Characteristics of histamine tachyphylaxis in canine tracheal smooth muscle

Histamine receptors rapidly desensitize without altering nerve-evoked contractions in murine urinary bladder smooth muscle

Histamine has been implicated in urinary bladder dysfunction as an inflammatory mediator driving sensory nerve hypersensitivity. However, the direct influence of histamine on smooth muscle has not been thoroughly investigated. We hypothesized that histamine directly contracts urinary bladder smooth muscle (UBSM) independent of effects on nerves. Single cell quantitative RT-PCR determined that only histamine H1 and H2 receptors were expressed on UBSM cells. In isolated tissue bath experiments, histamine (200 µM) caused a highly variable and rapidly desensitizing contraction that was completely abolished by the H1 receptor antagonist fexofenadine (5 µM) and the Gq/11 inhibitor YM254890 (1 µM). Neither the muscarinic receptor antagonist atropine (1 µM), the Na+ channel blocker tetrodotoxin (1 µM), nor the transient receptor potential vanilloid type 1 antagonist capsazepine (10 µM) altered responses to histamine, suggesting that nerve activation was not involved. UBSM desensitization to histamine was not due to receptor internalization, as neither the cholesterol-depleting agent methyl-β-cyclodextrin (10 mM), the dynamin-mediated endocytosis inhibitor dynasore (100 µM), nor the clathrin-mediated endocytosis inhibitor pitstop2 (15 µM) augmented or prolonged histamine contractions. Buffer from desensitized tissues still contracted histamine-naïve tissues, revealing that histamine was not metabolized. Prolonged exposure to histamine also had no effect on contractions due to electrical field stimulation, suggesting that both efferent nerve and UBSM excitability were unchanged. Together, these data suggest that histamine, although able to transiently contract UBSM, does not have a lasting effect on UBSM excitability or responses to efferent nerve input. Thus, any acute effects of histamine directly on UBSM contractility are unlikely to alter urinary bladder function.NEW & NOTEWORTHY Histamine is commonly associated with inflammatory bladder pathologies. We sought to investigate the role of histamine on urinary bladder contractility. Histamine contracts the bladder, but this response is highly variable and desensitizes completely in minutes. This desensitization is not due to internalization of the receptor or metabolism of histamine. Because nerve-evoked contractions are also not increased in the presence of histamine, our findings suggest that histamine is not directly acting to change contractility.

Effects of COX inhibitors on responsiveness of the tracheal tract to acetylcholine and histamine and their relationship with LTC4 and PGE2 levels of bronchoalveolar lavage fluid in allergic Guinea pigs

Introduction: Nonsteroidal anti-inflammatory drugs (NSAIDs) intervene in the COX (cyclooxygenase) pathways which generate two important inflammation mediators, prostaglandins (PGs) and leukotriene (LTs). Contradictory claims regarding the effect of NSAIDs in asthmatic patients continues to be an issue. The present study investigated the effects of COX inhibitors on the responsiveness of the tracheal tract and on the levels of LTC4 and PGE2 in cells of the bronchoalveolar lavage fluid in an allergic guinea pig model.Materials and Methods: Adult male Dunkin-Hartley guinea pigs (250 - 300 g) were divided into seven groups of six animals each. Four COX inhibitors, aspirin (200 mg/kg and 20 mg/kg), indomethacin (10 mg/kg), ketoprofen (10 mg/kg), and celecoxib (25 mg/kg), were given orally on day 17 to allergy induced guinea pigs at 0, 12, and 24 h before ovalbumin challenge on day 18. PGF2 and LT4 were measured in the bronchoalveolar lavage fluid as well as inflammatory cell count and total protein. Tracheal responsiveness to acetylcholine (Ach) and histamine (His) also was evaluated.Results: An augment in the response of the trachea to Ach and His, as well as overt allergenic signs including short breath, wheezing and sneezing, was observed. The most significant increase in tracheal hyper-responsiveness was observed in the ketoprofen-treated group with similar but less pronounced changes observed in the indomethacin-treated group. Although some variables increased with the aspirin and celecoxib treatments, overall the tracheal sensitivity was reduced. Inflammatory cells including eosinophils and neutrophils corresponded to the changes observed for each treatment group.Conclusion: Ketoprofen and indomethacin increased the tracheal sensibility to Ach and His; therefore, their administration is not recommended in patients susceptible to allergy.

Investigation into the mechanism of anti-asthmatic action of Moringa oleifera

We have studied the possible mechanism of the antiasthmatic action of Moringa oleifera seed kernel ethanolic extract (EXMO, 400 mg/kg). EXMO produced an increase in the Preconvulsion Dyspnea time induced by histamine and acetylcholine aerosol, a significant reduction in the elevated eosinophil and monocyte counts in the Broncho-Alveolar Lavage fluid of sensitized guinea pigs, reduction in the paw edema volume as compared to the control rats, and decrease in the elevated histamine release from the sensitized guinea pig lungs. The pD(2) values of histamine in tracheal chain and taenia coli were significantly greater and that in lung strip was lower in the sensitized animals and treatment with EXMO significantly decreased pD(2) values in all three preparations. Our data suggest inhibition of the immediate hypersensitive reaction, histamine release, and the infiltration of various inflammatory cells as possible antiasthmatic mechanisms of action.

Mechanism underlying histamine-induced desensitization of amylase secretion in rat parotid glands

1. Histamine acted on H2 receptors in rat parotid tissues and induced the amylase secretion. Immunoblot analysis by using anti-H2 receptor protein antiserum demonstrated that histamine induced the increase and decrease in the amounts of H2 receptor proteins in basolateral and intracellular membranes, respectively. 2. Short-term treatment with histamine resulted in decreases in amylase secretion, the density of H2 receptors and their affinity for the agonists during further incubation with histamine, but showed an unaltered secretory response to isoproterenol, indicating that the histamine-induced desensitization was confined to H2 receptors. 3. This treatment triggered a 20% decrease in the histamine-stimulated adenylate cyclase activity and a 40% decrease in the phosphorylation level of Gi2alpha protein in the tissues, resulting in an increase in pertussis toxin (IAP)-catalyzed ADP-ribosylation of the protein. An enhancement of cholera toxin-catalyzed ADP-ribosylation of Gs protein was observed only during the first incubation with histamine. 4. This treatment triggered a 30% decrease and a 60% increase in the histamine-stimulated activities of protein kinase A and protein phosphatase 2A in the tissues, respectively. 5. Pretreatment with okadaic acid completely blocked the histamine-induced decrease in amylase secretion and increase in IAP-catalyzed ADP-ribosylation of Gi protein. The levels of Gi2alpha and Gs alpha proteins in the tissues were not modified by histamine treatment and the level of Gi2alpha protein was not affected by pretreatment with okadaic acid, as assessed by immunoblot analyses with anti-Gi2alpha and anti-Gs alpha protein antiserum. 6. The regulation of Gi2alpha protein phosphorylation in parotid tissues plays an important role in the histamine-induced desensitization of amylase secretion.

Postnatal expression of H1-receptor mRNA in the rat brain: correlation to L-histidine decarboxylase expression and local upregulation in limbic seizures

Histamine is implicated in the regulation of brain functions through three distinct receptors. Endogenous histamine in the brain is derived from mast cells and neurons, but the importance of these two pools during early postnatal development is still unknown. The expression of histamine H1-receptor in the rat brain was examined using in situ hybridization during postnatal development and in adults. For comparison, the expression of L-histidine decarboxylase (HDC) in the two pools was revealed. H1-receptor was evenly expressed throughout the brain on the first postnatal days, but resembled the adult, uneven pattern already on postnatal day 5 (P5). HDC was expressed in both mast cells and tuberomammillary neurons from birth until P5, after which the mast cell expression was no more detectable. In adult rat brain, high or moderate levels of H1-receptor expression were found in the hippocampus, zona incerta, medial amygdaloid nucleus and reticular thalamic nucleus. In most areas of the adult brain the expression of H1-receptor mRNA correlates well with binding data and histaminergic innervation. A notable exception is the hypothalamus, with high fibre density but moderate or low H1-receptor expression. Systemic kainic acid administration induced increased expression of H1-receptor mRNA in the caudate-putamen and dentate gyrus, whereas no change was seen in the hippocampal subfields CA1-CA3 or in the entorhinal cortex 6 h after kainic acid injections. This significant increase supports the concept that histaminergic transmission, through H1-receptor, is involved in the regulation of seizure activity in the brain.

Molecular pharmacological aspects of histamine receptors

In this article, we review the recent developments in the field of histamine research. Besides the description of pharmacological tools for the H1, H2 and H3 receptor, specific attention is paid to both the molecular aspects of the receptor proteins, including the recent cloning of the receptor genes, and their respective signal transduction mechanisms.

Effect of magnitude of airway responsiveness and therapy with inhaled corticosteroid on histamine tachyphylaxis in asthma

Histamine challenge testing is used to measure airway responsiveness in asthma. Histamine tachyphylaxis has been demonstrated after repeated challenges in mild asthmatics not using inhaled corticosteroid. Other studies, using subjects with variable severity of asthma, have not demonstrated histamine tachyphylaxis. Forty patients with stable asthma were studied and stratified according to severity of airway hyperresponsiveness and use of inhaled corticosteroid, to examine the effects of these factors on histamine tachyphylaxis. Airway responsiveness was measured as the histamine provocative concentration causing a 20 percent fall in FEV1 (PC20). Twenty subjects had mildly increased airway hyperresponsiveness (PC20 > 1 mg/ml), of whom 10 were using inhaled corticosteroid. Twenty subjects had moderate to severely increased airway hyperresponsiveness (PC20 < 1 mg/ml), of whom 10 were using inhaled corticosteroid. On each of two study days, 1 week apart, two histamine challenges were performed 1 h apart. Histamine tachyphylaxis was found for the entire group on both study days. The geometric mean PC20 increased from 1.0 mg/ml (percent SEM 1.2) to 1.3 mg/ml (percent SEM 1.2) 1 h later on day 1 (p < 0.0005), and 1.1 mg/ml (percent SEM 1.2) to 1.3 mg/ml (percent SEM 1.2) 1 h later on day 2 p < 0.05). Subgroup analysis demonstrated that tachyphylaxis only occurred consistently in subjects with mildly increased airway hyperresponsiveness not receiving inhaled corticosteroid. In this group, the PC20 increased from 2.2 mg/ml (percent SEM 1.2) to 3.2 mg/ml (percent SEM 1.2) on day 1 (p < 0.001), and from 2.5 mg/ml (percent SEM 1.3) to 3.4 mg/ml (percent SEM 1.2) on day 2 (p < 0.05). This study confirms that histamine tachyphylaxis occurs in asthmatics, but is consistently present only in mild, noncorticosteroid-dependent asthmatics.

Exercise-induced refractoriness in asthmatic subjects involves leukotriene and prostaglandin interdependent mechanisms

Exercise-induced bronchoconstriction is caused, in part, by leukotriene (LT)D4 release in asthmatic airways. Asthmatics become refractory to exercise bronchoconstriction with repeated challenges, due to inhibitory prostaglandin release. The purpose of this study was to test the hypothesis that exercise refractoriness is caused by LTD4-induced inhibitory prostaglandin release. Fourteen stable asthmatic subjects with exercise-induced bronchoconstriction were studied. On the first 2 days, subjects underwent two challenges, 1 h apart, with either exercise or inhaled LTD4. Eight subjects then took part in three double-blind, randomized, placebo-controlled, crossover studies with flurbiprofen, a prostaglandin synthetase inhibitor, to determine whether cross refractoriness occurs between exercise and LTD4, whether flurbiprofen attenuates this effect, and whether flurbiprofen attenuates LTD4 tachyphylaxis. There was a reduction in the intensity of bronchoconstriction to the second challenge both with exercise (refractoriness) and with LTD4 (tachyphylaxis). The degrees of refractoriness and tachyphylaxis were correlated (r = 0.72, p = 0.005). Flurbiprofen attenuated LTD4 tachyphylaxis. Cross refractoriness occurred between exercise and LTD4, and flurbiprofen treatment also attenuated this effect. One hour after LTD4 challenge, the mean fall in FEV1 after exercise was 12.3% (%SEM 2.3) on placebo and 17.1% (%SEM 3.8) on flurbiprofen (p = 0.027). Similarly, 1 h after exercise, the LTD4 PC20 increased to 0.73 (%SEM 1.4) microgram/ml on placebo and 0.30 (%SEM 1.8) microgram/ml on flurbiprofen (p = 0.026). These results suggest that LTD4 released in asthmatic airways as a result of exercise stimulates inhibitory prostaglandin release, resulting in exercise refractoriness.

Reduced activity of acetylcholinesterase in canine tracheal smooth muscle homogenates after active immune-sensitization

Previous investigations have suggested that immune-sensitization increases airway smooth muscle responsiveness to cholinomimetic stimulation by reducing the rate of degradation of acetylcholine. To examine the hypothesis that increased cholinomimetic responsiveness of tracheal smooth muscle (TSM) caused by immune-sensitization results from inhibition of acetylcholinesterase (AChase) activity, we developed a method for direct measurement of AChase activity in homogenates of TSM obtained from mongrel dogs actively sensitized in vivo to ragweed pollen extract (n = 7) and sham-sensitized littermate controls (n = 7). For both sensitized and control specimens, saturation of AChase was obtained at approximately 3.12 mM substrate (acetylthiocholine); however, maximal enzyme activity in homogenates of ragweed-sensitized tissues was significantly less (0.862 +/- 0.088 absorbance units/min/mg protein [AU/min/mg]) compared to control homogenates (1.590 +/- 0.129 AU/min/mg; P less than 0.001). Kinetic analysis (Eadie-Hofstee plot) indicated similar Michaelis constants (Km) for AChase from ragweed-sensitized (0.360 +/- 0.063) and control (0.336 +/- 0.062) homogenates (P = NS). The concentration of physostigmine eliciting half-maximal inhibition (Ki) of AChase activity also was similar for tissues from sensitized (-7.92 +/- 0.032 log M) and control animals (-7.86 +/- 0.012 log M; P = NS). Pretreatment with selected mediators of anaphylaxis (10(-4) M histamine, 10(-6) M serotonin, 10(-5) M prostaglandin E2, 10(-6) M prostaglandin F2 alpha, and 10(-7) M leukotriene D4) did not affect AChase activity. Our data demonstrate reduced AChase activity in homogenates of canine TSM after active immune-sensitization in vivo. This corresponds to functional augmentation of cholinomimetic contraction in actively sensitized tissues.

Different profiles of desensitization dynamics in guinea-pig jejunal longitudinal smooth muscle after stimulation with histamine and methacholine

1. In the present study we investigated desensitization phenomena of guinea-pig jejunal longitudinal smooth muscle responses after stimulation with 100 microM histamine or methacholine, using a superfusion method. 2. Histamine H1-receptor-mediated contractions appear to be rapidly reduced after application of 100 microM histamine. Muscarinic responses were not affected following desensitization with 100 microM histamine, indicating a homologous desensitization. 3. Initial contractions to 0.3 microM histamine were reduced by 90%, recovered quickly, but did not reach control levels within 1 h. Desensitization of histamine responses could be separated into two phases; a rapid, but transient, desensitization and a more sustained desensitization. As a consequence of this sustained effect the pD2 for histamine shifted from 6.7 +/- 0.1 (control) to 6.1 +/- 0.1 (desensitized). 4. Desensitization with 100 microM methacholine caused a heterologous desensitization, reflected by the development of a refractory period, in which neither histamine nor methacholine was able to elicit a contraction. After a few minutes responses to both agents recovered to control levels. 5. During the refractory period after methacholine desensitization, muscle strips were still responsive to 40 mM KCl but did not contract in response to 10 mM caffeine, suggesting that the heterologous desensitization is caused by a modification of an intracellular Ca2(+)-store, which is used by both histamine and methacholine. 6. The recovery of the responses after methacholine desensitization was not dependent on extracellular Ca2+, suggesting that the recovery is not dependent on refilling of the intracellular Ca2+ store with extracellular Ca2+. 7. The protein kinase C activator, phorbol-12,13-dibutyrate, concentration-dependently inhibited histamine- and methacholine-induced contractions. Protein kinase C seems therefore not to be implicated in the observed homologous H,-receptor desensitization. 8. These data suggest that different forms of desensitization can be distinguished in this model, each with a different time course and dependent on the applied stimulus.

The excitatory effect of dopamine on isolated canine tracheal smooth muscle

The effect of exogenous dopamine on canine tracheal smooth muscle has been studied in-vitro. Dopamine at concentrations over 10(-5)M induced contractions of tracheal muscle strips and repeated exposures resulted in desensitization (tachyphylaxis) of the muscle. The sensitivity of the response varied dramatically among muscle strips. At lower concentrations, dopamine caused neither muscle relaxation nor inhibition of contractions evoked by 10(-6)M acetylcholine. Both a dopaminergic antagonist, haloperidol (10(-5) and 10(-4)M), and an alpha-adrenoceptor antagonist, phentolamine (10(-7) to 10(-5)M), attenuated the contraction to 10(-3)M dopamine. The beta-adrenoceptor antagonist, propranolol (10(-8) to 10(-6)M), enhanced the contraction. However, the contraction could only be abolished by phentolamine at 10(-4) M. Thus, in canine tracheal smooth muscle, the contractile response to dopamine is predominantly through the activity of alpha-adrenoceptors and the role of dopaminergic receptors is vague. It is suggested that the weakness of the dopamine-induced contraction results from an antagonism between alpha- and beta-adrenoceptor effects and the dopamine tachyphylaxis may reflect a gradually decreased activation of the alpha-adrenoceptor mechanism in comparison with the beta-adrenoceptor mechanism.

Effects of esculetin (6,7-dihydroxycoumarin) on guinea-pig tracheal chains in vitro

Esculetin (6,7-dihydroxycoumarin) was tested in isolated guinea-pig tracheal chains from normal and ovalbumin-sensitized animals. The drug showed a concentration-dependent relaxant effect, with an EC50 of 60 microM. Pretreatment with esculetin (30-60 microM) was not able to reduce the contractile response induced by either carbachol (0.1 microM), histamine (10 microM), BaCl2 (10 mM) or K+ (80 mM). Esculetin increased the contraction evoked by PGF2 alpha and by histamine, but reduced A23187-induced contraction. Mepacrine (10 microM), a phospholipase A2 inhibitor, reduced the relaxant effect of the lower esculetin concentrations (3-60 microM). Indomethacin (0.1 microM) increased the relaxation induced by esculetin (3-10 microM). In tracheal chains from ovalbumin-sensitized guinea-pigs, esculetin (60 microM) significantly reduced the duration of the recovery phase after antigen challenge contraction. In conclusion, in our in vitro conditions, this natural coumarin derivative seems to act by interfering with the arachidonic acid cascade.

Prednisone pretreatment leads to histaminic airway hyporeactivity soon after resolution of the immediate allergic response

We assessed the effect of prednisone pretreatment (50 mg/day for three days) on the development of the early increase in histamine reactivity that occurs soon after resolution of the immediate response in allergic humans. Four allergic subjects who were known to develop only isolated immediate responses upon Kentucky bluegrass inhalation, as well as four mild allergic asthmatic subjects known to develop typical dual phase responses, were evaluated. All testing was done more than nine weeks after the grass pollen season had ended. Allergen inhalation produced an immediate response in all subjects. However, upon resolution of the immediate response to allergen in these pretreated subjects, the PC200His in all dual responding asthmatics and in three of the four isolated immediate responders had substantially increased above baseline values. We conclude that prednisone pretreatment leads to histaminic hyporeactivity soon after resolution of the immediate allergic response in both dual responding asthmatics and isolated immediate responders. It would seem that this prednisone effect is independent of its potential influence on the influx of inflammatory cells into diseased airways.

Indomethacin blocks airway tolerance to repetitive exercise but not to eucapnic hyperpnea in asthmatic subjects

We have examined the effects of indomethacin (I) on tolerance to the bronchomotor effects of repetitive challenge with exercise (EX) and eucapnic hyperpnea (EH) in 7 asthmatic subjects. Each subject was studied on 4 separate days. EH was performed for 4 min at a minute ventilation found previously to increase specific airway resistance (SRaw) by 8 units (cm H2O/L/s). All exercise challenges were performed on a cycle ergometer for 5 min at a constant work load. Subjects breathed room temperature, dry air for both stimuli. SRaw was serially measured before and after each stimulus. Tolerance was examined by giving up to 3 repetitions of EH or EX, allowing a return of SRaw to within 1 unit of baseline between repetitions. Placebo (P) or I (25 mg four times a day for 7 doses) was administered in a single-blind manner. The timing between stimulus repetitions on the P day was matched to that of the I day. After P, the initial rise in SRaw was similar for both EX and EH, with a significant and progressive decrease in this rise after each stimulus repetition (p = 0.032 for EX, p = 0.006 for EH). After I, tolerance was still demonstrated to EH (p = 0.002), but not to EX (p = 0.231). This finding indicates that EH and EX are not identical stimuli, since there is an I-sensitive mechanism (possibly a bronchodilating prostaglandin) associated with the development of tolerance to EX but not to EH. Our data also suggest a possible additional bronchoconstricting mechanism associated with EX and not with EH.

Characterization of nicotine-induced contraction in the canine bronchus

1. The modes of action of nicotine on the dog bronchial smooth muscle preparation was investigated, in order to compare with those on the bronchial preparations from the guinea-pig, rabbit and monkey. 2. Nicotine induced a contraction in the dog bronchial preparation, and this response was abolished by hexamethonium and atropine and potentiated by physostigmine. 3. These findings suggest that the contractile response to nicotine was mediated through an action on the nicotinic receptors and due to the release of acetylcholine. 4. Tetrodotoxin did not inhibit the contractile response to nicotine in the dog bronchial preparation, suggesting that the nicotine-induced response may be produced mainly through a sodium action potential-independent process. 5. The present observations in the dog bronchial preparations coincided with those in the rabbit and monkey bronchi but not with the findings in the guinea-pig bronchus.

Effects of ergometrine on airway smooth muscle contractile responses

A 26-year-old asthmatic female developed severe asthma within a few hours of receiving three oral doses of 0.4 mg ergometrine maleate for the control of postpartum haemorrhaging. This experience and two previous reports of bronchospasm in asthmatic subjects following ergometrine suggested that ergometrine altered airway smooth muscle tone. In the present investigation the effect of ergometrine was studied on canine tracheal smooth muscle strips. Ergometrine (10(-9) M-10(-4) M) induced contraction of canine tracheal smooth muscle. The concentration causing 50% of maximal contraction (EC50) was 4.73 X 10(-8) M. The acetylcholine EC50 was not altered by ergometrine (10(-9) M or 10(-8) M); however, acetylcholine (10(-4) M and 10(-3) M) induced contractions were enhanced by ergometrine (10(-8) M). The data suggest that ergometrine maleate may cause broncho-constriction in some patients with asthma.

Tetrodotoxin-resistant response to nicotine in rabbit bronchial preparation

The mode of action of nicotine was studied in a rabbit bronchial preparation. Nicotine (3 X 10(-5)-10(-3) M) produced a phasic contraction. No inhibitory response to nicotine was observed. The contractile response to nicotine was inhibited by hexamethonium, pentolinium and atropine but not by tetrodotoxin. Nicotine increased the efflux of tritium from preparations which had been labelled with [3H]choline. Tetrodotoxin did not inhibit the nicotine-evoked tritium release from the bronchial preparation. The results indicate that the release of acetylcholine evoked by nicotine was not influenced by tetrodotoxin in this preparation, and that the nicotine-induced response may be produced mainly through a sodium action potential-independent process. We could not rule out a contribution by acetylcholine released from the acetylcholine store in the smooth muscle in the contractile mechanisms for nicotine.

Sequential histamine inhalations cause increased bronchial histamine reactivity in guinea pigs: role of platelets, thromboxanes and prostacyclin

Groups of 6-15 guinea pigs sensitized to ovalbumin were challenged by repeated inhalations of a constant histamine dose at time 0, 10, 20, 60 and 70 min. Bronchial obstruction was measured by whole body plethysmography. The degree of bronchial obstruction increased from one challenge to the other reaching maximal values after 70 min. This increase of bronchial responsiveness to histamine after repeated histamine challenges was reduced by pretreatment with clemastine (histamine H1-receptor antagonist, 0.12 mg/kg i.p., n = 7, P less than 0.05) and more effectively by combined clemastine/cimetidine pretreatment (combined H1-H2-receptor antagonists, 0.12 resp. 10 mg/kg, n = 7, P less than 0.001); pretreatment with acetylsalicylic acid (10 mg/kg orally) accelerated the increase of bronchial responsiveness to histamine (n = 9, P less than 0.01 at the second challenge), inhalation of prostacyclin (1 microgram) prior to each histamine inhalation prevented the increase of bronchial histamine sensitivity totally (n = 10, P less than 0.001), whereas inhibition of thromboxane biosynthesis (imidazol, 10 mg/kg i.p., n = 6; 4-[2-(1H-imidazol-1-yl)ethoxy]benzoic acid, 10 mg/kg i.p., n = 9; imidazo(1,5-a)pyridine-5-hexanoic acid, 1 mg/kg i.p., n = 8) as well as immunologic platelet depletion were ineffective in our test system. We conclude that prostacyclin inhibits the increase of bronchial responsiveness to histamine after sequential histamine inhalation challenges by a platelet independent mechanism. 1-(3-phenyl-2-propenyl)-1H-imidazol, the fourth type of thromboxane synthetase inhibitor tested (10 mg/kg i.p., n = 15) showed specific effects which may be attributed to antihistamine functions.

Influence of histamine and prostaglandin on desensitization to neurotensin in rat blood pressure

Triphasic depressor-pressor-depressor blood pressure responses to neurotensin (NT: 1.67 micrograms/kg i.v.) in anesthetized rats were not elicited when the second dose of NT was administered 20 min after the first injection. Pretreatment of animals with histamine markedly reduced the depressor response to NT, and vice versa. The triphasic blood pressure pattern remained unaffected with acetylcholine and serotonin treatment, and hypotensive effects of acetylcholine and serotonin were not modified by NT. Attenuation of depressor response induced by the second injection of NT was antagonized by pretreatment with prostaglandin synthesis inhibitors such as indomethacin, mefenamic acid and acetylsalicylic acid. These results suggest that histamine and prostaglandins play a role in the development of desensitization to NT in rat blood pressure.

Specific and non-specific desensitization of guinea-pig ileal smooth muscle

The effects of cis-2-methyl-4-dimethylaminomethyl-1-3-dioxolane methiodide (CD), a muscarinic agonist, histamine, substance P and K+-stimulation on the mechanical responses, Ca2+-dependence and desensitization in guinea-pig ileal longitudinal smooth muscle have been studied. The mechanical responses to all four stimulants are highly dependent upon extracellular Ca2+(Ca2+EXT) and are blocked by the Ca2+ channel antagonist nicardipine. The tonic (slow) components of response are more dependent on Ca2+EXT and are more sensitive to nicardipine (IC50 values 5.0 X 10(-8) - 2.5 X 10(-9)M) than are the phasic (fast) components of response. Tissue exposure to CD (5 X 10(-7)M, 10 min) or histamine (3 X 10(-6)M and 3 X 10(-4)M, 10 min) produces short term nonspecific desensitization but substance P (5 X 10(-8)M, 10 min) produces only specific desensitization. K+-induced responses neither desensitize nor are desensitized. Desensitization is concentration- and time-dependent for both specific and nonspecific processes. Nonspecific desensitization is protected by elevation of K+ concentration (5.36mM) in the incubating medium, by dithiothreitol and by inhibitors (mepacrine,p-bromophenacyl bromide and phenylgloxal) of phospholipase A2 and is potentiated by mellitin, an activator of phospholipase A2. Desensitization produced by the muscarinic agonist CS is protected by Gpp(NH)p (10(-4)M), but histamine-induced desensitization is unaffected. There is no loss of muscarinic receptors, measured by [3H]QNB binding following tissue exposure to low concentrations of CD (5.0 X 10(-7)M) for up to 72 h. However, an apparent loss of receptors (20-30%) is measured following 10-90 min exposure of tissue to 10(-3)M CD. It is suggested that contractions of guinea-pig ileal longitudinal smooth muscle elicited by CD, histamine, substance P or K+ mobilize a common pool of Ca2+ through a Ca2+ channel antagonist (nicardipine) sensitive pathway. However, the existence of short term nonspecific desensitization (CD and histamine), specific desensitization (substance P) or no desensitization (K+ stimulation) indicates that significant differences exist in the pathways linking initial stimulus to mechanical response. The ability of elevated K+ to protect against nonspecific desensitization suggest that post stimulus membrane hyperpolarization may represent one contributing component to nonspecific desensitization. Products of phospholipid degradation may also contribute to desensitization since inhibitors or activators of phospholipase A2 prevented or potentiated respectively, nonspecific desensitization.

The site of action of Ptychodiscus brevis toxin within the parasympathetic axonal sodium channel h gate in airway smooth muscle

The red tide toxin produced by Ptychodiscus brevis ( PBTX ) may cause cough, sneezing, and asthma. Previous in vitro studies with isolated canine tracheal smooth muscle demonstrated that PBTX stimulates sodium channels of parasympathetic nerve endings and thus causes a contractile response. The present study investigated the mechanism of the PBTX effect on canine tracheal smooth muscle. Repeated exposure of the muscle strip to PBTX (final concentration 46 micrograms/ml) followed by washout of the toxin resulted in reestablishment of baseline tension but a failure of contraction on further addition of PBTX . However, veratridine and scorpion toxin (SCT), which are voltage-sensitive sodium channel activators, still induced contraction. Furthermore, the contraction caused by veratridine was enhanced by a high dose of PBTX , whereas contraction caused by SCT was not. Responses to veratridine and SCT as well as PBTX (previously reported) were blocked by tetrodotoxin (a sodium channel blocker), while acetylcholine responsiveness remained intact. These results indicate that PBTX receptors in parasympathetic nerves influence Na+ flux at the h gate, that these receptors differ from the veratridine and SCT receptors, and that the conformational change in the receptors induced by PBTX affects the tissue response to veratridine.

In vivo allergen tachyphylaxis in guinea pig lung is mediated by endogenous prostaglandin E biosynthesis

The term "allergen tachyphylaxis" (AT) describes a progressively decreased bronchial reactivity to allergen exposition after repeated allergen challenge [in our test system measured by sequential inhalative antigen challenge of sensitized guinea pigs (GP)]. The hypothesis that AT is mediated by endogenous prostaglandin E (PGE) biosynthesis was tested in vivo on GP sensitized to ovalbumin (OA). Different groups of animals were challenged with simultaneous inhalation of OA (repeatedly at time 0, 10, 20, 60 and 70 min) together with inhibitors of PGE-biosynthesis ( parachloromercurobenzoic acid = PCMB, copper sulfate = Cu, copper dithiotreitol complex = CuDTT , dithiotreitol = DTT and dimercaptopropanol = DMP) or agents increasing PGE production (aurothioglucose = Au, zinc dithiotreitol complex = ZnDTT and reduced glutathion = GSH). Bronchial obstruction was measured by whole body plethysmography . PCMB, Cu, CuDTT , DTT and DMP inhibited AT, whereas Au and ZnDTT enhanced AT. Acetylsalicylic acid (ASA) treatment prevented AT. Aerosols of PGE2, but not of prostacyclin or prostaglandin D2 restored AT in ASA treated animals. In addition to these in vivo experiments in vitro investigations showed that PCMB, Cu and DTT decreased while ZnDTT increased PGE biosynthesis of allergen challenged GP lungs. It is concluded that AT, an important self-protecting mechanism of GP bronchial asthma, is mediated at least in part via endogenous PGE.

Indomethacin enhances the effect of histamine on airways resistance in the anaesthetized guinea-pig

In anaesthetized guinea-pigs intravenous histamine caused an increase in airways resistance (RA) and a fall in dynamic compliance (CDyn). Indomethacin (1 mg kg-1, i.v.) significantly enhanced the effect of histamine on RA. Indomethacin also increased the basal RA and the RA response to a histamine infusion. The effect of indomethacin on CDyn was less consistent but here also there was a trend for an increased response to histamine. Sodium carbonate (the vehicle for indomethacin, 0.05 ml 100 mM solution) had no effect on RA or CDyn in control experiments. Propranolol (0.1 mg kg-1, i.v.) enhanced the effect of histamine on RA in animals pretreated with either indomethacin or Na2CO3 vehicle, but the effect was more consistent in indomethacin pretreated animals. Indomethacin also tended to enhance the effect of histamine on RA in animals pretreated with reserpine or BW755c but it had little effect on the CDyn response to histamine. The results show that indomethacin augments the responsiveness of the airways to histamine in the anaesthetized guinea-pig. The results with propranolol and reserpine suggest that an operational beta-adrenergic system is not required for the effect of indomethacin on RA. No confirmation for lipoxygenase involvement was obtained with the lipoxygenase inhibitor, BW755.

The effect of prostaglandin E2 on histamine-stimulated calcium mobilization as a possible explanation for histamine tachyphylaxis in canine tracheal smooth muscle

Isolated strips of canine tracheal smooth muscle rapidly lost their responsiveness to histamine when placed in a zero calcium Krebs buffer. Responsiveness to acetylcholine, however, was not rapidly lost, and following 120 min of incubation in zero calcium buffer with frequent washes, 10% of the contractile response still remained. The kinetics of each loss of response suggest that primarily a loosely bound source of calcium is mobilized by histamine and a more tightly bound source is mobilized by acetylcholine. Consistent with these data were the effects of the calcium antagonist verapamil. In normal calcium Krebs solution, dose-response curves to histamine were markedly reduced by verapamil while acetylcholine responses were relatively unaffected. In calcium depleted tracheal strips, indomethacin potentiated the calcium dose-response curve, determined by incremental readdition of calcium in the presence of histamine (10(-4) M), with comparatively little effect on the calcium dose-response curve in the presence of acetylcholine (10(-6) M). Also, in indomethacin pretreated tracheal strips, a reduction in the histamine-calcium dose-response curve could be produced by exogenous addition of 2.8 X 10(-9) M and 2.8 X 10(-8) M PGE2. In the acetylcholine-calcium responses there was a significant reduction only at 2.8 X 10(-8) M PGE2. These data suggest that histamine mobilizes primarily a loosely bound, possibly extracellular source of calcium necessary for contraction, and this histamine-stimulated calcium mobilization is sensitive to the effects of PGE2.

Desensitization of beta-adrenoreceptors in guinea-pig trachea: a prostaglandin mediated phenomenon

1 The formation and release of PGE2-like material (PGE2-lm) from guinea-pig isolated trachea and human bronchi following relaxation with isoprenaline (I) was investigated. 2 When airway smooth muscle precontracted with pilocarpine is relaxed by I, PGE2-lm is released in the bathing fluid. Under conditions of beta-adrenoreceptor desensitization, a greater amount of PGE2-lm is formed and the responsiveness of guinea-pig tracheal spirals to cumulative doses of I is diminished. 3 Inhibition of cyclo-oxygenase with indomethacin does not modify the relaxation induced by I but prevents the occurrence of refractoriness to I as well as formation and release of PGE2-lm. Addition of exogenous PGE2 to guinea-pig tracheas which were treated with indomethacin is able to restore the capacity of I to cause tachyphylaxis. 4 It is concluded that PGE2 which does not mediate the relaxation induced by I in airway smooth muscle, is responsible for the onset of desensitization of the beta-adrenoreceptor.

Involvement of catecholamines in Haemophilus influenzae induced decrease of beta-adrenoceptor function

The deeper airways of patients with asthmatic bronchitis are often infected with Haemophilus influenzae. Vaccination of guinea pigs with H. influenzae resulted in a significant impairment of the isoproterenol induced relaxation of isolated tracheal spirals by approximately 50% 4 days following vaccination. In the present study we further investigated the effects of some drugs affecting catecholamine release on the H. influenzae induced functional desensitization of tracheal spirals. Benserazide, an inhibitor of dopa-decarboxylase, completely prevented the reduction in isoproterenol-induced relaxation after H. influenzae vaccination, while no effect on relaxation of tracheal spirals from control animals was detected. On the other hand, inhibiting the re-uptake of catecholamines with desipramine did not influence the relaxation in the H. influenzae vaccinated tracheal spirals. Treatment of control animals with desipramine however resulted in a decreased relaxation of the isolated spirals by 40%. One day following vaccination with H. influenzae the level of norepinephrine in lung tissue was significantly elevated by 71%, and in plasma by 77%, while after 4 days no significant effects were observed. The spontaneous release of norepinephrine, epinephrine and dopamine of tracheal incubates was increased at days 1 and 4 following vaccination. The release of catecholamines from minced lung incubates of H. influenzae pretreated guinea pigs did not differ from that of controls. On the basis of these results it may be suggested that catecholamine metabolism is changed in lungs from H. influenzae vaccinated animals. Catecholamines, accordingly may play a role in the desensitization of beta-adrenoceptors by H. influenzae.

The effect of inhibitors of arachidonic acid metabolism on drug-induced contractions in isolated tracheal smooth muscle of the pig

1 The regulation of drug-induced tone in airways smooth muscle was examined in an isolated preparation of swine tracheal smooth muscle. 2 The trachea contracted (isometric) to histamine, 2-pyridylethylamine (2-PEA), acetylcholine and K+ but no response to histamine H2-receptor agonists were observed. 3 Histamine-induced contractions (100 microM) were potentiated by 213.3% by indomethacin (1 microM) and by 126.9% by sodium salicylate (250 microM). These inhibitors had only slight or no effects on acetylcholine-induced tone. 2-PEA responses were also potentiated by indomethacin but there were no changes in the response to H2-receptor agonists in the presence of indomethacin. The indomethacin-mediated potentiation of histamine was blocked by 5, 8, 11, 14-eicosatetraynoic acid (10 microM). FPL 55712 had no effect on these responses. 4 Mepacrine (100 microM) inhibited responses to histamine but not those to acetylcholine. No effect was observed with dexamethasone (up to 100 microM). 5 Prostaglandin E2 caused relaxation but arachidonic acid did not. 6 The possibility that histamine H1-agonist-induced contractions are regulated by contractile products of the arachidonic acid lipoxygenase pathway is discussed.

Decreased Schultz-Dale reaction in airways from sensitized guinea pigs treated with threshold concentrations of egg albumin in vivo

Treatment of egg albumin sensitized guinea pigs with repeated concentrations of egg albumin changed the Schultz-Dale response. The dose-response curve of egg albumin on the trachea was shifted to the right. Both the amount of SRS-A and histamine released by antigen exposure from sensitized or desensitized guinea pig lungs were almost similar. Regarding the number of H1-receptors in an isolated membrane fraction from the lungs of sensitized and desensitized guinea pigs, no significant difference was observed. on the contrary, the receptor-response to SRS-A was found to be decreased in desensitized trachea in comparison with sensitized trachea. The treatment of guinea pigs with repeated threshold concentrations of histamine did not change the contractile response neither to histamine nor to egg albumin. The decreased Schultz-Dale response after repeated egg albumin treatment in vivo might depend on desensitization of a hypothetical SRS-A receptor.

Allergen tachyphylaxis of guinea pigs in vivo; a prostaglandin E mediated phenomenon?

Ovalbumin (OA) sensitized guinea pigs were repeatedly challenged with 1% OA in saline nebulized ultrasonically at the 0, 10, 20, 60 and 70th min. The intensity of bronchial obstruction was measured by body plethysmography. The first three challenges (0. 10, 20 min) caused strong asthmatic reactions in all animals, the last two (60, 70 min) only mild one in 10 out of 15 animals. The development of this "tachyphylaxis" was markedly reduced by pretreatment of the animals with cyclooxygenase inhibitors (indomethacin 10 mg/kg intraperitoneally resp, acetylsalicylic acid 10 mg/kg orally 2 h before tests). The effect of both inhibitors (i.e. inhibition of tachyphylaxis) was abolished by supplementing prostaglandin E2 as aerosol simultaneously to the allergen (100-200 ng per inhalation). The results suggest that allergen tachyphylaxis we have observed in vivo might be due to synthesis of cyclooxygenase products, e.g. prostaglandin E.