Mechanism of action of nicotine in isolated urinary bladder of guinea‐pig

Open Access
Publisher Website
Google Scholar
Abstract
1 Nicotine produced a transient contraction of isolated strips of guinea-pig urinary bladder. The response to nicotine was antagonized by the nicotinic receptor antagonist, hexamethonium but was insensitive to tetrodotoxin. 2 The nicotine-induced contraction was potentiated by the cholinesterase inhibitor, physostigmine, and was reduced to 50% and 70% by the muscarinic cholinoceptor antagonist, atropine and the sympathetic neurone blocking drug, guanethidine, respectively. Chemical denervation with 6-hydroxydopamine abolished the inhibitory effect of guanethidine. Simultaneous treatment with atropine and guanethidine did not abolish the response to nicotine, but the degree of inhibition was comparable to that obtained with atropine alone. 3 The nicotine-induced contraction was insensitive to bunazosin and yohimbine (α1- and α2-adrenoceptor antagonists, respectively), and exogenously applied noradrenaline did not cause a contraction even in the presence of blockade of noradrenaline uptake mechanisms with desipramine and normetanephrine and of β-adrenoceptors with propranolol, suggesting a non-adrenergic nature of the sympathomimetic effect of nicotine in this tissue. 4 The nicotine-induced contraction in the presence of atropine was abolished after desensitization of P2-purinoceptors with α, β-methylene adenosine 5′-triphosphate, a slowly degradable ATP analogue selective for P2-purinoceptors. By this desensitization, the response to ATP, but not to histamine, was also abolished. 5 A cyclo-oxygenase inhibitor flurbiprofen partially inhibited the nicotine-induced contraction. The degree of the inhibition was more pronounced in the presence of atropine than in its absence. Flurbiprofen antagonized the response to exogenously applied ATP in an unsurmountable manner, but not that to carbachol. 6 The present results suggest that nicotine might induce a contraction through an interaction with nicotinic receptors located on the terminals of, possibly, (i) parasympathetic cholinergic, (ii) sympathetic non-adrenergic and (iii) non-sympathetic purinergic nerves in guinea-pig detrusor preparations, and that a portion of the contraction due to the purine nucleotide released is possibly potentiated by intramural prostaglandin(s). Parasympathetic cholinergic output might be modulated by an unknown excitatory substance released by nicotine from sympathetic nerve. 7 Nicotine reveals a latent excitatory effect of the sympathetic hypogastric nerve which innervates guinea-pig detrusor.