Pertussis toxin‐sensitive muscarinic relaxation in the rat iris dilator muscle

Abstract

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 microM 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.