Breakdown of phosphoinositides in airway smooth muscle: lack of influence of anti-asthmatic drugs

Ion channel regulation of intracellular calcium and airway smooth muscle function

Airway hyper-responsiveness associated with asthma is mediated by airway smooth muscle cells (SMCs) and has a complicated etiology involving increases in cell contraction and proliferation and the secretion of inflammatory mediators. Although these pathological changes are diverse, a common feature associated with their regulation is a change in intracellular Ca(2+) concentration ([Ca(2+)](i)). Because the [Ca(2+)](i) itself is a function of the activity and expression of a variety of ion channels, in both the plasma membrane and sarcoplasmic reticulum of the SMC, the modification of this ion channel activity may predispose airway SMCs to hyper-responsiveness. Our objective is to review how ion channels determine the [Ca(2+)](i) and influence the function of airway SMCs and emphasize the potential of ion channels as sites for therapeutic approaches to asthma.

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.

Comparison of the effects of selective inhibitors of phosphodiesterase types III and IV in airway smooth muscle with differing beta-adrenoceptor subtypes

1. The relaxant properties of the type IV adenosine 3',5'-cyclic monophosphate phosphodiesterase (cyclic AMP PDE) inhibitor, rolipram and the beta 2-selective and non-selective beta-adrenoceptor agonists salbutamol and isoprenaline, were compared on the guinea-pig, bovine, and mouse trachea and porcine bronchus all precontracted with methacholine (EC30). 2. Rolipram and both beta-agonists produced concentration-dependent reversal of the methacholine-induced tone in the four airway preparations. 3. Isoprenaline and salbutamol were similar in potency on the guinea-pig (-log10IC50:8.43, 8.06) and bovine (-log10 IC50:8.52, 8.40) airways. In contrast, salbutamol was much less potent than isoprenaline on the mouse trachea (> 1000 fold) and the porcine bronchus (> 100,000 fold). 4. The potency of rolipram approached that of isoprenaline on the guinea-pig and bovine trachea (beta 2-adrenoceptors predominate). However, rolipram was significantly less active than isoprenaline on the porcine bronchus (1000 fold) and mouse trachea (> 2000 fold) where beta 2-adrenoceptors predominate. 5. Siguazodan, the type III cyclic AMP PDE inhibitor, produced concentration-dependent relaxations of the porcine bronchus and guinea-pig trachea contracted with methacholine. Siguazodan was 100 fold more active than rolipram in pig tissues indicating the type III isoenzyme may be of greater functional significance in this tissue. In contrast, siguazodan was 15 times less potent that rolipram in guinea-pig airways suggesting a greater role for the type IV PDE. 6. These findings may reflect a possible relationship between the beta 2-adrenoceptor subtype and the functional importance of the type IV PDE isoenzyme. A similar relationship may exist between beta 1-adrenoceptors and the PDE type III isoenzyme.

Theophylline produces over-additive relaxation of canine tracheal smooth muscle when combined with beta-agonists: The dose-response relationship

The interaction between theophylline (T) and the beta-agonists albuterol (A) and isoproterenol (I) was examined using canine cervical tracheal smooth muscle devoid of epithelium contracted with 0.1 or 0.3 microM methacholine. Greater functional antagonism with beta-agonists vs. T was confirmed and an ability of T to potentiate beta-agonist relaxation was demonstrated. The EC50 for T increased from 0.13 +/- 0.02 to 0.37 +/- 0.07 mM (mean +/- SEM) in preparations contracted with 0.1 or 0.3 microM methacholine, respectively, while that for I increased from 0.036 +/- 0.008 to 0.17 +/- 0.03 microM, a significantly larger change (P < 0.025). In tissues contracted with 0.3 microM methacholine and pretreated with 10 micrograms/ml of T IC50 values from composite concentration-response curves for I and A were displaced to the left and Emax was increased (56.6 to 71.5% for I, 44 to 61% for A, P < 0.0002). Addition of 10 micrograms/ml T resulted in relaxations which exceeded that calculated by the fractional product method for additive, independent action (P < 0.0001 for I, P < 0.0002 for A at 0.3 microM methacholine), suggesting that at least part of T's action was over-additive. Five, 10 and 20 micrograms/ml T enhanced the effectiveness of single concentrations of I by factors of 1.47 +/- 0.14 (P < 0.05), 2.72 +/- 0.26 (P < 0.01) and 5.34 +/- 0.55 (P < 0.01), respectively, in preparations contracted with 0.1 microM methacholine: I enhanced the effectiveness to a lesser degree. Using two approaches, positive interaction or over-additivity between T and beta-agonists has been demonstrated.

Inhibitory effects of cyclic AMP-elevating agents on norepinephrine-induced phosphatidylinositide hydrolysis and contraction in rat aorta

1. Exposure of rat aorta to forskolin, dibutyryl cyclic AMP, rolipram or isobutylmethylxanthine partially prevented the increased inositol monophosphate accumulation due to norepinephrine, while the contractile responses to norepinephrine were almost completely inhibited. 2. Inhibition of the increased phosphatidylinositide synthesis due to norepinephrine could not account for the decreased inositol monophosphate accumulation. 3. Although the increased inositol monophosphate accumulation due to norepinephrine was partially inhibited by the cyclooxygenase inhibitor, indomethacin, the inhibitory effects of the cyclic AMP-elevating agents were still observed in the presence of indomethacin. 4. Inhibition of agonist-induced phosphatidylinositide hydrolysis may contribute, at least in part, to the vasodilatory effects of the cyclic AMP-elevating agents.

Beta-adrenoceptor stimulation inhibits histamine-stimulated inositol phospholipid hydrolysis in bovine tracheal smooth muscle

1. Histamine and carbachol produced concentration-related increases in the accumulation of 3H-inositol phosphates in slices of bovine tracheal smooth muscle. 2. Noradrenaline alone produced a small stimulation of 3H-inositol phosphate accumulation which was inhibited by the alpha-adrenoceptor antagonist phentolamine. In contrast, when noradrenaline (0.1 mM) was added simultaneously with histamine it significantly reduced the inositol phosphate response to high (greater than or equal to 0.1 mM) concentrations of histamine. However, noradrenaline had no inhibitory effect on the carbachol-induced inositol phosphate response. 3. The non-selective beta-agonist isoprenaline (IC50 = 0.08 microM) and the beta 2-selective agonist salbutamol (IC50 = 0.29 microM) both produced a dose-related inhibition of the inositol phosphate response to 0.1 mM histamine. The inhibitory effect of salbutamol was antagonized by propranolol (KA = 2.4 x 10(9) M-1) and the beta 2-selective adrenoceptor antagonist ICI 118551 (KA = 1.7 x 10(9) M-1). 4. The accumulation of 3H-inositol phosphates induced by histamine increased steadily over a 40 min period after an initial lag period of 3-4 min. Following the simultaneous addition of histamine and salbutamol there was a further delay of 3-4 min before the appearance of the inhibitory effect of salbutamol. 5. The effect of histamine on inositol phosphate accumulation was accompanied by a stimulation of [3H]-inositol incorporation into membrane phospholipids which was reduced by the presence of salbutamol. However, when histamine was used to stimulate maximally [3H]-inositol incorporation during the prelabelling period, salbutamol produced a marked inhibition of histamine-stimulated 3H-inositol phosphate accumulation under conditions in which there was no change in the level of incorporation.

Differential inhibitory effects of forskolin, isoproterenol, and dibutyryl cyclic adenosine monophosphate on phosphoinositide hydrolysis in canine tracheal smooth muscle

A characteristic feature of airway smooth muscle is its relative sensitivity to relaxant effects of beta adrenergic agonists when contracted by inflammatory mediators, such as histamine, vs. resistance to these relaxant effects when contracted by muscarinic agonists. Because contractions presumably depend upon the hydrolysis of membrane phosphoinositides (PI) and the generation of inositol phosphates (IP), our goal was to test for the effects of forskolin, isoproterenol, and dibutyryl cAMP on histamine- vs. methacholine-induced IP accumulation in canine tracheal smooth muscle. Methacholine (10(-3) M) was a more effective stimulant of IP accumulation (9.6 +/- 2.1-fold increase) than equimolar histamine (3.6 +/- 0.5-fold increase) in this tissue. When responses to equieffective methacholine (4 x 10(-6) M) and histamine (10(-3) M) were compared, neither forskolin, isoproterenol, nor dibutyryl cAMP significantly decreased IP accumulation in response to methacholine. In contrast, each of these three agents significantly decreased responses to histamine (by 56 +/- 9, 52 +/- 2, and 61 +/- 2%, respectively). We concluded that, in canine tracheal smooth muscle, increased cAMP is associated with inhibition of PI hydrolysis in response to histamine but not methacholine. The findings suggest a novel mechanism for selective modulation by cAMP of receptor-mediated cellular activation.

The drug therapy of asthma: directions for the 21st century

There are several novel pharmacological approaches to asthma therapy which have resulted from a further understanding of airway smooth muscle function and inflammatory mechanisms involved in asthma. The most effective bronchodilators currently available are beta 2-adrenoceptor agonists and drugs with a prolonged duration of action after inhalation will be useful. Other possible bronchodilator approaches include vasoactive intestinal peptide, prostaglandins, novel xanthines, selective muscarinic antagonists, drugs interfering with intracellular calcium release and K+-channel activators. The most effective anti-inflammatory treatment is corticosteroids and efforts are being made to improve the topical potency of these drugs. Many other ways of reducing inflammatory effects in asthma are being explored, such as mediator antagonists (especially thromboxane and PAF antagonists), inhibitors of inflammatory cell activation (especially of eosinophils) and inhibition of neurogenic inflammation. Use of anti-inflammatory therapies earlier in the disease should become more widespread.