K. Tanaka et al., Differential distribution of ionic channels and muscarinic receptors at the cat tracheal neuromuscular junction, AUTON NEURO, 84(1-2), 2000, pp. 30-39
The ionic and pharmacological properties of atropine-sensitive excitatory j
unction potentials (EJPs) were investigated by the double sucrose gap and m
icroelectrode membrane potential recording methods, and compared with those
of inward currents evoked by carbachol (I-CCh), in cat tracheal smooth mus
cle. A single and repetitive field stimulation (10-30 V, 50 mu s, 20 Hz) ev
oked atropine-inhibitable EJPs and associated twitch-like contractions. Red
uction in external Nai concentration strongly, but in the external Cl- conc
entration, decreased the EJP amplitude after 5 min super-fusion, although p
rolonged exposure to low Cl- solutions attenuated the EJPs modestly. Chlori
de channel blockers such as 9-AC and niflumic acid (each 100 mu M), at conc
entrations high enough to inhibit I-CCh almost completely, failed to abolis
h the EJPs. Pirenzepine, AF-DX116 and 4-DAMP all effectively inhibited the
EJPs at their concentrations to block respective muscarinic receptor subtyp
es relatively specifically, while depletion of internal stores by 10 mM caf
feine and/or 3 mu M ryanodine caused only a partial decrease in the EJP amp
litude. These properties are considerably different from those of I-CCh whi
ch is activated exclusively through activation of the M-3 receptor/IP3-medi
ated Ca2+ release pathway and reflects mostly a Ca2+ -dependent Cl- current
, and suggests the differential distribution of muscarinic receptors and io
nic channels inside and outside of the cholinergic neuromuscular junction o
f this muscle. (C) 2000 Elsevier Science B.V. All rights reserved.