Functional role of nitric oxide in guinea pig tracheal epithelium

Innervation of equine airways

Equine obstructive pulmonary disease (COPD), or heaves or recurrent airway obstruction, is a common equine pulmonary disease similar to human asthma and/or CODP. Since bronchospasm and inflammation are the key features in heaves, the purpose of this paper is to review the contribution of neural mechanism that may be relevant to this disease. Equine airway receive cholinergic and adrenergic innervation, as well as observed in many species. It was suggested that the autonomic neural control in asthma might be defective with an imbalance between excitatory and inhibitory pathways, resulting in excessively twitchy airways. Moreover, the recognition that, in addition to classical adrenergic and cholinergic pathway there are non-adrenergic-non-cholinergic inhibitory (iNANC) and excitatory (eNANC) innervation and many mediators, which have potent effects on airway function, has revived interest in neural control of airway.

The protective role of epithelium-derived nitric oxide in isolated bovine trachea

Airway epithelial cells from bovine airways can release relaxant factors such as nitric oxide (NO) and prostaglandin E(2) and the removal of airway epithelium results in an increased responsiveness of smooth muscle to spasmogen stimuli. In this study, we assessed whether or not epithelial NO modulates the contractile response of bovine trachea in vitro.Cumulative concentration-response curves to acetylcholine (ACh), histamine (Hist) and 5-hydroxytryptamine (5-HT) were obtained in both intact and epithelium denuded tracheal strips in the presence of indomethacin (10 microM).In intact, but not in epithelium denuded strips, preincubation with the NO synthase inhibitor L-N((G))-Nitro-arginine methyl ester (L-NAME), but not with D-NAME, shifted to the left the concentration-response curve to ACh (pD(2) values in the absence and in the presence of L-NAME were 3.47+/-0.1 and 4.60+/-0.1, respectively; P<0.05) and to Hist (pD(2) in the absence and in the presence of L-NAME: 3.89+/-0.1 and 4.54+/-0.1, respectively; P<0.05). This effect was reversed by L-arginine (1mM), but not by D-arginine. The contractile response to 5-HT was not affected by L-NAME in either intact or epithelium denuded strips. These data suggest that NO is an epithelial relaxant factor modulating airway cholinergic and histaminergic contraction of bovine trachea and that the activation of the epithelial NO synthase is a mediator-specific process.

Nitric oxide is generated in smooth muscle layer by neurokinin A and counteracts constriction in guinea pig airway

It has been reported that several bronchoconstrictors generate nitric oxide (NO), counteracting bronchoconstriction, and removal of bronchial epithelia reduces NO production. However, it has not been elucidated whether neurokinin A (NKA), a potent bronchoconstrictor liberated from nerve terminals, generates NO. Specific questions in this study were (1) does NKA also generate NO, (2) does NO counteract NKA-induced bronchoconstriction, and (3) does the NO generation require bronchial epithelial cells? In an in vivo study exogenous as well as endogenous (capsaicin-induced) NKA increased airway opening pressure (P(ao)) and the exhaled NO level, and both were inhibited by an antagonist selective for NK(2) receptor (a receptor for NKA), SR48968. The exhaled NO level became negligible with an inhibitor of NO synthase (NOS) type 1-3 (N(G)-nitro-L-arginine methyl ester, L-NAME) with increased P(ao), but not with a NOS type 2 inhibitor. In an in vitro study, NKA increased the nitrite/nitrate level in superfused fluid of tracheal segments. Removing smooth muscle reduced nitrite/nitrate in the fluid to negligible levels, while the level was unchanged with removal of the epithelia. Pretreatment with l-NAME enhanced the tension of epithelia-removed tracheal segments. These findings indicate that (1) NKA generates NO, (2) NO counteracts NKA-induced bronchoconstriction, and (3) NKA activates NOS in the muscle layer, independently of bronchial epithelia.

Prostaglandin-release impairment in the bladder epithelium of streptozotocin-induced diabetic rats

Isolated epithelial layer preparations were obtained from urinary bladders of 4-week streptozotocin-diabetic rats and used for endogenous prostaglandins E(2) and F(2alpha) determination. Tissues were incubated in modified Krebs solution under basal conditions, or in the presence of either indomethacin (5x10(-7) M), ATP (10(-5) and 10(-3) M) or bradykinin (10(-7) and 10(-5) M), and samples of incubation medium were collected at 15 and 30 min. In the presence of indomethacin, the release of prostaglandins in the incubation medium was under the detection limit of the enzyme immunoassay (EIA). The epithelium from diabetic rat urinary bladders was thicker and heavier and the absolute amount of endogenous prostaglandins E(2) and F(2alpha) was higher than for control animals, but when prostaglandin production was expressed as a fraction of tissue weight, it was reduced in diabetic epithelium. ATP and bradykinin has significantly increased the endogenous release of both prostaglandins from the epithelium when compared with the release under basal conditions. This increase was time-dependent and was higher in diabetic than in control tissues. ATP evoked a phasic and tonic contraction in bladder strips that was abolished by epithelium removal. Concentration-response curves for ATP did not differ among groups. Bradykinin evoked a long-lasting tonic contraction that was reduced significantly by epithelium removal in diabetic rat bladders only. Concentration-response curves for prostaglandin E(2) and F(2alpha) in diabetic rat bladder differed significantly from that in controls and epithelium removal did not alter these responses. It is suggested that bradykinin receptors and P2X nucleotide receptors already found in the smooth muscle detrusor might be present in the epithelial layer of the bladder. The prostaglandin-release impairment observed in this study might be responsible, in part, for bladder abnormalities observed in pathological conditions, such as diabetes.

Presence of constitutive endothelial nitric oxide synthase immunoreactivity in urothelial cells of hamster proximal urethra

Electrical field stimulation caused frequency-dependent relaxations in precontracted strips of hamster proximal urethra, which were attenuated by L-N(G)-nitroarginine methyl ester (10(-4) M) and completely blocked by tetrodotoxin (10(-6) M). Strips of hamster urethra devoid of urothelium showed reduced relaxant responses to electrical field stimulation which were abolished by L-N(G)-nitroarginine methyl ester (10(-4) M). Western blot analysis showed the presence of a constitutive endothelial nitric oxide synthase in the urothelial layer, suggesting that urothelium may release nitric oxide in response to electrical field stimulation and that this release is blocked by tetrodotoxin. It is suggested that the urothelium may contribute to relaxations of the smooth muscle of hamster urethra produced by nerve stimulation.

Possible functional modulation by acetylcholine of nitric oxide on guinea pig isolated trachea

The aim of this study was to evaluate whether acetylcholine induces NO release. We determined the responses on the cholinergic component of the response to electrical field stimulation (EFS) the effects of L-nitro-arginine-methyl-ester (L-NAME; 1 mM), an inhibitor of NO synthase, of L-Arginine (L-ARG; 1 mM), a precursor of NO synthesis, and methoctramine (0.01-0.1-1 microM), an antagonist of M2 receptors, alone or associated with L-NAME. The experiments were performed on guinea pig isolated intact- or denuded-epithelium tracheal rings contracted in a frequency-dependent manner to EFS. At the maximum frequency tested (30 Hz), the contractile response elicited was 60.36 +/- 0.61% of acetylcholine (100 microM) contraction, while the maximal relaxant effect induced by EFS was -28.40 +/- 0.61% in epithelium intact preparations. A pretreatment with L-NAME significantly (P<0.05) increased the contraction (76.08 +/- 1.39%) and reduced the relaxation elicited by EFS. L-NAME effect on both EFS induced responses were statistically (P<0.05) reversed by the association L-NAME + L-ARG. Methoctramine (1 microM) enhanced contractile (P<0.05) (79.20 +/- 2.21%), as well as relaxant responses (-38.73 +/- 0.99%) elicited by EFS in guinea pig epithelium-intact tracheal rings; in a separate series of experiments, performed on guinea pig epithelium-intact rings, L-NAME increased the contractile responses to methoctramine (82.6 +/- 2.31), but reduced the relaxant ones (26.38 +/- 1.29). In contrast, at the maximum frequency tested, it increased only the contractile response, but not modify the relaxant one, in epithelium denuded rings. In conclusion, the present data showed that the release of acetylcholine from postganglionic cholinergic nerves plays an important role on NO formation and this effect may be modulate by epithelium.

Nitric oxide regulatory role in sensitized guinea pig trachea

Nitric oxide (NO) has been cited to play an important regulatory role in airway function. Moreover, the NO synthase expression in models of inflammation is documented. The aim of this study was to investigate, in vitro, the NO modulation of cholinergic responses in sham-sensitized and ovalbumin-sensitized guinea pig trachea by using L-arginine (L-ARG), a precursor of NO synthesis, and L-Ng-nitro-arginine-methyl-ester (L-NAME), an inhibitor of NO synthase. Our results showed that NO's ability to modulate cholinergic responses in ovalbumin-sensitized guinea pig trachea is lost. Indeed L-ARG and L-NAME modify acetylcholine sensitivity in sham-sensitized guinea pig but not in ovalbumin-sensitized guinea pig.

A pharmacological and histochemical study of hamster urethra and the role of urothelium

1. Electrical field stimulation (EFS) of circular strips of hamster proximal urethra caused frequency-dependent relaxations at raised tone. Phentolamine (10(-6) M), propranolol (10(-6) M) and atropine (10(-6) M) were present throughout the experiment. Neurogenic relaxation was attenuated by L-NG-nitroarginine methyl ester (L-NAME) (10(-4) M), was restored by L-arginine (3 x 10(-3) M) but not by D-arginine (3 x 10(-3) M) and completely blocked by tetrodotoxin (10(-6) M). Neurogenic relaxation was also reduced by suramin (10(-4) M) and totally blocked by suramin together with L-NAME. Strips of hamster urethra devoid of urothelium showed little, if any, relaxant response to EFS. 2. An immunohistochemical study showed nitric oxide synthase-immunoreactive nerves in the smooth muscle layers and in the lamina propria, just beneath the urothelium, but no nitric oxide synthase (NOS) staining in the urothelial layer. 3. Noradrenaline elicited a significantly greater contraction in strips without urothelium than in control strips. L-NAME (10(-4) M) did not affect noradrenaline-induced contraction in both control and urothelium-free strips. The contractile response to acetylcholine was not dependent on the presence or absence of urothelium. Nevertheless the response induced by exogenous acetylcholine (10(-3) M) was increased by L-NAME (10(-4) M), both in intact and in urothelium-free strips. 4. Prostaglandin E2 (10(-8)-5 x 10(-6) M) and 2-methyl-thio-ATP (10(-9)-10(-5) M) relaxed proximal urethra. Suramin (10(-4) M) significantly inhibited the relaxation induced by 2-methyl-thio-ATP. The amplitude of these responses was not significantly different between intact and urothelium-free strips and was not blocked by L-NAME (10(-4) M). 5. These results suggest that nitric oxide (NO) is the principal transmitter involved in the non-adrenergic, non-cholinergic (NANC) relaxation of hamster proximal urethra possibly together with another inhibitory transmitter released from nerves. NO can be released from nerves located in the circular smooth muscle layer and in the lamina propria rather than in the urothelium. The reduced neurogenic relaxation in urothelium-free preparations suggests that a NO-dependent inhibitory factor is released from the urothelium. In addition, ATP and prostaglandin E2 may be involved, together with NO, in the urethra during micturition.