Inhibitory pathways in the circular muscle of rat jejunum

1. Conflicting data have been reported on the contribution of nitric oxide (NO) to inhibitory neurotransmission in rat jejunum. Therefore, the mechanism of relaxation and contribution to inhibitory neurotransmission of NO, adenosine 5′-triphosphate (ATP), vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) was examined in the circular muscle of Wistar–Han rat jejunum. 2. Mucosa-free circular muscle strips were precontracted with methacholine in the presence of guanethidine and exposed to electrical field stimulation (EFS) and exogenous NO, ATP, VIP and PACAP. All stimuli induced reduction of tone and inhibition of phasic motility. Only electrically induced responses were sensitive to tetrodotoxin (3 × 10(−6) M). 3. NO (10(−6)–10(−4) M)-induced concentration-dependent relaxations that were inhibited by the soluble guanylyl cyclase inhibitor 1H-[1,2,4]-oxadiazolo-[4,3-a]-quinoxalin-1-one (ODQ; 10(−5) M) and the small conductance Ca(2+)-activated K(+)-channel blocker apamin (APA; 3 × 10(−8) M). 4. Relaxations elicited by exogenous ATP (10(−4)–10(−3) M) were inhibited by the P2Y purinoceptor antagonist reactive blue 2 (RB2; 3 × 10(−4) M), but not by APA and ODQ. 5. The inhibitory responses evoked by 10(−7) M VIP and 3 × 10(−8) M PACAP were decreased by the selective PAC(1) receptor antagonist PACAP(6–38) (3 × 10(−6) M) and APA. The VPAC(2) receptor antagonist PG99-465 (3 × 10(−7) M) reduced relaxations caused by VIP, but not those by PACAP, while the VPAC(1) receptor antagonist PG97-269 (3 × 10(−7) M) had no influence. 6. EFS-induced relaxations were inhibited by the NO-synthase inhibitor N(ω)-nitro-L-arginine methyl ester (3 × 10(−4) M), ODQ and APA, but not by RB2, PG97-269, PG99-465 and PACAP(6–38). 7. These results suggest that NO is the main inhibitory neurotransmitter in the circular muscle of Wistar–Han rat jejunum acting through a rise in cyclic guanosine monophosphate levels and activation of small conductance Ca(2+)-dependent K(+) channels

Small intestinal motility in soluble guanylate cyclase α₁ knockout mice

Nitric oxide (NO) activates soluble guanylate cyclase (sGC) to produce guanosine-3',5'-cyclic-monophosphate (cGMP). The aim of this study was to investigate the nitrergic regulation of jejunal motility in sGC alpha(1) knockout (KO) mice. Functional responses to nitrergic stimuli and cGMP levels in response to nitrergic stimuli were determined in circular muscle strips. Intestinal transit was determined. Nitrergic relaxations induced by electrical field stimulation and exogenous NO were almost abolished in male KO strips, but only minimally reduced and sensitive to ODQ in female KO strips. Basal cGMP levels were decreased in KO strips, but NO still induced an increase in cGMP levels. Transit was not attenuated in male nor female KO mice. In vitro, sGC alpha(1)beta(1) is the most important isoform in nitrergic relaxation of jejunum, but nitrergic relaxation can also occur via sGC alpha(2)beta(1) activation. The latter mechanism is more pronounced in female than in male KO mice. In vivo, no important implications on intestinal motility were observed in male and female KO mice

Gastric motility in soluble guanylate cyclase alpha(1) knock-out mice

The principal target of the relaxant neurotransmitter nitric oxide (NO) is soluble guanylate cyclase (sGC). As the α(1)β(1)-isoform of sGC is the predominant one in the gastrointestinal tract, the aim of this study was to investigate the role of sGC in nitrergic regulation of gastric motility in male and female sGCα(1) knock-out (KO) mice. In circular gastric fundus muscle strips, functional responses and cGMP levels were determined in response to nitrergic and non-nitrergic stimuli. sGC subunit mRNA expression in fundus was measured by real-time RT-PCR; in vivo gastric emptying of a phenol red meal was determined. No changes were observed in sGC subunit mRNA levels between wild-type (WT) and KO tissues. Nitrergic relaxations induced by short trains of electrical field stimulation (EFS) were abolished, while those by long trains of EFS were reduced in KO strips; the latter responses were abolished by 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). The relaxations evoked by exogenous NO and the NO-independent sGC activator BAY 41-2272 were reduced in KO strips but still sensitive to ODQ. Relaxations induced by vasoactive intestinal peptide (VIP) and 8-bromo-cGMP were not influenced. Basal cGMP levels were decreased in KO strips but NO, long train EFS and BAY 41-2272 still induced a moderate ODQ-sensitive increase in cGMP levels. Gastric emptying, measured at 15 and 60 min, was increased at 15 min in male KO mice. sGCα(1)β(1) plays an important role in gastric nitrergic relaxation in vitro, but some degree of nitrergic relaxation can occur via sGCα(2)β(1) activation in sGCα(1) KO mice, which contributes to the moderate in vivo consequence on gastric emptying