, than the estimates in the iris sphincter. In absolute terms, however, our -log K(B) values of the THP-enantiomers, UH-AH 37 and clozapine in the iris sphincter were 0.8-2 log units lower than previous data on cloned m5 receptors suggesting receptor properties different from the presumed M5 receptor. In conclusion, the muscarinic receptors in the rabbit iris sphincter may represent a novel type differing from M1-M4 receptors.