Indirectly Gated Cl−-Dependent Cl−Channels Sense Physiological Changes of Extracellular Chloride in the Leech

Abstract

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 Ca2+. 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 Ca2+. 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.