Nonadrenergic, noncholinergic relaxation of bovine iris sphincter: Role of endogenous 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 apparatus. Contraction- relaxation response was elicited by electrical field stimulation (ES; 12 Hz, 50-msec duration, 70-80 V). Relaxation was arbitrarily defined as maximal decrease of tension below prestimulation baseline after cessation of ES. We also determined the tissue levels of cyclic guanosine monophosphate (cGMP) by radioimmunoassay. RESULTS. ES produced a biphasic response: contraction followed by relaxation. After cessation of ES, the muscle relaxed to below the initial baseline tension. Tetrodotoxin (TTX) abolished most of the contraction and all the relaxation response. Atropine blocked most of the contraction component, leaving the relaxation component unchanged. Prazosin and bupranolol (α₁-adrenergic and β-adrenergic antagonists, respectively) also did not affect the relaxation component of the response. Neither substance P nor its antagonist (N-acetyl-L-tryptophane 3,5-bis (trifluoromethyl)-benzyl ester; ATTB) inhibited or mimicked the response. The nitric oxide synthase (NOS) inhibitors N(ω)-nitro-L-arginine methyl ester (L-NAME), N(ω)-nitro-L-arginine (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 relaxation 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 decreased 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-cGMP cascade.