Subtype of muscarinic receptors mediating relaxation and contraction in the rat iris dilator smooth muscle

1. 1. Carbachol produced a relaxation of dilator muscle at a concentration lower than 1 μM and a contraction at a concentration higher than 1 μM. 2. 2. We studied the effects of the M 1-selective antagonist, pirenzepine, the M 2-selective antagonist, himbacine, the M 3-selective antagonist, 4-diphenyl-acetoxy- N-methylpiperidine methiodide (4-DAMP) and the non-selective antagonist, atropine, on carbachol-induced relaxation and contraction of the rat iris dilator smooth muscle. All the antagonists competitively inhibited both the responses to carbachol. 3. 3. In relaxation and contraction, the low affinity of pirenzepine and himbacine suggest that the rat iris dilator smooth muscle receptors are not of the M 1 and M 2 subtypes. In contrast, 4-DAMP potently inhibited the carbachol-induced relaxation and contraction with affinities similar to those reported for the M 3 subtype. 4. 4. Carbachol-induced relaxation and contraction of the rat iris dilator appears to be mediated through a homogeneous population of M 3 subtype.