Presynaptic effects of methoctramine on release of acetylcholine

Abstract

Stores of transmitter, labelled with [ 3H]choline, were used to study the negative feedback modulation of the release of acetylcholine by presynaptic M 2 muscarinic receptors. The release of acetylcholine was measured by radioassaying the electrical stimulation-evoked release of [ 3H]acetylcholine from slices of cerebral cortex of the rat and from the Auerbach plexus of the guinea pig ileum. Experimental conditions (2 Hz, 240 shocks) were chosen where the negative-feedback modulation by endogenous acetyleholine was not significant, therefore the presynaptic affinity constant for antagonists was not underestimated. The M 2 agonist oxotremorine inhibited the release of acetylcholine in a concentration-dependent manner in both preparations. The IC 50 values for oxotremorine were 10.8 ± 4.89 × 10 −6 M on the cortex and 5.89 ± 3.85 × 10 −8 M on the Auerbach plexus ( n = 4). The effect of oxotremorine was blocked by atropine, similarly to methoctramine, which is a cardioselective muscarinic receptor antagonist. The dose-ratio and dissociation constant were calculated by measuring the rightward shift that methoctramine and other antagonists produced on the inhibitory dose-effect curve for oxotremorine. The antagonist equilibrium dissociation constants (p k B) of methoctramine were 5.69 ± 0.27 and 5.51 ± 0.37 on the cortical and the myenteric plexus preparations, respectively ( n = 4). Postsynaptic antimuscarinic affinity (p A 2) of methoctramine on the smooth muscle of the guinea pig ileum was found to be 6.68 ± 0.11 n = 4). These findings indicate that, although methoctramine is a cardioselective compound, unlike pancuronium, it may not be a useful tool for discriminating between different presynaptic muscarinic receptors. However, methoctramine was able to distinguish pre- and postsynaptic muscarinic receptors, located at the Auerbach plexus-smooth muscle neuroeffector site.