Abstract
In canine lower esophageal sphincter, myogenic constitutive nitric-oxide (NO) synthase (NOS) in plasma membrane limits tone by opening large conductance Ca2+-dependent K+ channels (BKCa channels) and hyperpolarizing the membrane. We examined whether KV channels were involved and whether NO from enteric nerves and from NO donors used the same mechanisms. With nerves inactive, 100 nM iberiotoxin, likeN-nitro-l-arginine(l-NOARG), increased tone but less. 4-Aminopyridine (4-AP) at 5 mM behaved similarly. Tetraethyl ammonium (TEA) at 20 mM equaled the effect of l-NOARG and occluded any tone increase from any combination of these agents. More than iberiotoxin or 4-AP, TEA decreased relaxations in response to sodium nitroprusside (SNP) or 3-morpholino-sydnonimine (Sin-1) by ∼50%. In whole-cell patch-clamp recordings, TEA and 4-AP reduced outward K+currents additively by >90% at depolarization of +90 mV. Thus, K+ channels in addition to BKCa channels are opened by myogenic NO, and exogenous NO had relaxing effects both related and unrelated to K+ channel openings. TEA (20 mM) increased tone but did not inhibit relaxations to electrical field stimulation (EFS) of enteric nerves. 4-AP relaxed tone, an effect that was abolished and reversed by l-NOARG. 4-AP apparently released NO and acetylcholine from nerves. The putative Cl− channel blocker niflumic acid (NFA; 30–100 μM) dose dependently reduced tone, but tone, restored by 10−6 M carbachol or 20 mM TEA, was still relaxed by EFS and by SNP. 4,4′-Diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) at 500 to 1000 μM did not inhibit relaxation to EFS or SNP. The addition of TEA (20 mM) to DIDS (1000 μM) induced tonic and phasic activity and markedly inhibited relaxations to EFS. DIDS plus TEA reduced the relaxations to SNP like TEA alone. Reduction in extracellular [Cl−] by isethionate substitution reduced tone but did not reduce relaxations when tone was restored. The combination of reduced extracellular [Cl−] and TEA did not abolish relaxation to EFS until DIDS was added. Thus, multiple K+channels are opened by myogenic NO, and openings of these channels, as well as DIDS-sensitive, undefined mechanisms, are induced when NO is released from nerves or SNP.
Footnotes
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Send reprint requests to: E. E. Daniel, Ph.D., Room 4N51, Health Sciences Centre, McMaster University, 1200 Main St. W., Hamilton, Ontario L8N 3Z5, Canada. E-mail:daniele{at}fhs.csu.mcmaster.ca
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↵1 This study was supported by the Medical Research Council of Canada.
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↵2 Current address: Playfair Neuroscience Division, Toronto Hospital, Western Division, 399 Bathurst St., University of Toronto, Toronto, Ontario M5T 2S8, Canada.
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↵3 These are cooperative students from Hill Park High School and summer students. T.B. received Summer Scholarships from the Canadian Association for Gastroenterology. A.L. received Summer Scholarships from ASPET and the American Digestive Health Foundation. M.R. received a Summer Scholarship from the American Digestive Health Foundation. S.T., V.N., and V.M. were cooperative students from Hill Park High School, as were T.B. and M.R. initially.
- Abbreviations:
- LES
- lower esophageal sphincter
- 4-AP
- 4-aminopyridine
- BKCa channels
- large conductance Ca2+-dependent K+ channels
- DIDS
- 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid
- EFS
- electrical field stimulation
- Ibtx
- iberiotoxin
- l-NOARG
- N-nitro-l-arginine
- [Ca2+]i
- intracellular Ca2+concentration
- AUC
- area under the curve
- NO
- nitric oxide
- cNOS
- constitutive nitric-oxide synthase
- NOS
- nitric-oxide synthase
- NFA
- niflumic acid
- Sin-1
- 3-morpholino-sydnonimine
- ω-CTX(GVIA)
- ω-conotoxin (GVIA)
- SNP
- sodium nitroprusside
- PSS
- physiological salt solution
- ICC
- interstitial cells of Cajal
- TEA
- tetraethyl ammonium
- TTX
- tetrodotoxin
- Received September 3, 1999.
- Accepted March 14, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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