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

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Abstract
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. 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 μm or less) and a concentration at high doses. 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. 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. Pretreatment with PTX did not significantly affect contraction induced by noradrenaline or 5‐hydroxytryptamine or the relaxation induced by isoprenaline in dilator muscles. 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.