Abstract
The α3/β4 rat neuronal nicotinic acetylcholine receptor, stably transfected in human embryonic kidney cells, was examined using the whole-cell-clamp technique and 2-dimensional confocal imaging. Application of agonists (nicotine, cytisine, epibatidine) activated a large (100–200 pA/pF) inwardly rectifying monovalent current, with little current at voltages between 0 and +40 mV. Rapid application of nicotine and cytisine indicated EC50 values of ≅22 and ≅64 μM, respectively, and suggested second order binding kinetics (Hill coefficient ∼2). The time constant of desensitization (decay) of nicotine-activated current was concentration-dependent (typically ∼10 s at 30 μM versus ∼1.0 s at 100–1000 μM), but not voltage-dependent and was significantly smaller than the ∼200 s reported for the α3/β4 receptor expressed in Xenopus oocytes. Nicotine-activated current was rapidly and reversibly blocked by coapplication of mecamylamine andd-tubocurarine. At −80 mV holding potentials, the current was also suppressed by ∼25% either upon complete removal or elevation of Ca2+ to 10 mM. Total replacement of Na+ by Ca2+ also completely blocked the current. On the other hand, evidence for permeation of Ca2+was indicated by increased inward current at −40 mV upon elevation of Ca2+ from 2 to 10 mM, as well as a rise in the cytosolic Ca2+ proportional to the current carried by the receptor. These findings are consistent with the idea that Ca2+, in addition to its channel-permeating properties, may also regulate the receptor from an extracellular site. Our results suggest that the α3/β4 neuronal nicotinic acetylcholine receptor, when stably expressed in human embryonic kidney 293 cells, has desensitization kinetics and Ca2+ regulatory mechanisms somewhat different from those described for the receptor expressed inXenopus oocytes.
Footnotes
- Received November 4, 1998.
- Accepted March 15, 1999.
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Send reprint requests to: Dr. Martin Morad, Georgetown University Medical Center, Department of Pharmacology, 3900 Reservoir Road N.W., Washington, DC 20007. E-mail: moradm{at}gunet.georgetown.edu
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Supported by National Institutes of Health Grants HL16152 and DA06486.
- The American Society for Pharmacology and Experimental Therapeutics
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