Immunohistochemical demonstration of a paracrine role of nitric oxide in bronchial function

Airway epithelium is a predominant source of endogenous airway GABA and contributes to relaxation of airway smooth muscle tone

Chronic obstructive pulmonary disease and asthma are characterized by hyperreactive airway responses that predispose patients to episodes of acute airway constriction. Recent studies suggest a complex paradigm of GABAergic signaling in airways that involves GABA-mediated relaxation of airway smooth muscle. However, the cellular source of airway GABA and mechanisms regulating its release remain unknown. We questioned whether epithelium is a major source of GABA in the airway and whether the absence of epithelium-derived GABA contributes to greater airway smooth muscle force. Messenger RNA encoding glutamic acid decarboxylase (GAD) 65/67 was quantitatively measured in human airway epithelium and smooth muscle. HPLC quantified GABA levels in guinea pig tracheal ring segments under basal or stimulated conditions with or without epithelium. The role of endogenous GABA in the maintenance of an acetylcholine contraction in human airway and guinea pig airway smooth muscle was assessed in organ baths. A 37.5-fold greater amount of mRNA encoding GAD 67 was detected in human epithelium vs. airway smooth muscle cells. HPLC confirmed that guinea pig airways with intact epithelium have a higher constitutive elution of GABA under basal or KCl-depolarized conditions compared with epithelium-denuded airway rings. Inhibition of GABA transporters significantly suppressed KCl-mediated release of GABA from epithelium-intact airways, but tetrodotoxin was without effect. The presence of intact epithelium had a significant GABAergic-mediated prorelaxant effect on the maintenance of contractile tone. Airway epithelium is a predominant cellular source of endogenous GABA in the airway and contributes significant prorelaxant GABA effects on airway smooth muscle force.

Guanylyl cyclases, nitric oxide, natriuretic peptides, and airway smooth muscle function

Airway smooth muscle (ASM) plays an important role in asthma pathophysiology through its contractile and proliferative functions. The cyclic nucleotides adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) are second messengers capable of mediating the effects of a variety of drugs and hormones. There is a large body of evidence to support the hypothesis that cAMP is a mediator of the ASM's relaxant effects of drugs, such as beta2-adrenoceptor agonists, in human airways. Although most attention has been paid to this second messenger and the signal transduction pathways it activates, recent evidence suggests that cGMP is also an important second messenger in ASM with important relaxant and antiproliferative effects. Here, we review the regulation and function of cGMP in ASM and discuss the implications for asthma pathophysiology and therapeutics. Recent studies suggest that activators of soluble and particulate guanylyl cyclases, such as nitric oxide donors and natriuretic peptides, have both relaxant and antiproliferative effects that are mediated through cGMP-dependent and cGMP-independent pathways. Abnormalities in these pathways may contribute to asthma pathophysiology, and therapeutic manipulation may complement the effects of beta2-adrenoceptor agonists.

Type I nitric oxide synthase is decreased in the fetal pulmonary circulation of hypertensive lambs

The nitric oxide (NO)/guanosine 3',5'-cyclic monophosphate (cGMP) pathway plays an essential role in mediating pulmonary vasodilatation during transition of the pulmonary circulation at birth. We used immunoblot analysis (Western) and semiquantitative immunohistochemistry to study the presence, distribution, and relative amounts of type I nitric oxide synthase (NOS-I). Immunoblots were performed on normal fetal sheep lungs, whereas immunohistochemistry for NOS-I was compared between lungs from normal fetal lambs vs. fetal lambs with persistent pulmonary hypertension of the newborn (PPHN) induced by ligation of the ductus arteriosus. Western blot analysis using a polyclonal antibody detected NOS-I protein in homogenates of normal fetal sheep lungs. Abundant NOS-I immunoreactivity was observed exclusively in the precapillary resistance vessels, i.e., terminal bronchiole-associated arteries (TA) and respiratory bronchiole-associated arteries (RA) in normal fetal lung. In marked contrast, immunoreactivity for NOS-I was significantly reduced in the TA and RA of hypertensive lungs. We conclude that there is a heterogeneous distribution of NOS-I in the normal fetal sheep lung, but that NOS-I staining is significantly reduced in lambs with PPHN.

Regulation of murine airway responsiveness by endothelial nitric oxide synthase

Nitric oxide (NO) is a potent vasodilator, but it can also modulate contractile responses of the airway smooth muscle. Whether or not endothelial (e) NO synthase (NOS) contributes to the regulation of bronchial tone is unknown at present. Experiments were designed to investigate the isoforms of NOS that are expressed in murine airways and to determine whether or not the endogenous release of NO modulates bronchial tone in wild-type mice and in mice with targeted deletion of eNOS [eNOS(-/-)]. The presence of neuronal NOS (nNOS), inducible NOS (iNOS), and eNOS in murine trachea and lung parenchyma was assessed by RT-PCR, immunoblotting, and immunohistochemistry. Airway resistance was measured in conscious unrestrained mice by means of a whole body plethysmography chamber. The three isoforms of NOS were constitutively present in lungs of wild-type mice, whereas only iNOS and nNOS were present in eNOS(-/-) mice. Labeling of nNOS was localized in submucosal airway nerves but was not consistently detected, and iNOS immunoreactivity was observed in tracheal and bronchiolar epithelial cells, whereas eNOS was expressed in endothelial cells. In wild-type mice, treatment with N-nitro-L-arginine methyl ester, but not with aminoguanidine, potentiated the increase in airway resistance produced by inhalation of methacholine. eNOS(-/-) mice were hyperresponsive to inhaled methacholine and markedly less sensitive to N-nitro-L-arginine methyl ester. These results demonstrate that the three NOS isoforms are expressed constitutively in murine lung and that NO derived from eNOS plays a physiological role in controlling bronchial airway reactivity.

Neuronal nitric oxide synthase: expression in rat parietal cells

Nitric oxide synthases (NOS) are enzymes that catalyze the generation of nitric oxide (NO) from L-arginine and require nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor. At least three isoforms of NOS have been identified: neuronal NOS (nNOS or NOS I), inducible NOS (iNOS or NOS II), and endothelial NOS (eNOS or NOS II). Recent studies implicate NO in the regulation of gastric acid secretion. The aim of the present study was to localize the cellular distribution and characterize the isoform of NOS present in oxyntic mucosa. Oxyntic mucosal segments from rat stomach were stained by the NADPH-diaphorase reaction and with isoform-specific NOS antibodies. The expression of NOS in isolated, highly enriched (>98%) rat parietal cells was examined by immunohistochemistry, Western blot analysis, and RT-PCR. In oxyntic mucosa, histochemical staining revealed NADPH-diaphorase and nNOS immunoreactivity in cells in the midportion of the glands, which were identified as parietal cells in hematoxylin and eosin-stained step sections. In isolated parietal cells, decisive evidence for nNOS expression was obtained by specific immunohistochemistry, Western blotting, and RT-PCR. Cloning and sequence analysis of the PCR product confirmed it to be nNOS (100% identity). Expression of nNOS in parietal cells suggests that endogenous NO, acting as an intracellular signaling molecule, may participate in the regulation of gastric acid secretion.

Inhibition of neuronal M(2) muscarinic receptor function in the lungs by extracellular nitric oxide

1. These experiments were carried out to test whether neuronal M(2) muscarinic receptor function in the lungs is affected by nitric oxide (NO) and whether the source of the NO is epithelial or neuronal. 2. In pathogen free, anaesthetized guinea-pigs, the muscarinic agonist pilocarpine inhibited vagally induced bronchoconstriction demonstrating functional neuronal M(2) muscarinic receptors. In the presence of the NO donor, 3-morpholino-sydnonimine (SIN-1), pilocarpine no longer inhibited vagally induced bronchoconstriction. In contrast, inhibiting endogenous NO with N(G)-monomethyl-L-arginine methyl ester (L-NMMA) did not affect the ability of pilocarpine to decrease vagally induced bronchoconstriction. 3. In isolated tracheas, pilocarpine inhibited contractions induced by electrical field stimulation demonstrating that neuronal M(2) muscarinic receptors function in vitro. As in the anaesthetized guinea-pigs, SIN-1 shifted the pilocarpine dose response curve to the right, demonstrating decreased neuronal M(2) receptor function. However, in vitro, L-NMMA shifted the pilocarpine dose response curve to the left, demonstrating that endogenous NO was inhibiting the ability of the M(2) receptors to decrease acetylcholine (ACh) release. 4. Both haemoglobin (Hb), which scavenges NO, and epithelial removal also shifted the pilocarpine dose response curve to the left, demonstrating that the NO inhibiting neuronal M(2) receptor function was extracellular and probably of epithelial origin. 5. In conclusion, extracellular NO appears to inhibit the ability of the M(2) receptors to decrease ACh release from the parasympathetic nerves in the lungs in vivo and in vitro in pathogen free guinea-pigs. However, while the neuronal M(2) receptors will respond to NO (from SIN-1) in vivo, there does not appear to be an endogenous source of NO since L-NMMA had no effect in vivo.