Different muscarine receptors mediate the prejunctional inhibition of [3H]-noradrenaline release in rat or guinea-pig iris and the contraction of the rabbit iris sphincter muscle.

To investigate the muscarine receptor type mediating inhibition of [3H]-noradrenaline release from the isolated rat and guinea-pig iris we have determined the potency of antimuscarinic drugs to antagonize the methacholine-induced inhibition of [3H]-noradrenaline overflow evoked by field stimulation (3 Hz, 2 min). The prejunctional apparent affinities were compared with those obtained for postjunctional muscarine receptors mediating the methacholine-induced contraction of the isolated rabbit iris sphincter muscle. Prejunctional apparent affinity constants of pirenzepine (6.67), himbacine (8.51), methoctramine (7.92), 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP, 8.00), hexahydro-difenidol enantiomers (6.92, (R); 5.77, (S)) in the rat iris and methoctramine (7.58) in the guinea-pig iris indicate the presence of M2 receptors. Although the postjunctional affinity constants in the rabbit iris sphincter of methoctramine (5.93), gallamine (3.92), and 4-DAMP (9.07) confirm our previous suggestions of the presence of M3-like receptors, the results obtained with the hexahydro-difenidol enantiomers do not agree with that concept. The postjunctional affinity constants of the hexahydro-difenidol enantiomers were not different from the prejunctional values (6.86, (R); 5.55, (S)), indicating a similar and low degree of stereoselectivity for these stereoisomers at both receptor sites (14 and 17, (R)/(S)-ratios, respectively). Hence, the postjunctional muscarine receptor in the rabbit iris sphincter fails to exhibit the high degree of stereoselectivity observed for hexahydro-difenidol enantiomers at M3 receptors on other smooth muscles.