Evidence for a role for nitric oxide in relation of the frog oesophageal body to electrical field stimulation

Prejunctional modulation of non-adrenergic non-cholinergic (NANC) inhibitory responses in the isolated guinea-pig gastric fundus

The inhibitory neurotransmission of the stomach was investigated in isolated guinea-pig gastric fundus. In preparations treated with guanethidine (1 micro mol L-1) and p-fluoro-hexahydro-sila-difenidol (1 micro mol L-1), electrical stimulation evoked neurogenic inhibitory responses not modified by hexamethonium (100 micro mol L-1), suggesting that inhibitory postganglionic non-adrenergic non-cholinergic (NANC) nerve fibres are involved. The nitric oxide (NO)-synthase inhibitor Nomega-nitro-l-argininine-methyl-ester hydrochloride (1-100 micro mol L-1) and the soluble guanylyl cyclase inhibitor ODQ (0.1-3 micro mol L-1) also abolished such relaxant response, suggesting the involvement of NO/Cyclic Guanosine 3',5' monophosphate (cGMP) system as the final mechanism of muscle relaxation. The alpha2-adrenoceptor agonist, UK 14 304 (10 nmol L-1-10 micro mol L-1) did not influence the electrical field stimulation (EFS)-evoked NANC responses. These latter responses were also refractory to a variety of receptor agonists and antagonists, acting at Gamma Aminobutyric Acid (GABA), serotonin 5HT1a, opioid micro , delta and kappa, muscarinic M1 and M2, histamine H2 and H3 and cannabinoid receptors. The NANC response was insensitive to the P/Q-type Ca2+-channel blocker omega-agatoxin TK (1 nmol L-1-0.1 micro mol L-1), but partially inhibited by the N-type Ca2+-channel blocker omega-conotoxin GVIA (0.1 nmol L-1-0.1 micro mol L-1), and by the L-type Ca2+-channel blockers nifedipine and calcicludine (0.1 nmol L-1-0.1 micro mol L-1). These data suggest that the NANC relaxation of the isolated guinea-pig gastric fundus is mediated by NO as the final inhibitory (neuro)transmitter at the longitudinal smooth muscle cells. The mechanism(s) promoting NO production is/are Ca2+-dependent, but apparently insensitive to presynaptic modulation. Both N- and L-type channels seem to occur in nitrergic nerve endings, where they contribute to trigger NO diffusion at the synaptic cleft.

Effects of diabetes on non-adrenergic, non-cholinergic relaxation induced by GABA and electrical stimulation in the rat isolated duodenum

1. The effects of streptozotocin (STZ)-induced experimental diabetes on nitrergic-mediated responses to GABA and electrical field stimulation (EFS) have been evaluated in rat isolated duodenum. 2. In the presence of noradrenergic and cholinergic blockade, EFS (60 V, 1 ms, 0.1-32 Hz) induced frequency dependent relaxations of the preparation. GABA also caused submaximal relaxation of the rat duodenum. The relaxations induced by GABA and EFS were reduced in duodenal tissues from diabetic rats compared with control rats. 3. Neither ATP- nor sodium nitroprusside-induced relaxations were altered in diabetic duodenal tissues. GABA- and EFS-induced relaxations were inhibited by NG-nitro-L-arginine methyl ester (L-NAME; 300 mmol/L) in both diabetic and control rats. Although the inhibition caused by L-NAME of GABA- and EFS-induced relaxation was partially reversed by L-arginine (1 mmol/L), L-arginine alone had no effect on GABA- and EFS-induced relaxation in diabetic rats. 4. These results suggest that STZ-induced diabetes impairs non-adrenergic, non-cholinergic relaxation induced by EFS and GABA. Impairment of nitrergic innervation of the rat duodenum may contribute to the abnormalities of intestinal motility abnormalities associated with diabetes.

Neuronal NOS provides nitrergic inhibitory neurotransmitter in mouse lower esophageal sphincter

To identify the enzymatic source of nitric oxide (NO) in the lower esophageal sphincter (LES), studies were performed in wild-type and genetically engineered endothelial nitric oxide synthase [eNOS(-)] and neuronal NOS [nNOS(-)] mice. Under nonadrenergic noncholinergic (NANC) conditions, LES ring preparations developed spontaneous tone in all animals. In the wild-type mice, electrical field stimulation produced frequency-dependent intrastimulus relaxation and a poststimulus rebound contraction. NOS inhibitor N(omega)-nitro-L-arginine methyl ester (100 microM) abolished intrastimulus relaxation and rebound contraction. In nNOS(-) mice, both the intrastimulus relaxation and rebound contraction were absent. However, in eNOS(-) mice there was no significant difference in either the relaxation or rebound contraction from the wild-type animal. Both nNOS(-) and eNOS(-) tissues showed concentration-dependent relaxation to NO donor diethylenetriamine-NO and there was no difference in the sensitivity to the NO donor in nNOS(-), eNOS(-), or wild-type animals. These results indicate that in mouse LES, nNOS rather than eNOS is the enzymatic source of the NO that mediates NANC relaxation and rebound contraction.

NANC relaxation of the circular smooth muscle of the oesophagus of the Agama lizard involves the L-arginine-nitric oxide synthase pathway

On carbachol (CCh; 10-30 microM) pre-contracted circular muscle strips of the Agama lizard oesophagus, electrical field stimulation evoked frequency-dependent relaxations in the presence of guanethidine (1 microM) and indomethacin (1 microM). These non-adrenergic inhibitory responses were concentration-dependently inhibited by the nitric oxide synthase (NOS) inhibitor N omega-nitro-L-arginine methyl ester (L-NAME) within a concentration range of 30-300 microM but not D-NAME (up to 300 microM), although a component remained at 4-16 Hz even with 300 microM L-NAME. The inhibition by L-NAME (300 microM) was completely prevented when L-arginine (L-Arg; 15 mM) but not D-Arg (up to 15 mM) was applied simultaneously with L-NAME (300 microM). Increasing the L-NAME concentration to 1 mM had no additional inhibitory effect. Sodium nitroprusside (SNP) concentration-dependently relaxed pre-contracted oesophageal strips, L-NAME (up to 300 microM) had no effect. Neither adenosine 5'-triphosphate (up to 0.1 mM) nor vasoactive intestinal polypeptide (up to 0.1 microM) caused the pre-contracted oesophagus to relax. This study has shown that the NANC inhibitory response of the Agama lizard oesophagus circular muscle largely involves the L-Arg-NOS pathway as seen by the effect of L-NAME, L-Arg and SNP. The identity of the L-NAME-resistant component(s) and the lack of effect of tetrodotoxin (up to 3 microM) and omega-conotoxin GVIA (up to 0.1 microM) in relation to the nature of the inhibitory response are discussed.