Myogenic NOS in canine lower esophageal sphincter: enzyme activation, substrate recycling, and product actions.

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). N G-nitro-l-arginine (l-NNA, 10-4 M), IbTX (10-8 M), or buffering intracellular Ca2+ to 8 nM decreased outward currents >80%. Sodium nitroprusside (NaNP, 10-4 M) restoredl-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 afterl-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, likel-arginine, reversedl-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.