Differential distribution of ionic channels and muscarinic receptors at the cat tracheal neuromuscular junction

Abstract

The ionic and pharmacological properties of atropine-sensitive excitatory junction potentials (EJPs) were investigated by the double sucrose gap and microelectrode membrane potential recording methods, and compared with those of inward currents evoked by carbachol ( I CCh), in cat tracheal smooth muscle. A single and repetitive field stimulation (10–30 V, 50 μs, 20 Hz) evoked atropine-inhibitable EJPs and associated twitch-like contractions. Reduction in external Na + concentration strongly, but in the external Cl − concentration, decreased the EJP amplitude after 5 min superfusion, although prolonged exposure to low Cl − solutions attenuated the EJPs modestly. Chloride channel blockers such as 9-AC and niflumic acid (each 100 μ M), at concentrations high enough to inhibit I CCh almost completely, failed to abolish the EJPs. Pirenzepine, AF-DX116 and 4-DAMP all effectively inhibited the EJPs at their concentrations to block respective muscarinic receptor subtypes relatively specifically, while depletion of internal stores by 10 mM caffeine and/or 3 μM ryanodine caused only a partial decrease in the EJP amplitude. These properties are considerably different from those of I CCh which is activated exclusively through activation of the M 3 receptor/IP 3-mediated Ca 2+ release pathway and reflects mostly a Ca 2+-dependent Cl − current, and suggests the differential distribution of muscarinic receptors and ionic channels inside and outside of the cholinergic neuromuscular junction of this muscle.