Mechanical and electrophysiological effects of endothelin-1 on guinea-pig isolated lower oesophageal sphincter circular smooth muscle.
1. The effects of endothelin-1 (ET-1) on guinea-pig lower oesophageal sphincter (LOS) circular smooth muscle were investigated by using intracellular microelectrodes and isometric tension recording techniques. 2. ET-1 produced biphasic mechanical responses; an initial transient relaxation followed by a sustained contraction. The initial relaxation was not inhibited by either tetrodotoxin (TTX, 1 microM) or L-N(G)-nitroarginine (L-NOARG, 100 microM). The sustained contraction was greatly attenuated by nifedipine (1 microM). 3. ET-1 (1 - 30 nM) induced a concentration-dependent hyperpolarisation that was unaffected by TTX or L-NOARG. The ET(A) receptor antagonist, BQ123 (0.3 microM) abolished the ET-1-induced hyperpolarisation, whereas the ET(B) receptor antagonist, BQ788 (0.3 microM) had no detectable effect. Sarafotoxin S6c (10 nM) did not change the membrane potential. 4. The ET-1-induced hyperpolarisation was abolished by apamin (0.1 microM). Interestingly, apamin abolished the ET-1-induced transient relaxation but potentiated the sustained contraction. 5. In Ca(2+)-free Krebs solution, the ET-1-induced hyperpolarisation was greatly attenuated and returned to the control value when the tissue was reperfused with Krebs solution containing Ca(2+). The ET-1-induced hyperpolarisation was insensitive to nifedipine but was attenuated by SK&F 96365 (1 - [beta-[3-(4 - methoxy - phenyl)propoxy] - 4 - methoxyphenethyl] - 1H-imidazole hydrochloride, 50 microM), an inhibitor of receptor-mediated Ca(2+) entry. The residual component of the ET-1-induced hyperpolarisation was sensitive to thapsigargin (1 microM). 6. These results demonstrate that, in guinea-pig LOS circular smooth muscle, ET-1 hyperpolarizes the membrane by activating apamin-sensitive K(+) channels, mainly as a result of receptor-mediated Ca(2+) entry and partly by Ca(2+) release from intracellular stores. The hyperpolarisation triggers the initial transient relaxation, which acts to oppose the sustained contraction.
Imaeda
Kenro
K
Department of Pharmacology, University of Oxford, Mansfield Road, Oxford OX1 3QT.
Trout
Stephen J
SJ
Cunnane
Thomas C
TC
D000818:Animals; D001030:Apamin / Q000494:pharmacology; D001685:Biological Factors / Q000378:metabolism; D001696:Biomechanical Phenomena; D002118:Calcium / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D065128:Endothelin Receptor Antagonists; D019332:Endothelin-1 / Q000494:pharmacology*; D004730:Endothelium, Vascular / Q000502:physiology; D004943:Esophagogastric Junction / Q000187:drug effects* / Q000502:physiology; D006168:Guinea Pigs; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009119:Muscle Contraction / Q000187:drug effects; D009126:Muscle Relaxation / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D026902:Potassium Channel Blockers / Q000494:pharmacology; D017466:Receptors, Endothelin / Q000378:metabolism
D001685:Biological Factors; D065128:Endothelin Receptor Antagonists; D019332:Endothelin-1; D026902:Potassium Channel Blockers; D017466:Receptors, Endothelin; C067410:endothelium-dependent hyperpolarization factor; D001030:Apamin; D011188:Potassium; D002118:Calcium