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Playfair Neuroscience Unit, Toronto Hospital (Western Division), Toronto M5T 2S8; Department of Biomedical Sciences, Faculty of Health Sciences, McMaster University, Hamilton, Ontario, Canada L8N 3Z5; and Department of Molecular Genetics, Kumamoto University School of Medicine, Kumamoto 862, Japan
Depolarization elicited outward
K+ currents from canine lower
esophageal sphincter (LES) muscle cells, primarily through iberiotoxin (IbTX)- and tetraethylammonium-sensitive
Ca2+-dependent
K+ channels. Current magnitudes
varied with pipette Ca2+
concentration (EC50 = 108.5 nM).
NG-nitro-L-arginine
(L-NNA,
10
4 M), IbTX
(108 M), or buffering
intracellular Ca2+ to 8 nM
decreased outward currents >80%. Sodium nitroprusside (NaNP,
104 M) restored
L-NNA-inhibited or low
intracellular Ca2+ concentration
(not IbTX)-inhibited currents.
L-NNA or IbTX
application depolarized LES cells from 
43 to 35 mV. NaNP
restored the membrane potential to 46 mV after
L-NNA but not after IbTX
application. Nifedipine (30 µM) reduced outward currents and
abolished or reduced L-NNA or
NaNP effects, respectively. Immunocytochemistry revealed the presence
of both argininosuccinate synthetase and argininosuccinate lyase in LES
muscle cells. L-Citrulline, like
L-arginine, reversed L-NNA inhibition of outward
currents; only L-arginine
reversed inhibition of outward currents by an antibody to
argininosuccinate synthetase. Therefore, endogenous nitric oxide
production, activated by Ca2+
entrance involving L-type
Ca2+ channels, may continuously
enhance outward currents to modulate LES muscle cell membrane potential
and excitability.
nitric oxide synthase; calcium-dependent potassium channel; L-citrulline recycling; lower esophageal sphincter tone; sarcoplasmic reticulum; smooth muscle
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