Abstract
The pre- and postjunctional affinity constants of a series of muscarinic antagonists were determined in guinea pig and rabbit irises. Field stimulation-evoked [ 3H]noradrenaline release from superfused isolated irises was concentration dependently inhibited by (±)-methacholine, confirming the presence on the iris noradrenergic nerves of prejunctional inhibitory muscarinic receptors. The affinity constants of the antagonists at the pre- and postjunctional receptors are compatible with the coexistence in the iris of two different M 2 receptors: the cardiac (M 2α) subtype on the noradrenergic nerves and the smooth muscle (M 2β) subtype on the iris sphincter muscle. The rank order of potency of the antagonists studied at the prejunctional site was: atropine > himbacine . AF-DX 116 > pirenzepine > hexahydrosiladifenidol. The order of potency at the postjunctional receptors mediating the methacholine-induced isotonic contraction of the isolated rabbit iris sphincter was: atropine > hexahydrosiladifenidol > pirenzepine > himbacine > AF-DX 116.
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