Nonadrenergic, noncholinergic relaxation of bovine iris sphincter: role of endogenous nitric oxide.
OBJECTIVE
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 (alpha1-adrenergic and beta-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 Nomega-nitro-L-arginine methyl ester (L-NAME), Nomega-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.
Pianka
P
P
Department of Ophthalmology, The Tel Aviv Sourasky Medical Center, Israel.
Oron
Y
Y
Lazar
M
M
Geyer
O
O
D018663:Adrenergic Agents / Q000494:pharmacology; D000818:Animals; D002417:Cattle; D018678:Cholinergic Agents / Q000494:pharmacology; D006152:Cyclic GMP / Q000378:metabolism; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors; D007498:Iris / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology*; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009569:Nitric Oxide / Q000502:physiology*; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D011680:Pupil / Q000502:physiology
D018663:Adrenergic Agents; D018678:Cholinergic Agents; D004791:Enzyme Inhibitors; D009569:Nitric Oxide; D019001:Nitric Oxide Synthase; D006162:Guanylate Cyclase; D006152:Cyclic GMP