Pharmacological "cross-inhibition" of connexin hemichannels and swelling activated anion channels.
The study of ion channels has relied heavily on the use of pharmacological blocking agents. However, many of these agents have multiple effects, which may compromise interpretation of results when the affected mechanisms/pathways mediate similar functions. Volume regulated anion channels (VRAC) and connexin hemichannels can both mediate the release of glutamate and taurine, although these channels have distinct activation stimuli and hemichannels, but not VRAC, are permeable to Lucifer Yellow (LY). It has been reported that some anion channel blockers may inhibit connexin hemichannels. We further examined the effects of classic gap junction/hemichannel blockers and anion channel blockers on these channels. The typical VRAC blockers, NPPB, IAA-94, and tamoxifen blocked low divalent cation-induced glutamate and taurine release and LY loading, presumed due to hemichannel opening. The blocking action of these compounds on hemichannels was concentration dependent and fell within the same range where the drugs classically block VRACs. Conversely, carbenoxolone (CBX), the most widely used gap junction/hemichannel blocker, was an effective blocker of VRAC-mediated glutamate and taurine release, and blocked these channels at similar concentrations at which it blocked hemichannels. The CBX effect on VRACs was verified using astrocytes from connexin 43 knock out (Cx43 KO) animals. In these cells, the hypotonic induced amino acid flux was retained whereas the low divalent cation solution-induced flux was lost. These results extend our knowledge about "cross-inhibition" of VRACs and gap junctions/hemichannels by certain pharmacological agents. Given the overlap in function of these two types of channels, great care must be exerted in using pharmacological blockers to identify one channel from the other.
Department of Neurology, University of Washington School of Medicine, Seattle, Washington 98104, USA. zcye@u.washington.edu
Zu-Cheng
ZC
Ye
Nancyann
N
Oberheim
Helmut
H
Kettenmann
Bruce R
BR
Ransom
D000596:Amino Acids / Q000378:metabolism; D000704:Analysis of Variance; D020533:Angiogenesis Inhibitors / Q000494:pharmacology; D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes / Q000187:drug effects*; D002229:Carbenoxolone / Q000494:pharmacology; D048429:Cell Size / Q000187:drug effects; D002478:Cells, Cultured; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D018031:Connexin 43 / Q000037:antagonists & inhibitors / Q000172:deficiency / Q000187:drug effects / Q000502:physiology*; D006034:Glycyrrhetinic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology; D007038:Hypotonic Solutions / Q000494:pharmacology; D007189:Indans / Q000494:pharmacology; D015640:Ion Channel Gating / Q000187:drug effects / Q000235:genetics / Q000502:physiology*; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007546:Isoquinolines / Q000378:metabolism; D051379:Mice; D018345:Mice, Knockout; D009579:Nitrobenzoates / Q000494:pharmacology; D051381:Rats; D013629:Tamoxifen / Q000494:pharmacology
D000596:Amino Acids; D020533:Angiogenesis Inhibitors; D018031:Connexin 43; D007038:Hypotonic Solutions; D007189:Indans; D007473:Ion Channels; D007546:Isoquinolines; D009579:Nitrobenzoates; C060056:indanoxyacetic acid 94; D013629:Tamoxifen; C119129:18alpha-glycyrrhetinic acid; C058176:5-nitro-2-(3-phenylpropylamino)benzoic acid; C017475:lucifer yellow; D002229:Carbenoxolone; D006034:Glycyrrhetinic Acid