Positive and negative coupling of the endothelin ETA receptor to Ca2+-permeable channels in rabbit cerebral cortex arterioles.
1. Arteriolar segments were isolated from pial membrane and studied within 10 h. Current-clamp and voltage-clamp measurements were made by patch-clamp recording from smooth muscle cells within arterioles. [Ca2+]i was measured from the smooth muscle cell layer by digital imaging of emission from fura-PE3 which was loaded into arterioles by pre-incubation with the acetoxymethyl ester derivative. The external diameter of arterioles was measured using a video-dimension analyser. 2. Endothelin-1 (ET1) was a potent constrictor of isolated arterioles and induced a sustained depolarization up to -27 mV and reduced membrane resistance (EC50 140-170 pm). At a constant holding potential of -60 mV ET-1 induced a transient followed by a sustained inward current. ET1 inhibited L-type voltage-dependent Ca2+ current. 3. ET1 induced a transient followed by sustained elevation of [Ca2+]i. The sustained effect was dependent on extracellular Ca2+. It occurred at a constant holding potential of -60 mV and was not inhibited by the Ca2+ antagonists nicardipine (1 microM) or D600 (10 microM). Thapsigargin (1 microM) completely depleted Ca2+ from caffeine- and ET1-sensitive sarcoplasmic reticulum but did not inhibit the ET1-induced sustained elevation of [Ca2+]i. ET1 effects on [Ca2+]i were prevented by the ETA receptor antagonist BQ123 (cyclo-D-Asp-Pro-D-Val-Leu-D-Trp). 4. The data suggest that ETA receptors are negatively coupled to L-type Ca2+ channels and positively coupled to receptor-operated Ca2+-permeable channels. Inhibition of L-type Ca2+ channel activity may suppress autoregulation, and Ca2+ influx through receptor-operated channels may have a major functional role in the potent long-lasting constrictor effect of endothelin-1 in the cerebral microcirculation.
Guibert
C
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School of Biomedical Sciences, Worsley Building, University of Leeds, Leeds LS2 9JT, UK.
Beech
D J
DJ
D000818:Animals; D001160:Arterioles / Q000502:physiology; D015220:Calcium Channels / Q000378:metabolism*; D002536:Cerebral Arteries / Q000502:physiology*; D002540:Cerebral Cortex / Q000098:blood supply*; D002560:Cerebrovascular Circulation / Q000502:physiology; D004558:Electric Stimulation; D004594:Electrophysiology; D019332:Endothelin-1 / Q000494:pharmacology; D005711:Gallopamil / Q000494:pharmacology; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000502:physiology; D018408:Patch-Clamp Techniques; D011817:Rabbits; D044022:Receptor, Endothelin A; D017466:Receptors, Endothelin / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism
D015220:Calcium Channels; D019332:Endothelin-1; D044022:Receptor, Endothelin A; D017466:Receptors, Endothelin; D005711:Gallopamil