Non-quantal release of acetylcholine in guinea-pig airways: Role of choline transporter

Abstract

In the resting state, motor neurons continuously release ACh through quantal and non-quantal mechanisms, the latter through vesicular ACh transporter (VAChT) and choline transporter (ChT). Although in skeletal muscle these mechanisms have been extensively studied, the non-quantal release (NQR) from parasympathetic neurons of airway smooth muscle has not been described. Here we corroborated that the organophosphate paraoxon (acetylcholinesterase inhibitor) induced a contraction blocked by atropine (muscarinic antagonist) in guinea-pig tracheal rings. This contraction was not modified by two blockers of evoked quantal release, tetrodotoxin (voltage-dependent Na+ channel blocker) and ω-conotoxin GVIA (N-type Ca2+ channel blocker), nor by the nicotinic blocker hexamethonium, suggesting that acetylcholine NQR could be responsible of the paraoxon-induced contraction. We confirmed that tetrodotoxin, and to some extent ω-conotoxin, abolished the evoked quantal ACh release induced by electrical field stimulation. Hemicholinium-3 (ChT inhibitor), but not vesamicol (VAChT inhibitor), caused a concentration-dependent inhibition of the response to paraoxon. The highest concentration of hemicholinium-3 left ~75% of the response to electrical field stimulation, implying that inhibition of paraoxon-induced contraction was not due to depletion of neuronal vesicles. Non-neuronal sources of ACh released through organic cation transporters were discarded because their inhibition by quinine or corticosterone did not modify the response to paraoxon. Calcium-free medium abolished the effect of paraoxon, and NiCl2, 2-aminoethyl diphenyl-borate and SKF 96365 partly inhibited it, suggesting that non-specific cation channels were involved in the acetylcholine NQR. We concluded that a Ca2+-dependent NQR of ACh is present in cholinergic nerves from guinea-pig airways, and that ChT is involved in this phenomenon. © 2011 The Authors. Journal compilation © 2011 The Physiological Society.