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Upregulation of muscarinic receptors by long-term nitric oxide inhibition in the rat ileum

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1. The aim of the present study was to examine the effects of long-term nitric oxide (NO) blockade on contractions of the rat ileum induced by muscarinic agonists.

2. Male Wistar rats received the NO synthesis inhibitor NG-nitro-l-arginine methyl ester (l-NAME; 20 mg/rat per day) in drinking water for 7, 15, 30 and 60 days. Concentration–responses curves to methacholine and carbachol were obtained and pEC50 values were calculated. Saturation binding assays were performed in membranes prepared from rat ileum after 60 days ofl-NAME treatment and the dissociation constant (KD) and maximal number of binding sites (Bmax) were determined by Scatchard analysis.

3. The NO synthase activity of the ileum was markedly reduced in alll-NAME-treated groups. At 60 days afterl-NAME treatment, a significant increase in the potency of methacholine (fourfold) and carbachol (threefold) was observed. In binding studies, we found a significant increase in Bmax for [3H]-quinuclidinyl benzilate of approximately 57% in thel-NAME treated group without any significant change in KD values. The contractile response to methacholine was not modified by the soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (3 µmol/L). No morphological alterations in the rat ileum were observed inl-NAME-treated rats.

4. Our findings suggest that treatment withl-NAME for 60 days induces a marked increase in the potency of methacholine and carbachol, as well as an increase in receptor number in the rat ileum.
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Keywords: NG-nitro-l-arginine methyl ester; contractile response; nitric oxide synthase; non-adrenergic non-cholinergic fibres

Document Type: Research Article

Affiliations: 1: Department of Pharmacology, Faculty of Medical Sciences, UNICAMP, Campinas, 2: Department of Pharmacology, INFAR, UNIFESP, São Paulo and 3: Department of Physical Education, Biosciences Institute, UNESP, Rio Claro, Brazil

Publication date: March 1, 2003

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