Effects of acetylcholinesterase inhibitor paraoxon denote the possibility of non-quantal acetylcholine release in myocardium of different vertebrates

Abstract

Effects of organophosphorous acetylcholinesterase inhibitor paraoxon were studied in the isolated atrial and ventricular myocardium preparations of a fish (cod), an amphibian (frog) and a mammal (rat) using the microelectrode technique. Incubation of isolated atrium with paraoxon (5 × 10(-6)-5 × 10(-5) M) caused significant reduction of action potential duration and marked slowing of sinus rhythm. These effects were abolished by muscarinic blocker atropine and therefore are caused by acetylcholine, which accumulates in the myocardium due to acetylcholinesterase inhibition even in the absence of vagal input. Hemicholinium III is a blocker of high affinity choline-uptake transporters, which are believed to mediate non-quantal release of acetylcholine from cholinergic terminals in different tissues. In the atrial myocardium of all the three studied species, hemicholinium III (10(-5) M) significantly suppressed all the effects of paraoxon. Blocker of parasympathetic ganglionic transmission hexamethonium bromide (10(-4) M) and inhibitor of vesicular acetylcholine transporters vesamicol (10(-5) M) failed to attenuate paraoxon effects. Among ventricular myocardium preparations of three species paraoxon provoked marked cholinergic effects only in frog, hemicholinium III abolished these effects effectively. We conclude that paraoxon stops degradation of acetylcholine in the myocardium and helps to reveal the effects of acetylcholine, which is continuously secreted from the cholinergic nerves in non-quantal manner. Thus, non-quantal release of acetylcholine in the heart is not specific only for mammals, but is also present in the hearts of different vertebrates.

Both neuronal and non-neuronal acetylcholine take part in non-quantal acetylcholine release in the rat atrium

“…As we have previously demonstrated, cholinergic effects of AChE inhibitors (armin, neostigmine and paraoxon) in the heart are due to the accumulation of non-quantal ACh (Abramochkin et al, 2010(Abramochkin et al, , 2012. According to the present results, AChE inhibitor paraoxon produces cholinergic effects in the atrial myocardial preparations of adult rats, newborns and 19-day rat embryos (Figs.…”

“…These findings were confirmed in the experiments on non-mammalian vertebrate species (Abramochkin et al, 2012). While quantal ACh release is based on exocytosis of ACh-containing vesicle from parasympathetic postganglionic intramural neurons and therefore depends on the impulse activity of these neurons, non-quantal release is a continuous leakage of ACh through the membrane independent on impulse activity of postganglionic neurons.…”

“…According to our previous results (Abramochkin et al, 2012), 5 × 10 − 6 M is the most effective concentration of paraoxon, producing cholinergic effects by means of ACh accumulation in the myocardium. These effects were completely abolished by 3 × 10 − 6 M atropine and therefore are mediated by accumulating ACh.…”

Luminescent nanoparticles for rapid monitoring of endogenous acetylcholine release in mice atria

“…Reliability of the sensing was also confirmed by the lack of any luminescence response to ACh when the enzyme AChE was inhibited by paraoxon. It is worth noting that paraoxon should be very efficient in inhibition of AChE expressed in atria, because dense parasympathetic innervation of atria is the well known cause of the significant effect of paraoxon on electrogenesis in atrial myocytes …”

“…It is worth noting that paraoxon should be very efficient in inhibition of AChE expressed in atria, because dense parasympathetic innervation of atria is the well known cause of the significant effect of paraoxon on electrogenesis in atrial myocytes. [25] 2 | EXPERIMENTAL…”