Nonadrenergic, noncholinergic relaxation of bovine iris sphincter: Role ofendogenous nitric oxide

PURPOSE. TO investigate the role of endogenously generated nitric oxide (NO ) in the relaxation of bovine iris sphincter. METHODS. Isolated bovine sphincters were mounted on an isometric tension ap paratus. Contraction-relaxation response was elicited by electrical field s timulation (ES; 12 Hz, 50-msec duration, 70-80 V). Relaxation was arbitrari ly defined as maximal decrease of tension below prestimulation baseline aft er cessation of ES. We also determined the tissue levels of cyclic guanosin e monophosphate (cGMP) by radioimmunoassay. RESULTS. ES produced a biphasic response: contraction followed by relaxatio n. After cessation of ES, the muscle relaxed to below the initial baseline tension. Tetrodotoxin (TTX) abolished most of the contraction and all the r elaxation response. Atropine blocked most of the contraction component, lea ving the relaxation component unchanged. Prazosin and bupranolol (alpha(1)- adrenergic and beta-adrenergic antagonists, respectively) also did not affe ct the relaxation component of the response. Neither substance P nor its an tagonist (N-acetyl-L-tryptophane 3,5-bis (trifluoromethyl)-benzyl ester; AT TB) inhibited or mimicked the response. The nitric oxide synthase (NOS) inh ibitors N-omega-nitro-L-arginine methyl ester (L-NAME), N-omega-nitro-L-arg inine (L-NNA), and aminoguanidine dose-dependently inhibited the relaxation response by 50% to 70%. The free radical scavenger 2-(4-carboxyphenyl)-4,4 ,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (carboxy-PTIO) and the guanylyl cyclase inhibitor methylene blue also abrogated 70% and 45% of the relaxat ion response, respectively. ES caused an increase in muscle cGMP from 2.3 /- 0.3 to 3.9 +/- 0.5 picomoles per muscle. L-NNA or L-NAME significantly d ecreased the tissue cGMP content (to 1.2 +/- 0.1 picomoles per muscle) and prevented the increase caused by ES. CONCLUSIONS. The relaxation component of the iris sphincter response to ES is a distinct nonadrenergic, noncholinergic, ES-induced event. Most of the relaxation is mediated by the endogenously generated NO-guanylyl cyclase-cG MP cascade.