Indirectly gated Cl(-)-dependent Cl(-) channels sense physiological changes of extracellular chloride in the leech.
The maintenance of ion homeostasis requires adequate ion sensors. In leeches, 34 nephridial nerve cells (NNCs) monitor the Cl(-) concentration of the blood. After a blood meal, the Cl(-) concentration of leech blood triples and is gradually restored to its normal value within 48 h after feeding. As previously shown in voltage-clamp experiments, the Cl(-) sensitivity of the NNCs relies on a persistent depolarizing Cl(-) current that is turned off by an increase of the extracellular Cl(-) concentration. The activation of this Cl(-)-dependent Cl(-) current is independent of voltage and of extra- and intracellular Ca(2+). The transduction mechanism is now characterized on the single-channel level. The NNC's sensitivity to Cl(-) is mediated by a slowly gating Cl(-)-dependent Cl(-) channel with a mean conductance of 50 pS in the cell-attached configuration. Gating of the Cl(-) channel is independent of voltage, and channel activity is independent of extra- and intracellular Ca(2+). Channel activity and the macroscopic current are reversibly blocked by bumetanide. In outside-out patches, changes of the extracellular Cl(-) concentration do not affect channel activity, indicating that channel gating is not via direct interaction of extracellular Cl(-) with the channel. As shown by recordings in the cell-attached configuration, the activity of the channels under the patch is instead governed by the Cl(-) concentration sensed by the rest of the cell. We postulate a membrane-bound Cl(-)-sensing receptor, which-on the increase of the extracellular Cl(-) concentration-closes the Cl(-) channel via a yet unidentified signaling pathway.
Wenning
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A
Fakultät für Biologie, Universität Konstanz, D-78457 Konstanz, Germany. awenning@biology.emory.edu
Erxleben
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Calabrese
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RL
D000818:Animals; D002034:Bumetanide / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D018118:Chloride Channels / Q000502:physiology*; D002712:Chlorides / Q000493:pharmacokinetics; D004232:Diuretics / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D005110:Extracellular Space / Q000378:metabolism; D015640:Ion Channel Gating / Q000502:physiology*; D007865:Leeches; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000502:physiology*; D009997:Osmotic Pressure; D018408:Patch-Clamp Techniques; D015398:Signal Transduction / Q000502:physiology
D002614:Chelating Agents; D018118:Chloride Channels; D002712:Chlorides; D004232:Diuretics; D002034:Bumetanide; D004533:Egtazic Acid; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid