Inositol phosphates and airway smooth muscle

Influence of epithelium on beta-adrenoceptor desensitization of guinea pig tracheal smooth muscle

The effect of tissue incubation with a beta2-agonist of denuded and intact epithelium trachea on the responsiveness to isoprenaline and beta-receptor blocked by propranolol (CR-1) was examined in this study. We examined the effect of epithelium removal on the beta-adrenoceptor desensitization resulting from incubation of guinea pig trachea in the beta-adrenergic agonist isoprenaline (10 microM). Desensitization was measured as the change in EC50, the concentration of beta-agonist that produced 50% relaxation of tracheal rings contracted with methacholine. As a second measure of desensitization, we measured the shift in EC50 resulting from incubation of tracheal rings with the beta-adrenoceptor antagonist propranolol (20 nM), expressed as CR-1 ([post-propranolol EC50/baseline EC50]-1). Initially, we measured desensitization immediately after incubation in isoprenaline; subsequently, we repeated the protocol and allowed a 30 min rest between the end of incubation and the measurement. The sensitivity of denuded epithelium trachea to isoprenaline and (CR-1) was significantly higher than that of intact epithelium only in non-incubated preparations (p<0.05 to p<0.001). Incubation to isoprenaline caused a significant reduction in the tracheal response to isoprenaline in both the denuded groups (p<0.005 for both cases) and intact epithelium groups (p<0.05 for both cases). Incubation to isoprenaline also caused a significant reduction in (CR-1) value in both the denuded groups (p<0.005 for group 2 and p<0.001 for group 4) and intact epithelium only in group 1 (p<0.05). However, the changes in EC50 due to tissue incubation with isoprenaline were significantly greater in denuded than intact epithelium trachea (p<0.05 for all cases) and for CR-1 value only in groups 1 and 2 (p<0.05). These results indicate decrease in both tracheal response to beta-agonist (tolerance) and CR-1 (due to incubation of tissues with isoprenaline), which were greater in denuded epithelium groups.

Influence of epithelium and isoprenaline incubation on responsiveness of guinea-pig trachea to methacholine

There are reports regarding harmful effect of long-term use of beta2-agonist drugs on asthma severity and airway responsiveness. In the present study, the responses of guinea pig trachea with intact and denuded epithelium (groups 1 and 2, n = 10) to methacholine as EC50 were measured in tissues nonincubated or incubated with 10 mumol/l isoprenaline during the resting period. The same protocol was performed in groups 3 and 4 (n = 5 for each group) with an additional 30 min rest time after isoprenaline incubation. The response of trachea with denuded epithelium (groups 2 and 4) to methacholine was significantly higher than that with intact epithelium both in incubated and nonincubated conditions (groups 1 and 3, p < 0.05 to p < 0.001). Incubation with isoprenaline caused a significant reduction in the tracheal response to methacholine in both the denuded groups (p < 0.005 and p < 0.001) and intact epithelium groups (p < 0.005 for both cases). The reduction in tracheal responsiveness to methacholine due to incubation in epithelium denuded trachea (groups 2 and 4) was nonsignificantly greater than that of intact epithelium tissues. There was no difference between groups 3 and 4 with those of groups 1 and 2 in both incubated and non incubated conditions. The maximum contractility response to methacholine was not different between tracheal chains with denuded and intact epithelium and did not change due to incubation with isoprenaline. The results of this study indicate reduction of tracheal response to methacholine due to incubation of tissues with isoprenaline, which was relatively more pronounced in epithelium denuded trachea.

Increased tracheal responsiveness to beta-adrenergic agonist and antagonist in ovalbumin-sensitized guinea pigs

Despite the controversy of bronchial responsiveness to beta2-agonist drugs in asthma, in a previous study we have shown increased responsiveness of asthmatic tracheobronchial tree to isoprenaline. Therefore, in the present study, tracheal responsiveness to isoprenaline and also beta-adrenergic receptor blockade were studied in sensitized guinea pigs. An experimental model of asthma was induced in guinea pigs by sensitization of animals with injection and inhalation of ovalbumin (OA). The responses of tracheal chains of sensitized and control animals to cumulative concentrations of isoprenaline (I) in the absence and presence of 10 nmol/l propranolol were measured, and the effective concentration of I causing 50% of maximum response (EC50 I) was obtained. The propranolol blockade (CR - 1) was calculated by: (post-propranolol EC50 I/EC50 I) - 1. Tracheal responses of sensitized and control animals to cumulative concentrations of methacholine (M) were also measured and EC50 M were obtained. The tracheal responses of sensitized guinea pig to isoprenaline was significantly higher than that of the control animals (EC50 I for sensitized and control animals were 0.24 +/- 0.04 and 0.58 +/- 0.07 micromol/l, respectively; p < 0.001). The beta-adrenergic receptor blockade by propranolol (CR - 1) was also significantly higher in sensitized guinea pigs than that of the control animals (p < 0.001). The results of this study indicate an increased tracheal response to beta-adrenergic-stimulating drug and enhancement of beta-adrenergic blockade by propranolol in the sensitized guinea pig.

Expression of transient receptor potential C6 and related transient receptor potential family members in human airway smooth muscle and lung tissue

Elevation of the intracellular free Ca(2+) concentration regulates many functional responses in airway smooth muscle, including contraction, proliferation, adhesion, and cell survival. This increase in calcium can be achieved by a release from internal stores (sarcoplasmic reticulum) and/or entry across the cell membrane from the extracellular environment. The molecular identity of this calcium influx pathway in human airway smooth muscle (HASM) remains unclear. Functional studies using Fluo 4-loaded HASM suggest the presence of a histamine H(1) receptor-activated Ca(2+) entry pathway with characteristics similar to those seen with transient receptor potential (TRP) family homologs. Using a range of molecular and cell biological approaches we defined the expression pattern of transient receptor potential classics (TRPC) homologs in airway cells and tissue. Here we show that HASM and human bronchial epithelial cells both express TRPC1, -4, and -6, with HASM also expressing TRPC3 at the mRNA level. Identification of TRPC6 protein by western blot and confocal microscopy indicated that the protein is localized in specific cell types, suggesting that it plays an important role in regulating key functions in airway cells. These data demonstrate the expression of a range of TRPC homologs in the airway and the presence of a functional Ca(2+) entry pathway with characteristics typical of TRPC family members. TRPC homologs may provide an important novel target for the treatment of airway disease.

Pharmacology of airway smooth muscle

Airway smooth muscle contributes to changes in airway caliber not only through the variations in its tone but also through its contribution to thickness of the airway wall. Until recently, most attention was paid to the agents that altered airway smooth muscle tone, their receptors, the signal transduction pathways they activated, and the mechanisms of contraction and relaxation themselves. Lately, the regulation of smooth muscle proliferation has received increasing attention, and, most recently, the possible role of smooth muscle as a source of inflammatory mediators has been recognized. Airway smooth muscle cells are now seen as playing an important interactive role with inflammatory and structural cells in the response to injury and repair of the airways.

Modulation of airway smooth muscle beta-adrenoceptor function by a muscarinic agonist

We have investigated the effect of a muscarinic agonist and protein kinase C (PKC) activation on beta-adrenoceptors and their coupling to adenylyl cyclase in bovine tracheal smooth muscle. There was a significant reduction in maximum binding capacity of [125I]iodocyanopindolol ([125I]ICYP) after exposure to carbachol and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA). Similarly both carbachol and PMA inhibited the 5'-guanylylimidodiphosphate-induced shift in [125I]ICYP binding by isoproterenol and significantly decreased isoproterenol-induced cyclic AMP accumulation. A phorbol ester, 4 alpha-phorbol 12,13-didecanoate which does not activate PKC had no effect on beta-receptor binding or coupling. These results suggest that PKC activation directly via a phorbol ester and indirectly via muscarinic receptor stimulation may lead to reduction and uncoupling of beta-receptors in airway smooth muscle. We suggest that this mechanism may be relevant to the reduction in beta-receptor coupling in asthmatic airways as an effect of PKC activation by inflammatory mediators and neurotransmitters.

Leukotriene D4 receptors are not negatively coupled to adenylyl cyclase in guinea-pig lung parenchyma

1. The possibility that receptors for the peptide-containing leukotrienes may be negatively coupled to adenylyl cyclase in guinea-pig lung parenchyma was investigated by comparing the effect of leukotriene D4 (LTD4) on the intracellular cyclic nucleotide (cyclic AMP and cyclic GMP) content and on the activity of cyclic AMP-dependent protein kinase (PKA). In addition, the potential association between changes in the cyclic nucleotide content and the ability of LTD4 to increase lung parenchymal tone was also evaluated. 2. Non-cumulative challenge of parenchymal lung strips with LTD4 elicited concentration-dependent contractions (pD2 = 8.23) that were paralleled by concentration-related increases in the intracellular level of cyclic AMP and cyclic GMP, and in the activation state of PKA (Kact = 33 nM). Temporally, these biochemical effects of LTD4 were transient, peaking after approximately 5 min drug contact thereafter decaying, despite the continued generation of tone. Both the biochemical and mechanical effects of LTD4 were antagonized by the LTD4-receptor blocking drug, ICI 198,615 (1 microM for 60 min), indicating that they were receptor-mediated events. 3. Challenge of guinea-pig lung with LTD4 (200 nM; EC100 for tension generation) stimulated a 150 and 70 fold increase in the elaboration of thromboxane B2 (TXB2) and 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha) respectively, relative to that generated spontaneously. 4. Pretreatment of lung strips for 60 min with an irreversible inhibitor of cyclo-oxygenase, flurbiprofen,at a concentration (8 microM) that abolished both basal and LTD4 (200 nM)-induced TXB2 and 6-keto-PGF1alpha release, relaxed rapidly the spontaneous tone of the tissues, reduced the cyclic AMP content by ~50%and lowered the PKA activity ratio from 29% to 17%. In addition, flurbiprofen abolished the ability of LTD4 (200 nM) to increase the cyclic AMP content and to activate PKA. Functionally, the magnitude of LTD4 (200 nM)-induced tone and the increase in cyclic GMP content were attenuated by approximately 20% and 50% respectively in flurbiprofen-treated tissues.5. In flurbiprofen-treated tissues, isoprenaline (10 microM for 10 min) increased the cyclic AMP content(from 4 to 27 pmol mg-1 protein) and activated PKA (from 15% to 26%). Preincubation (30 s or 5 min)of lung with LTD4 (200 nM) did not inhibit (or enhance) these isoprenaline-induced effects.6. Pretreatment of lung strips for 60 min with the thromboxane synthetase inhibitor, dazmegrel (10 microM),relaxed the spontaneous tone of the tissues, abolished the LTD4 (200 nM)-stimulated release of TXB2 and significantly enhanced (~two fold) the elaboration of 6-keto-PGF1alpha. In addition, dazmegrel attenuated (by ~50%) LTD4 (200 nM)-induced cyclic GMP accumulation but approximately doubled both the cyclic AMP content and PKA activity ratio. LTD4-induced contractions, in contrast, were not affected by dazmegrel.7. EP 092 (1 microM for 60 min), a selective TP-receptor blocking drug, had no effect on spontaneous tone,eicosanoid formation or on the cyclic GMP content of guinea-pig lung parenchymal strips. Likewise,EP 092 exerted no significant mechancial effect in lung challenged with LTD4 (200 nM) although it did potentiate, to a small extent, the ability of LTD4 (200 nM) to increase the cyclic AMP content.8. It is concluded that LTD4 can increase the intracellular level of cyclic AMP in guinea-pig parenchyma and activate PKA by a leukotriene-receptor-mediated mechanism sensitive to ICI 198,615. However,these biochemical actions of LTD4 are induced indirectly by an arachidonic acid-derived cyclo-oxygenase product(s) other than TXA2. Thus, contrary to reports of other investigators, no evidence was found to corroborate the finding that stimulation of leukotriene receptors on guinea-pig lung parenchyma results in a rapid lowering of the cyclic AMP content even in cyclo-oxygenase-blocked tissues. These data,therefore, do not support the hypothesis that leukotriene-induced tension generation is dependent upon a prior reduction in the cyclic AMP content.

Biochemistry of asthma

Asthma is a chronic condition of the airways that involves many types of inflammatory cell and the release of many mediators and neurotransmitters that have multiple effects on the various target cells in the airway. There have been important advances in understanding the biochemical events involved in signal transduction in inflammatory cells, in mediator synthesis and release, in contraction and relaxation of airway smooth muscle and in neural mechanisms. This may lead to more effective and specific therapies for this common but complex disease.

Beta-adrenoceptor induced inhibition of muscarinic receptor-stimulated phosphoinositide metabolism is agonist specific in bovine tracheal smooth muscle

The ability of the beta-adrenoceptor agonist isoprenaline to inhibit agonist-stimulated phosphoinositide metabolism was examined in bovine tracheal smooth muscle slices prelabelled with [3H]inositol. Accumulation of [3H]inositol phosphates was enhanced by the muscarinic agonists carbachol, oxotremorine and pilocarpine although the latter were only partial agonists for this response. Histamine stimulation of [3H]inositol phosphates was sensitive to mepyramine but maximal responses were only comparable to those of pilocarpine. Preincubation of tracheal slices with isoprenaline reduced the maximal phosphoinositide response to histamine and pilocarpine but the responses to carbachol and oxotremorine were unaffected. The inhibitory effect of isoprenaline (IC50 = 0.04 microM) was reversed competitively by 1 microM propranolol. The non-selective phosphodiesterase inhibitor isobutylmethylxanthine (IBMX) (1 mM) resulted in a more severe suppression of the histamine and pilocarpine responses and also produced a significant suppression of the maximal response to oxotremorine and a small shift in the carbachol dose-response curve. The different susceptibility of agonist-stimulated phosphoinositide hydrolysis to isoprenaline and IBMX are discussed in relation to the relative intrinsic activity of the agonists and/or the role of different muscarinic receptor subtypes.

Lack of effect of zaprinast on methacholine-induced contraction and inositol 1,4,5-trisphosphate accumulation in bovine tracheal smooth muscle

1. The effects of zaprinast (M&B 22948), a selective guanosine 3':5'-cyclic monophosphate (cyclic GMP) phosphodiesterase inhibitor, and sodium nitroprusside on cyclic GMP content, phosphoinositide hydrolysis and airway smooth muscle tone were examined in flurbiprofen pretreated bovine tracheal smooth muscle (BTSM). 2. Anion-exchange chromatography of the soluble fraction of BTSM homogenates resolved three peaks of Ca2+/calmodulin-independent phosphodiesterase (PDE) activity that corresponded to type Ia (cyclic GMP-specific, zaprinast-inhibitable), type II (cyclic GMP-stimulated) and type IV (Ro 20 1724-inhibitable) PDE isoenzymes. Zaprinast caused a selective inhibition of the type Ia PDE isoenzyme (IC50 0.94 microM) with respect to the type II and IV (IC50 s 93 microM and 197 microM respectively) isoenzymes. 3. Pretreatment of BTSM strips with zaprinast (10 microM) for 20 min affected neither the initial rate of force development, nor the resultant magnitude of contraction induced by methacholine (10 microM). In addition, zaprinast (10 microM; 20 min) did not affect the cumulative concentration-response relationship induced by methacholine. In contrast, sodium nitroprusside (300 microM) either alone, or in combination with zaprinast (10 microM), significantly attenuated tone induced by low, but not high concentrations of methacholine. This resulted in a non-parallel, rightwards shift of the methacholine concentration-response curves (nitroprusside: 4.0 fold; nitroprusside/zaprinast: 4.8 fold at the EC50 values), without a reduction in the maximum tone generated. 4. In BTSM slices, zaprinast (10 or 100 microM) did not influence basal or methacholine (10 microM)-stimulated cyclic GMP accumulation or inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation over a 60s incubation period, although it did significantly increase cyclic GMP content over longer (30 min) stimulation periods. 5. In [3H]-inositol prelabelled BTSM slices, stimulated in the presence of 5mM LiCl, methacholine (10 microM) caused a marked increase in total [3H]-inositol phosphate accumulation. This effect was not inhibited by zaprinast (10 microM), sodium nitroprusside (300 microM), or a combination of these drugs despite these agents markedly increasing tissue cyclic GMP content. 6. These findings demonstrate that despite zaprinast being a potent and selective inhibitor of the type Ia PDE isoenzyme in a cell-free system, this drug only increases cyclic GMP content in BTSM following prolonged agonist-stimulation. This may explain its lack of inhibitory effect on methacholine-induced tone. The inability of drugs which increase tissue cyclic GMP content and exhibit anti-spasmogenic activity to inhibit methacholine-stimulated Ins(1,4,5)P3 formation suggests that, unlike vascular smooth muscle, cyclic GMP-dependent mechanisms do not regulate receptor-mediated phosphoinositide hydrolysis in BTSM.

Effect of frusemide on airway smooth muscle contractility in vitro

Frusemide, an inhibitor of sodium-potassium-chloride (Na-K-Cl) cotransport, has been shown to inhibit the airway response to several constrictor stimuli in asthmatic subjects. The protection seen with frusemide in these studies could be due to an effect on epithelium, inflammatory cells, neural pathways, or airway smooth muscle. To determine whether frusemide inhibits airway smooth muscle contraction, experiments were performed in bovine and human airways in vitro. Fresh bovine tissue was obtained from the abattoir and human tissue from thoracotomy. The effect of 10(-5)M frusemide on histamine, potassium chloride, and hyperosmolar saline induced contractions was studied in bovine tracheal strips without epithelium. Frusemide, at a concentration that specifically inhibits Na-K-Cl cotransport, did not inhibit contraction caused by any of these agents. Frusemide was also without effect on hypertonic saline induced contractions of bovine and human bronchial rings with epithelium intact. These results suggest that modification of Na-K-Cl cotransport does not alter airway smooth muscle contractility and that the protective effect of frusemide on induced bronchoconstriction in vivo is unlikely to be due to a direct effect on airway smooth muscle.

Modulation of carbachol-induced inositol phosphate formation in bovine tracheal smooth muscle by cyclic AMP phosphodiesterase inhibitors

An investigation was made of a range of agents capable of elevating tissue cyclic AMP levels, or acting as a stable analogue of cyclic AMP, upon carbachol induced inositol phosphate responses in bovine tracheal smooth muscle slices. Whereas the beta 2 adrenoceptor agonist salbutamol (1 microM) and the membrane permeable analogue of cyclic AMP, 8-bromo-cyclic AMP (1 mM) were without effect upon total [3H]inositol phosphate formation induced by carbachol, 3-iso-butyl-1-methylaxanthine (IBMX) (EC50 140 microM), the high Km, cyclic AMP selective phosphodiesterase inhibitor rolipram (EC50 41 microM) and theophylline (EC50 76 microM) all inhibited the inositol phosphate response to low (1 microM) concentrations of carbachol. IBMX (IC50 13 microM), rolipram (IC50 4.6 microM) and theophylline (IC50 180 microM) all relaxed bovine tracheal muscle strips precontracted with methacholine (1 microM). The adenylate cyclase activator forskolin (1 microM), produced a much smaller (10% inhibition) effect upon inositol phosphate formation induced by carbachol. Carbachol (1 microM-1 mM) did not inhibit forskolin induced [3H]cyclic AMP formation. An inhibitor of the cyclic GMP preferring phosphodiesterase isozyme, M&B 22948 (1-100 microM), was without effect upon either carbachol induced inositol phosphate formation or trachealis tone. It is concluded that IBMX, rolipram and theophylline inhibit carbachol stimulated inositol phosphate formation, possibly through a cyclic AMP independent mechanism.