Nitrergic and cholinergic vagal pathways involved in the regulation of canine proximal gastric tone: an in vivo study

To better understand the relationship between cholinergic and nitrergic (NO ) innervation in the regulation of proximal gastric (fundic) tone in vivo, the effects of nitric oxide synthase blockade on fundic tone were studied i n conscious dogs using vagal cooling and an electronic barostat. Vagal cool ing, atropine (0.05 mg kg(-1) i.v. bolus) and hexamethonium (1 mg kg(-1) i. v. bolus) all markedly decreased fundic tone as reflected by increased intr agastric volume, indicating a significant contribution of vagal and enteric cholinergic pathways to the maintenance of canine fundic tone. Administrat ion of L-NNA (10 mg kg(-1) i.v. bolus) increased fundic tone and the effect s of L-NNA were completely prevented by prior vagal cooling or atropine adm inistration, but not by pretreatment with hexamethonium. The relaxation eff ects of neurally derived NO appear primarily related to inhibition of ongoi ng vagal cholinergic activity. The data are consistent with the primary sit e of action of nitrergic mechanisms on gastric fundic tone in conscious dog s being at a presynaptic site on vagal cholinergic efferent nerves.