Effect of Atropine on Vascular Adrenergic Neuroeffector Transmission

Abstract

Atropine, homatropine, scopolamine, procaine, lidocaine and phentolamine inhibited the contractile response of rabbit isolated pulmonary artery elicited by electrical-field stimulation. Methylatropine had no effect. The inhibition induced by atropine (2 × 10^–6–2 × 10^–4 m) had a rapid onset of action and then remained almost constant. The inhibition was slowly reversible. The potency of atropine as an inhibitor of responses to field stimulation was very much less than the potency of phentolamine. The inhibition was not antagonized by cocaine or (+)-amphetamine. Atropine (3 × 10^–5 and 3 × 10^–4 m) enhanced the electrical-field-stimulation-induced outflow of tritium from the pulmonary artery preloaded with ³H-(–)-noradrenaline. In contrast, atropine in a concentration-dependent manner either had no effect or slightly decreased the tyramine-induced outflow of tritium. Atropine reduced the contractile response of the pulmonary artery evoked by tyramine. Atropine (10^–4 and 3 × 10^–4 m) and phentolamine inhibited the arterial contractions elicited by exogenous (–)-noradrenaline in an apparently competitive manner. The contractions of rabbit isolated aorta elicited by (–)-noradrenaline, serotonin and histamine were inhibited by atropine (10^–5 and 10^–4 m). Atropine was very much less potent in antagonizing noradrenaline, histamine and serotonin than in antagonizing acetylcholine. The inhibitory potency of atropine, procaine and lidocaine on the accumulation of ³H-(–)-noradrenaline by rabbit aorta in vitro was much less than that of cocaine. The relationship between the aortic concentration of ³H-atropine and in vitro accumulation was almost linear. The accumulation was slightly higher at37 °C than at 1 °C. The results suggest that atropine blocks α-adrenoceptors, both presynaptically at the adrenergic neurone terminals and postsynaptically at the smooth muscle. In addition, atropine may possibly act in a nonspecific manner at postsynaptic sites.