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Molecular Pharmacology, Volume 52, Issue 5, 886-895
Research Institute of Toxicology, Utrecht University, NL-3508 TD Utrecht, The Netherlands
Atropine, the classic muscarinic receptor antagonist, inhibits ion
currents mediated by neuronal nicotinic acetylcholine receptors expressed in Xenopus laevis oocytes. At the holding
potential of 
80 mV, 1 µM atropine inhibits 1 mM acetylcholine-induced inward currents mediated by rat
2
2, 24, 32, 34, 42, 44, and 7
nicotinic receptors by 12-56%. Inward currents induced with a low
agonist concentration are equally inhibited (32, 34), less
inhibited (24, 7), or potentiated (42, 44) by 1 µM atropine. Effects on the more sensitive 44
nicotinic receptors were investigated in detail by systematic variation
of acetylcholine and atropine concentrations and of membrane potential.
At high agonist concentration, atropine inhibits 44 nicotinic
receptor-mediated ion current in a noncompetitive, voltage-dependent
way with IC50 values of 655 nM at 80 mV and
of 4.5 µM at 40 mV. At low agonist concentration, 1 µM atropine potentiates 44 nicotinic
receptor-mediated ion current. This potentiating effect is surmounted
by high concentrations of acetylcholine, indicating a competitive
interaction of atropine with the nicotinic receptor, and potentiation
is also reversed at high atropine concentrations. Steady state effects
of acetylcholine and atropine are accounted for by a model for combined
receptor occupation and channel block, in which atropine acts on two
distinct sites. The first site is associated with noncompetitive ion
channel block. The second site is associated with competitive
potentiation, which appears to occur when the agonist recognition sites
of the receptor are occupied by acetylcholine and atropine. The
apparent affinity of atropine for the agonist recognition sites of the 44 nicotinic acetylcholine receptor is estimated to be 29.9 µM.
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