The Effects of Nitric Oxide and Vasoactive Intestinal Peptide on the Smooth Muscle Relaxation of a Guinea Pig's Gastric Fundus

Park, Hyo Jin; Lee, Hye Young; Chung, Jun Pyo; Lee, Kwan Sik; Chon, Chae Yoon; Kang, Bok Soon; Park, In Suh

Abstract

BACKGROUND/AIMS: The autonomic nervous system distributed throughout the stomach consists of adrenergic, cholinergic, and non-adrenergic, non-cholinergic (NANC) nerves. Of these, NANC nerve fibers are known to be distributed all throughout the gastrointestinal tract and they provide the main inhibitory nerve supply to the gut smooth muscle. NANC nerve is known to release neurotransrnitters such as nitric oxide (NO), adenosine 5-triphosphate (ATP) and vasoactive intestinal peptide (VIP). However, the effect produced by each of the NANC neurotransmitter depends on an animal species and parts of gastrointestinal tract. Therefore, this experiment applied electrical field stirnulation at the circular musc]e of a guinea pigs gastric fundus to observe any relation of NANC nerve in muscu]ar relaxation.
METHODS
The effect of ATP, VIP, and NO on muscle re)axation was determined through the electrical field stimulation, and each neurotrans- mitters ability to regulate the muscle relaxation was compared.
RESULTS
Muscle strips were relaxed in response to electrical field stimulation that was applied. The relaxation was not inhibited by atropine or guanethidine, but was completely inhibited by tetrodotoxin. The muscle relaxation caused by NANC nerve stimulation was inhibited by L-NNA, VIP antagonist, and suramin. L-NNA was the most potent inhibitor among them. The muscle relaxation caused by electrical field stimulation was not further inhibited by VIP antagonist after the pretreatment of L-NNA, however, it was further inhibited by L-NNA following the treatment of VIP antagonist. The electrically stimulaed induced muscle relaxation was almost complete]y abolished by coadministration of L-NNA with VIP antagonist and suramin, The exogenous VIP-induced muscle relaxation was not affected by presence or absence of L-NNA. Oxyhemoglobin or methylene blue treatment inhibited the muscle relaxation induced by electrical field stimulation.
CONCLUSIONS
Non-adrenergic and non-cholinergic nerve has an inhibitory neurotransmission in a guinea pigs gastric fundus circular muscle and NO is the principal inhibitory neurotransmitter of NANC nerve. NO mediated muscle relaxation is thought to be associated with increase of cGMP in muscle cells.