1 The characteristics of muscarinic receptors mediating relaxation and/or contraction in the rat iris dilator muscle were examined. 2 Relaxation was induced in a dilator muscle by application of acetylcholine (ACh) at low doses (3 gM or less) and contraction was induced by high doses. Methacholine and carbachol also showed biphasic effects similar to those of ACh; in contrast, bethanechol, arecoline, pilocarpine and McN-A-343 induced mainly relaxation but no substantial contraction. 3 After parasympathetic denervation by ciliary ganglionectomy, the relaxant response to muscarinic agonists disappeared upon nerve stimulation. Application of McN-A-343 and pilocarpine induced only small contractions in denervated dilator muscles, indicating that these are partial agonists for contraction.4 pA2 values of pirenzepine, methoctramine, AF-DX 116, himbacine, and 4-DAMP for antagonism to pilocarpine-induced relaxation in normal dilator muscles and those for antagonism to ACh-induced contraction in denervated dilator muscles were determined. The pA2 values for antagonism to relaxation of all these antagonists were most similar to those for M3-type muscarinic receptors. 5 Although pA2 values for contraction of these antagonists, except for methoctramine, were very close to those for relaxation, contraction was not significantly antagonized by methoctramine. Contraction might be mediated by M3-like receptors which have a very low affinity for methoctramine. 6 In conclusion, ACh-induced biphasic responses in rat iris dilator muscles were clearly distinguished from each other by specific muscarinic agonists and parasympathetic denervation, whereas muscarinic receptors could not be subclassified according to the pA2 values of 5 specific antagonists only.

Effects of ethylcholine mustard aziridinium ion (AF64A) on contractile responses to agonists or transmural nerve stimulation (TNS) were examined in rat iris sphincter muscle.The responses to TNS of isolated sphincter muscle were abolished within 1 hr after the addition of 0.1 mM AF64A to the bathing solution, while responses to acetylcholine (ACh) and 5-hydroxytryptamine (5-HT) were not changed significantly. In sphincter muscles which were isolated 3 days after the micro-injection of 100 nmol AF64A into the anterior chamber of eyes in vivo, the response to TNS were decreased to about 30% of the control.The injection of AF64A at higher concentrations often resulted in serious chronic damage of the eye. When 100 nmol AF64A was injected twice with an interval of 3 days, the response to TNS was decreased to about 20% of the control. Maximum responses and sensitivities to ACh and 5-HT were markedly enhanced in sphincters from eyes which had been treated with AF64A twice. The sensitivity of depolarized sphincters to external Ca2+ was also increased significantly. Seven weeks after the second injection, responses to TNS recover ed to more than 50% of the control and the effects of AF64A mostly disappeared. In conclusion, the acute inhibition of parasympathetic transmission without postsynaptic changes can be achieved within 1 hr after bath-application of 0.1 mM AF64A. The reduc tion of parasympathetic nerve activity by treatment with AF64A in vivo induces nonspecific supersensitivity in iris sphincter, whereas effects of AF64A were mostly reversible under the present experimental conditions.

1 The effects of pertussis toxin (PTX) on contraction and/or relaxation induced by agonists or transmural nerve stimulation (TNS) were examined in the rat iris dilator and sphincter muscles. 2 TNS in the presence of phentolamine induced an atropine-sensitive biphasic response: initial contraction followed by relaxation in dilator muscles. Exogenously applied acetylcholine (ACh) elicited a large relaxation at low doses (3 gM or less) and a concentration at high doses.3 Only the ACh-induced relaxation was affected by injection of PTX (10 ng) into the anterior eye chamber. Relaxation was decreased 12 h after injection and had completely disappeared after 24 h. Relaxation recovered in part 3 weeks and almost completely 8 weeks after PTX treatment. A gradual decrease in muscarinic relaxation in a dilator muscle was also observed in vitro after addition of PTX to the bathing solution. 4 The pA2 values of muscarinic blockers, pirenzepine, AF-DX 116, 4-DAMP, and himbacine for competitive antagonism to ACh-induced contraction were 7.14, 6.53, 9.03, and 6.80, respectively, in PTX-pretreated dilator muscles. These values are comparable to those obtained in parasympathectomized dilator muscles and may indicate involvement of M3 or M3-like receptors in muscle contraction. 5 Pretreatment with PTX did not significantly affect contraction induced by noradrenaline or 5-hydroxytryptamine or the relaxation induced by isoprenaline in dilator muscles. 6 In conclusion, among several agonist-induced responses in the rat iris dilator and sphincter muscles, only muscarinic relaxation in dilator muscle occurs via activation of PTX-sensitive GTP binding proteins.

The reduction of parasympathetic nerve activity by the treatment with ethylcholine mustard aziridinium ion (AF64A) in vivo induced both specific and non-specific supersen sitivities in the rat iris sphincter (Tanaka et al., 1999). Changes in the expression of muscarinic receptor subtypes, which could be a cause of specific supersensitivity induced by the treatment with AF64A, were examined using competitive PCR techniques. Muscarinic receptor population is composed of m2, m3, and m4 subtypes in the rat iris (Furuta et al., 1998). Interestingly, m4 mRNA was much more abundantly expressed than m2 and m3 in the rat iris sphincter. The treatment with AF64A significantly increased the mRNA levels of m2 and m3 subtypes to 370 and 330% of the control but not that of m4 (approximately 90% of the control). In addition, the total protein contents were increased to approximately 125% of the control. The up-regulation of the mRNA levels of m2 and m3 subtypes by the treatment with AF64A was significant when they were compensated for the increase in total protein contents. The down-regulation of m4 mRNA expression was not significant even after being corrected for the protein content. These results suggest that the up-regulation of the mRNA levels of m2 and m3 subtypes may be, at least in part, responsible for the supersensitivity to muscarinic agonists after the treatment with AF64A in vivo.

No abstract