Depression by fenoprofen of the rebound contractions' elicited by vagal stimulation and arterial infusion of ATP in the rabbit stomach in vivo

Vagal control of nitric oxide and vasoactive intestinal polypeptide release in the regulation of gastric relaxation in rat

1. Gastric motility and neurotransmitter release in response to vagal stimulation were studied using a vascularly isolated perfused rat stomach. Gastric motor responses were recorded by a strain gauge force transducer implanted on the proximal stomach. 2. Electrical stimulation of vagal trunk (0.5-20 Hz) produced a triphasic response which was composed of a rapid transient relaxation (first phase) followed by a phasic contraction (second phase) and a delayed prolonged relaxation (third phase). Maximum responses of the first, second and third phase were observed at 2.5, 5 and 10 Hz, respectively. Intra-arterial infusion of tetrodotoxin (0.1 microM) or hexamethonium (100 microM) completely abolished the triphasic response. 3. The nitric oxide (NO) biosynthesis inhibitor NG-nitro-L-arginine (L-NNA; 100 microM) significantly antagonized the rapid relaxation but had no effect on the delayed relaxation, while vasoactive intestinal polypeptide (VIP) antagonist (1 microM) significantly reduced the delayed relaxation without affecting the rapid relaxation. 4. In response to vagal stimulation, NO production ([3H]citrulline formation in gastric tissue preloaded with [3H]arginine) was maximum at 2.5 Hz, whereas VIP release into the venous effluent was largest at 10 Hz. Hexamethonium abolished vagal-stimulated NO production and VIP release. L-NNA had no effect on VIP release in response to vagal stimulation. 5. The nicotinic receptor agonist 1,1-dimethyl-4-phenylpiperizinium (DMPP; 100 microM) also caused a triphasic response similar to that observed with vagal stimulation and produced a significant increase in VIP and NO formation. DMPP-evoked VIP release was not affected by L-NNA. Similarly, DMPP-evoked NO production was not antagonized by VIP antagonist. 6. These results suggest that vagus nerve stimulation evokes NO and VIP release via nicotinic synapses which cause different modes of relaxation of the stomach. There is no interaction between NO and VIP release in response to vagal stimulation.

The influence of the vagally induced rebound contractions on the non-adrenergic, non-cholinergic (NANC) inhibitory motility of the rabbit stomach and the role of prostaglandins

Factors influencing the vagally induced rebound contraction and its role in gastric inhibitory motility were studied in the anaesthetised rabbit. Gastric motility was assessed from measurements of gastric volume by means of an intragastric balloon. In the atropine- and guanethidine-treated animals, vagal stimulation caused biphasic motor responses: a rapid relaxation was followed by a rebound contraction. The latter, depending on experimental conditions, was able to restore and maintain gastric volume at the basal level. However, the rebound contraction was greatly influenced by the stimulation parameters and the basal gastric volume. Stimulation periods of less than 30 sec, or stimulation frequencies above 20 Hz, as well as basal gastric volumes above 70 ml could reduce the amplitude of the post-stimulus excitatory motility, and transformed the biphasic response into a triphasic one: a slow, long-lasting relaxation appeared after the rebound contraction. Prostaglandin-synthesis inhibitors of the non-steroidal anti-inflammatory group depressed the rebound contraction, and caused persistence of gastric relaxation, even after the offset of vagal stimulation. PGE2 evoked excitatory motor responses which closely mimicked the vagally induced rebound contraction. PGE2 also interrupted the rapid or slow, long-lasting relaxant responses. PGF2 alpha elicited tonic excitatory responses. These results suggest that PGE2 is involved in the mechanism underlying post-stimulus excitatory motility. They also suggest that the rebound contraction is a key factor in determining the inhibitory motility pattern of the rabbit stomach.