title: Feedback via Ca²⁺-activated ion channels modulates endothelin 1 signaling in retinal arteriolar smooth muscle.
abstract: To investigate the role of feedback by Ca²⁺-sensitive plasma-membrane ion channels in endothelin 1 (Et1) signaling in vitro and in vivo. Methods. Et1 responses were imaged from Fluo-4-loaded smooth muscle in isolated segments of rat retinal arteriole using two-dimensional (2-D) confocal laser microscopy. Vasoconstrictor responses to intravitreal injections of Et1 were recorded in the absence and presence of appropriate ion channel blockers using fluorescein angiograms imaged using a confocal scanning laser ophthalmoscope. Results. Et1 (10 nM) increased both basal [Ca²⁺](i) and the amplitude and frequency of Ca²⁺-waves in retinal arterioles. The Ca²⁺-activated Cl⁻-channel blockers DIDS and 9-anthracene carboxylic acid (9AC) blocked Et1-induced increases in wave frequency, and 9AC also inhibited the increase in amplitude. Iberiotoxin, an inhibitor of large conductance (BK) Ca²⁺-activated K⁺-channels, increased wave amplitude in the presence of Et1 but had no effect on frequency. None of these drugs affected basal [Ca²⁺](i). The voltage-operated Ca²⁺-channel inhibitor nimodipine inhibited wave frequency and amplitude and also lowered basal [Ca²⁺](i) in the presence of Et1. Intravitreal injection of Et1 caused retinal arteriolar vasoconstriction. This was inhibited by DIDS but not by iberiotoxin or penitrem A, another BK-channel inhibitor. Conclusions. Et1 evokes increases in the frequency of arteriolar Ca²⁺-waves in vitro, resulting in vasoconstriction in vivo. These responses, initiated by release of stored Ca²⁺, also require positive feedback via Ca²⁺-activated Cl⁻-channels and L-type Ca²⁺-channels.
authors.affiliation: Centre for Vision and Vascular Science, Queen's University of Belfast, Institute of Clinical Sciences, The Royal Victoria Hospital, Belfast, Northern Ireland.
authors.forename: Michael
authors.initials: M
authors.lastname: Stewart
authors.forename: Maurice
authors.initials: M
authors.lastname: Needham
authors.forename: Peter
authors.initials: P
authors.lastname: Bankhead
authors.forename: Tom A
authors.initials: TA
authors.lastname: Gardiner
authors.forename: C Norman
authors.initials: CN
authors.lastname: Scholfield
authors.forename: Tim M
authors.initials: TM
authors.lastname: Curtis
authors.forename: J Graham
authors.initials: JG
authors.lastname: McGeown
mesh_terms: D000818:Animals; D001160:Arterioles / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D002462:Cell Membrane / Q000378:metabolism; D019332:Endothelin-1 / Q000494:pharmacology*; D005451:Fluorescein Angiography; D066298:In Vitro Techniques; D058449:Intravitreal Injections; D008297:Male; D018613:Microscopy, Confocal; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012161:Retinal Artery / Q000378:metabolism*; D015398:Signal Transduction; D014661:Vasoconstriction
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D019332:Endothelin-1; D002118:Calcium