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

Stimulation of spinal dorsal horn β2-adrenergic receptor ameliorates neuropathic mechanical hypersensitivity through a reduction of phosphorylation of microglial p38 MAP kinase and astrocytic c-jun N-terminal kinase

The noradrenaline-adrenergic system has a crucial role in controlling nociceptive transduction at the spinal level. While α-adrenergic receptors are known to regulate nociceptive neurotransmitter release at the spinal presynaptic level, it is not entirely clear whether β-adrenergic receptors are involved in controlling pain transduction at the spinal level as well. The current study elucidated a role of β-adrenergic receptors in neuropathic pain in mice following a partial sciatic nerve ligation (PSNL). In addition, the cellular and intracellular signaling cascade induced by β-adrenergic receptors in neuropathic mice was elaborated. Intrathecal injection of isoproterenol (1 nmol), a nonselective β-adrenergic receptor agonist, briefly ameliorated hind paw mechanical hypersensitivity of PSNL mice. Isoproterenol's antinociceptive effect was mediated through β2-adrenergic receptors since pretreatment with ICI118551, a selective β2-adrenergic receptor antagonist, but not with CGP20712A, a selective β1-adrenergic receptor antagonist, significantly attenuated isoproterenol's effect. Furthermore, intrathecal treatment with a selective β2-adrenergic receptor agonist, terbutaline, but not a selective β1-adrenergic receptor agonist, dobutamine, also significantly ameliorated neuropathic pain. Fourteen days after PSNL, increased phosphorylation of both p38 Mitogen-activated protein kinase (MAPK) in microglia and c-jun N-terminal kinase (JNK) in astrocytes of ipsilateral spinal dorsal horn were observed. Phosphorylation of both microglial p38 MAPK and astrocytic JNK were downregulated by stimulation of the β2-adrenergic receptor. Together, these results suggest that spinal β2-adrenergic receptor have an inhibitory role in neuropathic nociceptive transduction at the spinal level through a downregulation of glial activity, perhaps through modulation of MAP kinases phosphorylation. Thus, targeting of β2-adrenergic receptors could be an effective therapeutic strategy in treating neuropathic pain.

An age-wise comparison of human airway smooth muscle proliferative capacity

We compared the proliferation of neonatal and adult airway smooth muscle cells (ASMC) with no/moderate lung disease, in glucose- (energy production by glycolysis) or glucose-free medium (ATP production from mitochondrial oxidative phosphorylations only), in response to 10% fetal calf serum (FCS) and PDGF-AA. In the presence of glucose, cell counts were significantly greater in neonatal vs. adult ASMC. Similarly, neonatal ASMC DNA synthesis in 10% FCS and PDGF-AA, and [Ca²⁺]i responses in the presence of histamine were significantly enhanced vs. adults. In glucose-free medium, cell proliferation was preserved in neonatal cells, unlike in adult cells, with concomitant increased porin (an indicator of mitochondrial activity) protein expression. Compared to adults, stimulated neonatal human ASMC are in a rapid and robust proliferative phase and have the capacity to respond disproportionately under abnormal environmental conditions, through increased mitochondrial biogenesis and altered calcium homeostasis.

The pharmacological rationale for combining muscarinic receptor antagonists and β-adrenoceptor agonists in the treatment of airway and bladder disease

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Muscarinic receptors increase smooth muscle tone in airways and urinary bladder.

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β-Adrenoceptors relax smooth muscle tone and oppose muscarinic contraction.

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Opposition involves transmitter release, signal transduction and receptor expression.

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This supports the combined use of muscarinic antagonists and β-adrenoceptor agonists.

Muscarinic receptor antagonists and β-adrenoceptor agonists are used in the treatment of obstructive airway disease and overactive bladder syndrome. Here we review the pharmacological rationale for their combination. Muscarinic receptors and β-adrenoceptors are physiological antagonists for smooth muscle tone in airways and bladder. Muscarinic agonism may attenuate β-adrenoceptor-mediated relaxation more than other contractile stimuli. Chronic treatment with one drug class may regulate expression of the target receptor but also that of the opposing receptor. Prejunctional β₂-adrenoceptors can enhance neuronal acetylcholine release. Moreover, at least in the airways, muscarinic receptors and β-adrenoceptors are expressed in different locations, indicating that only a combined modulation of both systems may cause dilatation along the entire bronchial tree. While all of these factors contribute to a rationale for a combination of muscarinic receptor antagonists and β-adrenoceptor agonists, the full value of such combination as compared to monotherapy can only be determined in clinical studies.

Active components of ginger potentiate β-agonist-induced relaxation of airway smooth muscle by modulating cytoskeletal regulatory proteins

β-Agonists are the first-line therapy to alleviate asthma symptoms by acutely relaxing the airway. Purified components of ginger relax airway smooth muscle (ASM), but the mechanisms are unclear. By elucidating these mechanisms, we can explore the use of phytotherapeutics in combination with traditional asthma therapies. The objectives of this study were to: (1) determine if 6-gingerol, 8-gingerol, or 6-shogaol potentiate β-agonist-induced ASM relaxation; and (2) define the mechanism(s) of action responsible for this potentiation. Human ASM was contracted in organ baths. Tissues were relaxed dose dependently with β-agonist, isoproterenol, in the presence of vehicle, 6-gingerol, 8-gingerol, or 6-shogaol (100 μM). Primary human ASM cells were used for cellular experiments. Purified phosphodiesterase (PDE) 4D or phospholipase C β enzyme was used to assess inhibitory activity of ginger components using fluorescent assays. A G-LISA assay was used to determine the effects of ginger constituents on Ras homolog gene family member A activation. Significant potentiation of isoproterenol-induced relaxation was observed with each of the ginger constituents. 6-Shogaol showed the largest shift in isoproterenol half-maximal effective concentration. 6-Gingerol, 8-gingerol, or 6-shogaol significantly inhibited PDE4D, whereas 8-gingerol and 6-shogaol also inhibited phospholipase C β activity. 6-Shogaol alone inhibited Ras homolog gene family member A activation. In human ASM cells, these constituents decreased phosphorylation of 17-kD protein kinase C-potentiated inhibitory protein of type 1 protein phosphatase and 8-gingerol decreased myosin light chain phosphorylation. Isolated components of ginger potentiate β-agonist-induced relaxation in human ASM. This potentiation involves PDE4D inhibition and cytoskeletal regulatory proteins. Together with β-agonists, 6-gingerol, 8-gingerol, or 6-shogaol may augment existing asthma therapy, resulting in relief of symptoms through complementary intracellular pathways.

Beta-Adrenergic Agonists

Inhaled β₂-adrenoceptor (β₂-AR) agonists are considered essential bronchodilator drugs in the treatment of bronchial asthma, both as symptoms-relievers and, in combination with inhaled corticosteroids, as disease-controllers. In this article, we first review the basic mechanisms by which the β₂-adrenergic system contributes to the control of airway smooth muscle tone. Then, we go on describing the structural characteristics of β₂-AR and the molecular basis of G-protein-coupled receptor signaling and mechanisms of its desensitization/ dysfunction. In particular, phosphorylation mediated by protein kinase A and β-adrenergic receptor kinase are examined in detail. Finally, we discuss the pivotal role of inhaled β₂-AR agonists in the treatment of asthma and the concerns about their safety that have been recently raised.

Potentiation of beta-adrenoceptor function in bovine tracheal smooth muscle by inhibition of protein kinase C

To examine the role of contractile agonist-induced activation of protein kinase C (PKC) in functional antagonism of airway smooth muscle contraction by beta-adrenoceptor agonists, we examined the effects of the specific PKC-inhibitor GF 109203X (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl) maleimide) on isoprenaline-induced relaxation of bovine tracheal smooth muscle contracted by various concentrations of methacholine and histamine. In the absence of GF 109203X, the potency of isoprenaline (pD(2)) was gradually reduced at increasing methacholine- and histamine-induced smooth muscle tones, but the maximal relaxation (E(max)) was decreased only at higher concentrations of methacholine. In the presence of GF 109203X, pD(2) values were significantly increased for both methacholine- and histamine-induced contractions. Moreover, isoprenaline E(max) values in the presence of high concentrations of methacholine were also increased. Although both methacholine- and histamine-induced contractions were slightly reduced by GF 109203X, the changes in isoprenaline pD(2) could only partially be explained by reduced contractile tone. In contrast to isoprenaline, forskolin-induced relaxations were not affected by GF 109203X. The results indicate that PKC activation contributes to the reduced beta-adrenergic responsiveness induced by methacholine and histamine, which may involve uncoupling of the beta-adrenoceptor from the effector system. Since many mediators and neurotransmitters in allergic airway inflammation can activate PKC, this cross talk may be important in the reduced bronchodilator response of patients with severe asthma.

Alpha 1-adrenoceptor effects mediated by protein kinase C alpha in human cultured prostatic stromal cells

1 We have investigated the effects of alpha(1)-adrenoceptor stimulation upon contractility, Ca(2+) influx, inositol phosphate production, and protein kinase C (PKC) translocation in human cultured prostatic stromal cells (HCPSC). 2 The alpha(1)-adrenoceptor selective agonist phenylephrine elicited contractile responses of HCPSC, i.e. a maximal cell shortening of 45+/-6% of initial cell length, with an EC(50) of 1.6+/-0.1 microM. The alpha(1)-adrenoceptor selective antagonists prazosin (1 microM) and terazosin (1 microM) both blocked contractions to phenylephrine (10 microM). The L-type calcium channel blocker nifedipine (10 microM), and the PKC inhibitors Gö 6976 (1 microM) and bisindolylmaleimide (1 microM) also inhibited phenylephrine-induced contractions. 3 Phenylephrine caused a concentration dependent increase in inositol phosphate production (EC(50) 119+/-67 nM). This response was blocked by terazosin (1 microM). 4 Phenylephrine caused the translocation of the PKC alpha isoform, but not the beta, delta, gamma, epsilon or lambda isoforms, from the cytosolic to the particulate fraction of HCPSC, with an EC(50) of 5.7+/-0.5 microM. 5 In FURA-2AM (5 microM) loaded cells, phenylephrine elicited concentration dependent increases in [Ca(2+)](i), with an EC(50) of 3.9+/-0.4 microM. The response to phenylephrine (10 microM) was blocked by prazosin (1 microM), bisindolymaleimide (1 microM), and nifedipine (10 microM). 6 In conclusion, this study has shown that HCPSC express functional alpha(1)-adrenoceptors, and that the intracellular pathways responsible for contractility may be largely dependent upon protein kinase C activation and subsequent opening of L-type calcium channels.

Tocolytic effects of a long-acting beta2-adrenoceptor agonist, formoterol, in rats

We have assessed the tocolytic activity of formoterol, a novel long-acting and potent beta2-adrenoceptor agonist, through its production of cyclic adenosine monophosphate, in comparison with ritodrine, a beta2-adrenoceptor agonist used clinically to counter premature delivery. Formoterol and ritodrine inhibited the amplitude and frequency of rat uterine contraction, with IC50 values of 3.8 x 10(-10) and 4.7 x 10(-7) M, respectively. Intravenous administration of formoterol or ritodrine caused inhibition of uterine motility and increased heart rate in a dose-dependent manner. Inhibition of uterine motility by oral administration of formoterol (0.3 and 1 mg kg(-1)) continued for at least 60 min, whereas that with ritodrine (100 mg kg(-1)) persisted for 15 min with rapid recovery thereafter in pregnant rats. The beta-adrenoceptor binding of [125I]iodopindolol to the myometrium of pregnant rats was competitive with formoterol and ritodrine, with Ki values of 0.04 and 6.10 nM, respectively. Formoterol (10(-6)-10(-4) M) and ritodrine (10(-6)-10(-4) M) increased the level of cyclic adenosine monophosphate in lymphocytes in a dose-dependent manner. The results suggested that formoterol caused relaxation of uterine motility through production of cyclic adenosine monophosphate. Thus, formoterol may be useful as a treatment to counter premature delivery.

Pharmacological characterization of muscarinic receptors implicated in rabbit detrusor muscle contraction and activation of inositol phospholipid hydrolysis in rabbit detrusor and parotid gland

In the present study, we evaluated the pharmacological characteristics of the functional muscarinic receptors implicated in rabbit detrusor contraction and coupled to inositol phospholipid turnover in rabbit detrusor and parotid gland. The selectivity of several muscarinic antagonists for detrusor vs. salivary gland muscarinic receptors was also examined. The affinities for the muscarinic m1-, m2- and m3-receptor subtypes were determined using membranes from human cloned receptors expressed in CHO-K1 cells using [3H]-N-methyl scopolamine as a radioligand. Anti-muscarinic activity was determined in isolated rabbit detrusor by measuring the displacement of the contractile response to carbachol, and in rabbit detrusor and rabbit parotid by measuring the displacement of inositol phospholipid hydrolysis (total inositol phosphate accumulation) to carbachol. A significant correlation was found between the potencies to antagonize carbachol-induced rabbit detrusor contraction (pK(B)) and the affinities (pKi) for the m3-receptor subtype (r = 0.93, P = 5 x 10(-6)). Lower, but significant, correlations [0.88 (P = 6.3 x 10(-5)), 0.72 (P = 4.6 x 10(-3))] were obtained with m1- or m2-receptor subtypes, respectively. Each muscarinic antagonist tested displayed similar potency to antagonize carbachol-stimulated inositol phospholipid hydrolysis in rabbit detrusor and parotid (r = 0.96, P = 8 x 10(-3)). A significant correlation was found between the potencies to antagonize carbachol-stimulated inositol phospholipid hydrolysis (pK(B)), determined in rabbit detrusor and rabbit parotid, and the affinities (pK(i)) for the m3-receptor subtype [r = 0.96 (P = 0.01), 0.99 (P = 5 x 10(-5)), respectively] and for the m1-receptor subtype [r = 0.98 (P = 3.5 x 10(-3)), 0.94 (P = 0.02), respectively] but not for the m2-receptor subtype [r = 0.33, 0.57, ns, respectively]. In each in vitro assay, methoctramine (preferential M2 selective antagonist) and pirenzepine (preferential M1 selective antagonist) were slightly potent. We suggest that the muscarinic receptor implicated in the response to carbachol in rabbit detrusor and parotid gland corresponds to the M3-subtype. None of the muscarinic antagonists studied in rabbit tissues displayed preferential affinity for the detrusor.

Effects of histamine and endothelin-1 on membrane potentials and ion currents in bovine tracheal smooth-muscle cells

We tested the effects of tetraethylammonium (TEA), acetylcholine (ACh), histamine, and endothelin-1 on single airway smooth-muscle cells from bovine trachea, using the patch-clamp technique. Resting membrane potential was -48 +/- 1 mV (n = 47). Both TEA and ACh significantly depolarized the membrane, by +28 +/- 4 mV (P < 0.001, n = 12) and +21 +/- 2 mV (P < 0.01, n = 7), respectively, in the whole-cell configuration. In contrast, both histamine and endothelin-1 hyperpolarized the membrane, by -21 +/- 6 mV (P < 0.01, n = 8) and -15 +/- 2 mV (P < 0.01, n = 8), respectively. Calcium-dependent large-conductance K+-channels (127 pS) and small-conductance K+ channels (21 pS) were identified in excised patches. The small-conductance K+ channel was inhibited by 4-aminopyridine and activated by both histamine and endothelin-1. Furthermore, TEA did not alter the membrane hyperpolarization by these agonists, suggesting that the small-conductance K+ channel or delayed-rectifier K+ channel was involved in the membrane hyperpolarization. Membrane hyperpolarization by histamine and endothelin-1 suggests that activation of voltage-dependent calcium channels (VDCCs) or of calcium influx does not contribute substantially to the contractile response of airway smooth-muscle contraction to these agonists.

A1 and A2 adenosine receptor modulation of contractility in the cauda epididymis of the guinea-pig

1. The effects of adenosine receptor agonists upon phenylephrine-stimulated contractility and [3H]-cyclic adenosine monophosphate ([3H]-cyclic AMP) accumulation in the cauda epididymis of the guinea-pig were investigated. The alpha1-adrenoceptor agonist, phenylephrine elicited concentration dependent contractile responses from preparations of epididymis. In the absence or presence of the L-type Ca2+ channel blocker, nifedipine (10 microM) the non-selective adenosine receptor agonist, 5'-N-ethylcarboxamidoadenosine (NECA, 1 microM) shifted phenylephrine concentration-response curves to the left (4 and 5 fold respectively). Following the incubation of preparations with pertussis toxin (200 ng ml(-1) 24 h) NECA shifted phenylephrine concentration-response curves to the right (5.7+/-0.9 fold). 2. In the presence of phenylephrine (1 microM), NECA and the A1 adenosine receptor selective agonists, N6-cyclopentyladenosine (CPA) and (2S)-N6-[2-endo-norbornyl]adenosine ((S)-ENBA) elicited concentration-responses dependent contractions from preparations of epididymis (pEC50 values 8.18+/-0.19, 7.79+/-0.29 and 8.15+/-0.43 respectively). The A3 adenosine receptor agonists N6-iodobenzyl-5'-N-methylcarboxamido adenosine (IBMECA) and N6-2-(4-aminophenyl) ethyladenosine (APNEA) mimicked this effect (but only at concentrations greater than 10 microM). In the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 30 nM) CPA concentration-response curves were shifted, in parallel to the right (apparent pKB 8.75+/-0.88) and the maximal response to NECA was reduced. 3. In the presence of DPCPX (100 nM) the adenosine agonist NECA and the A2A adenosine receptor selective agonist, CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-N-ethylcarboxamido adenosine), but not CPA, inhibited phenylephrine (20 microM) stimulated contractions (pIC50 7.15+/-0.48). This effect of NECA was blocked by xanthine amine congener (XAC, 1 microM) and the A2A adenosine receptor-selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-++ +ylamino]ethyl)phenol (ZM 241385; 30 nM). 4. (S)-ENBA (in the absence and presence of ZM 241385, 100 nM), but not NECA or CPA inhibited the forskolin (30 microM)-stimulated accumulation of [3H]-cyclic AMP in preparations of the epididymis of the guinea-pig (by 17+6% of control). In the presence of DPCPX (100 nM) NECA and CGS 21680, but not (S)-ENBA, increased the accumulation of [3H]-cyclic AMP in preparations of epididymis (pEC50 values 5.35+/-0.35 and 6.42+/-0.40 respectively), the NECA-induced elevation of [3H]-cyclic AMP was antagonised by XAC (apparent pKB 6.88+/-0.88) and also by the A2A adenosine receptor antagonist, ZM 241385 (apparent pKB 8.60+/-0.76). 5. These studies are consistent with the action of stable adenosine analogues at post-junctional A1 and A2 adenosine receptors in the epididymis of the guinea-pig. A1 Adenosine receptors potentiate alpha1-adrenoceptor contractility, an effect blocked by pertussis toxin, but which may not be dependent upon an inhibition of adenylyl cyclase. The epididymis of the guinea-pig also contains A2 adenosine receptors, possibly of the A2A subtype, which both inhibit contractility and also stimulate adenylyl cyclase.

Modulation of spasmogen-stimulated Ins(1,4,5)P3 generation and functional responses by selective inhibitors of types 3 and 4 phosphodiesterase in airways smooth muscle

1. The effects of isoenzyme-selective inhibitors of phosphodiesterases PDE3 and PDE4 on cyclic AMP concentration, two indices of phosphoinositide hydrolysis, and contractile responses to spasmogens have been investigated in bovine tracheal smooth muscle (BTSM). 2. Neither the PDE3-selective inhibitor ORG 9935, nor the PDE4-selective inhibitor rolipram increased cyclic AMP levels in BTSM. However, rolipram addition in the presence of PDE3 inhibition (ORG 9935; 1 microM) concentration-dependently (-log EC50 (M), 6.55+/-0.15; n = 3) increased cyclic AMP levels to about 70% of the maximal response to the beta-adrenoceptor agonist isoprenaline. 3. Rolipram per se inhibited histamine-stimulated [3H]-inositol (poly)phosphate ([3H]-InsP(X)) accumulation by > 80% (-log EC50 (M), 6.92+/-0.11; n = 3). Although ORG 9935 (1 microM) had little effect on histamine-stimulated [3H]-InsP(X) accumulation alone it greatly facilitated the inhibitory action of rolipram (-log EC50 (M), 8.82+/-0.39; n = 3). The effects of PDE3 and/or PDE4 inhibition on [3H]-InsP(X) accumulation stimulated by muscarinic acetylcholine (mACh) receptor activation were less marked. However, combined PDE3/4 inhibition significantly decreased this response at a submaximal concentration of mACh receptor agonist (carbachol; 1 microM). 4. The greater-than-additive effect of combined PDE3/4 inhibition was also observed at the level of contractile responses to histamine and carbachol. In experiments designed to investigate the effects of PDE3 and/or 4 inhibitors on the carbachol-mediated phasic contraction, additions of rolipram (10 microM) or ORG 9935 (1 microM) were without effect, whereas added together the inhibitors caused a significant (P < 0.01) 40% reduction in the peak phasic contractile response. 5. The effect on contraction correlated with a substantial inhibitory effect of PDE3/4 inhibition on the initial increase in inositol 1,4,5-trisphosphate (InsP3) accumulation stimulated by spasmogen. Thus, in the presence of ORG 9935 (1 microM) rolipram concentration-dependently inhibited carbachol-stimulated InsP3 accumulation by > or = 50% (-log EC50 (M), 6.77+/-0.21; n = 4). 6. Carbachol (100 microM) addition caused a rapid decrease (by 67% at 10 s) in BTSM cyclic AMP level in the presence of PDE3/4 inhibition. However, omission of Ca2+ from the incubation medium prevented the carbachol-evoked decrease in cyclic AMP and this coincided with a greater inhibition (> or = 80%) of the carbachol-stimulated InsP3 response. 7. These data indicate that combined PDE3 and PDE4 inhibition has greater-than-additive effects on second messenger and functional responses to spasmogens in BTSM. Furthermore, the ability of PDE3/4 inhibition significantly to attenuate mACh receptor-mediated contractile responses, may be, at least in part, attributed to an effect exerted at the level of InsP3 generation.

The influence of age on isolated tracheal responsiveness to spasmogens

The influence of animal age was examined on the responses of guinea-pig (birth-156 weeks) and rat (4-136 weeks) isolated tracheal tissue to the spasmogens histamine, 5-hydroxytryptamine (5-HT) and potassium ions (K+) using functional and biochemical techniques. Over the ages birth-12 weeks in the guinea-pig, K+ potency decreased 1.5 fold whereas histamine potency increased 2-fold between the ages of birth-2 weeks and then declined to original levels by age 20 weeks. 5-HT potency declined over the entire age range examined, resulting in a 25.1 fold decrease between the ages of 1 and 156 weeks. In the rat, 5-HT potency remained unchanged and a small but progressive increase in K+ potency was observed with respect to animal age. Significant age-related changes in inositol phosphate accumulation were observed in both unstimulated and histamine-stimulated isolated guinea-pig tracheal smooth muscle which did not correlate with the functional changes observed in response to spasmogenic stimulation. The results describe disparate age-related changes between two species of different spasmogenic agonists with the majority of age-related changes occurring during the maturation phase of growth of the guinea-pig.

Receptor-stimulated phospholipase C activity in human umbilical artery cultured endothelial cells grown in a low oxygen environment

Endothelial cells of the human umbilical blood vessels are widely cultured in an oxygen tension (21%) far above that in which they exist in vivo (3%). This study investigates the effect of the long term culture (ca. 1 month) of human umbilical artery endothelial cells in a reduced oxygen environment (3%: HUAEC3) in comparison to cells grown in a 'normoxic' environment (21%: HUAEC21). Despite reports of altered metabolic pathways and reduced membrane integrity in other cell types, the characteristics of HUAEC3 were found to be similar to those of HUAEC21 with respect to morphology, immunocytochemical profile and in vitro growth rates. Cellular glutathione was maintained in these cells although ATP levels in HUAEC3 were found to be significantly lower than those observed in HUAEC21. The phosphoinositide responses of the HUAEC3 to a variety of agonists were also found to be of similar magnitude to those observed in HUAEC21. In addition, the pharmacological characteristics of the phospholipase C-linked histamine H1 and P2y2 (P2U) receptors were not changed by culture of cells in a low oxygen environment.

Beta-adrenoceptor-mediated inhibition of alpha 1-adrenoceptor-mediated and field stimulation-induced contractile responses in the prostate of the guinea pig

1. The prostate of the guinea pig responds to electrical field-stimulation (2 s trains, 0.1 ms pulses at 3-60 Hz, supramaximal voltage) with contractile responses. At 18 Hz these responses were inhibited (82 +/- 2%) by the L-type Ca2+ channel blocker, nifedipine (10 microM) and (by 100%) by the neurotoxin, tetrodotoxin (500 nM). The alpha 1A-selective adrenoceptor antagonist, 5-methylurapidil, inhibited responses to field stimulation in the absence and presence of nifedipine (10 microM) with -log molar (p) IC50 (+/- s.e. mean) values of 7.95 +/- 0.14 and 7.01 +/- 0.07, respectively. 2. The non-selective beta-adrenoceptor agonist, isoprenaline, reduced (56 +/- 8%) field stimulation induced contractile responses (pEC50 6.91 +/- 0.11). The non-selective beta-adrenoceptor antagonist propranolol (50 nM) and the beta 1-adrenoceptor selective antagonist, atenolol (3 microM), but not the beta 2-adrenoceptor antagonist ICI 118,551 ((+/-)-1 -[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxyl]-3-[1-methylethyl)amino ]-2-butanol HCl; 100 nM) antagonized this effect (apparent pKB values 8.44 +/- 0.22 and 6.92 +/- 0.21, respectively) indicating an effect mediated through beta 1-like adrenoceptors. In the presence of nifedipine (10 microM) isoprenaline (up to 10 microM) did not inhibit the remaining response to field-stimulation. 3. Phenylephrine elicited contractile responses (pEC50 4.47 +/- 0.30) from preparations of guinea pig prostate which were reduced (63 +/- 25%) by nifedipine (10 microM). This response was antagonized by 5-methylurapidil (100 nM, apparent pKB 8.24 +/- 0.33), but was not affected by preincubation chloroethylclonidine (50 microM, 30 min). Responses to phenylephrine (30 microM) were inhibited (by up to 52 +/- 5%) by isoprenaline (pIC50 6.40 +/- 0.35, the beta 2-adrenoceptor selective agonist, salbutamol was weakly effective). Propranolol (300 nM), ICI 118,551 (100 nM) and atenolol (3 microM) shifted isoprenaline concentration-response curves to the right (apparent pKB +/- s.e. values 7.68 +/- 1.10; 8.00 +/- 0.72 and 6.62 +/- 0.95, respectively). In the presence of nifedipine (10 microM) responses to phenylephrine (30 microM,) were inhibited (by up to 51 +/- 4%) by isoprenaline (pIC50 6.88 +/- 0.17): propranolol (300 nM) and ICI 118,551 (100 nM), but not atenolol (3 microM) antagonized this effect (apparent pKB values 8.85 +/- 1.53 and 8.35 +/- 1.18, respectively). Thus beta 1-like and beta 2-like adrenoceptors may be involved in the isoprenaline-stimulated inhibition of phenylephrine concentration-response curves. 4. Phenylephrine stimulated [3H]-inositol phosphate accumulation (pEC50 4.47 +/- 0.83), an effect insensitive to chloroethylclonidine pre-treatment (50 microM, 30 min) and to nifedipine (10 microM), but inhibited by 5-methylurapidil (apparent pKD 7.90 +/- 0.22). Isoprenaline (up to 1 microM) did not affect the phenylephrine-stimulated maximal increase in [3H]-inositol phosphates but did increase [3H]-cyclic adenosine monophosphate ([3H]-cAMP) accumulation (pEC50 6.77 +/- 0.66); propranolol (30 nM) and ICI 118,551 (110 nM), but not atenolol (up to 3 microM), antagonized this effect. These responses may therefore be mediated through beta 2-like adrenoceptors. 5. These results show that the alpha 1-adrenoceptor mediated and field stimulation-induced contractions of the guinea pig prostate are partly dependent upon intracellular and extracellular sources of Ca2+. We conclude that both beta 1- and beta 2-like adrenoceptors inhibit responses to phenylephrine in the prostate of the guinea pig. The beta 1-like adrenoceptor-mediated inhibition of these responses is evident upon the field stimulation-induced and nifedipine-sensitive component of the response to phenylephrine and may not involve the activation of adenylyl cyclase. The beta 2-like adrenoceptor may inhibit both nifedipine sensitive and insensitive components of the response to phenylephrine, possibly through the activation of adenylyl cyclase, but not through the i

Inhibition of muscarinic-stimulated polyphosphoinositide hydrolysis and Ca2+ mobilization in cat iris sphincter smooth muscle cells by cAMP-elevating agents

The effects of carbachol (CCh) on inositol 1,4,5-trisphosphate (IP3) production and intracellular calcium ([Ca2+]i) mobilization, and their regulation by cAMP-elevating agents were investigated in SV-40 transformed cat iris sphincter smooth muscle (SV-CISM-2) cells. CCh produced time- and dose-dependent increases in IP3 production; the t1/2 and EC50 values were 68 s and 0.5 microM, respectively. The muscarinic agonist provoked a transient increase in [Ca2+]i which reached maximum within 77 s, and increased [Ca2+]i mobilization in a concentration-dependent manner with an EC50 of 1.4 microM. Thapsigargin, a Ca(2+)-pump inhibitor, caused a rapid rise in [Ca2+]i and subsequent addition of CCh was without effect. Both CCh-induced IP3 production and CCh-induced [Ca2+]i mobilization were more potently antagonized by 4-DAMP, an M3 muscarinic receptor antagonist, than by pirenzepine, an M1 receptor antagonist, suggesting that both responses are mediated through the M3 receptor subtype. Treatment of the cells with U73122, a phospholipase C (PLC) inhibitor, resulted in a concentration-dependent decrease in both CCh-stimulated IP3 production and [Ca2+]i mobilization. These data indicate close correlation between enhanced IP3 production and [Ca2+]i mobilization in these smooth muscle cells and suggest that the CCh-stimulated increase in [Ca2+]i could be mediated through increased IP3 production. Isoproterenol (ISO) inhibited CCh-induced IP3 production (IC50 = 80 nM) and [Ca2+]i mobilization (IC50 = 0.17 microM) in a concentration-dependent manner. Microsomal fractions isolated from SV-CISM-2 cells contained phospholipase C (PLC) which was stimulated by CCh (10 microM) and GTP gamma S (0.1 microM). Pretreatment of the cells with ISO or forskolin, 5 microM each, produced membrane fractions in which CCh-stimulated PLC activity was significantly attenuated. Furthermore, when microsomal fractions isolated from SV-CISM-2 cells were phosphorylated with Protein kinase A (PKA), the CCh- and GTP gamma S-stimulated IP3 production were significantly inhibited. It can be concluded from these studies that in SV-CISM-2 cells, activation of M3 muscarinic receptors results in stimulation of PLC-mediated PIP2 hydrolysis, generating IP3 which mobilizes [Ca2+]i. Furthermore, elevation of cAMP may inhibit IP3 production and [Ca2+]i mobilization through mechanisms involving PKA-dependent phosphorylation of PLC, G-proteins, IP3 receptor and/or IP3 metabolizing enzymes.

Dissociation between beta-adrenoceptor-mediated cyclic AMP accumulation and inhibition of histamine-stimulated phosphoinositide metabolism in airways smooth muscle

Spasmogen-stimulated phosphoinositide hydrolysis represents one of the major signalling pathways mediating pharmacomechanical coupling in airways smooth muscle (ASM), and cyclic AMP-induced inhibition of phosphoinositidase C has been proposed as an important mechanism underlying the bronchodilator properties of beta2-adrenoceptor agonists. To examine this hypothesis in more detail we have undertaken a direct comparison of the effects of salbutamol and salmeterol, short- and long-acting beta2-adrenoceptor agonists respectively, on cyclic AMP accumulation and histamine-stimulated [3H]-inositol phospholipid hydrolysis in bovine tracheal smooth muscle (BTSM) slices. Although salmeterol displayed a similarly greater potency over salbutamol for both stimulation of cyclic AMP, and inhibition of [3H]-inositol phosphate accumulation, there was a clear disparity between these agents with respect to both their efficacies and the duration of their effects. Hence while salmeterol caused a more protracted, but initially smaller increase in cyclic AMP accumulation compared to salbutamol, the inhibition of histamine-stimulated [3H]-inositol phosphate accumulation observed with salmeterol was of identical duration to salbutamol and was more marked than that of salbutamol at early time points. These data suggest that cyclic AMP accumulation is not the sole mechanism responsible for beta2-adrenoceptor-induced inhibition of phosphoinositide turnover in BTSM, and would support a recent proposal that cyclic AMP-dependent inhibition of agonist-stimulated Ca2+ mobilization in ASM may be mediated by factors independent of inositol phosphate generation.

Alpha-adrenoceptor mediated responses of the cauda epididymis of the guinea-pig

1. The subtypes of alpha-adrenoceptor mediating the contractile responses of the cauda epididymis of the guinea-pig were investigated. The alpha 1-adrenoceptor agonist phenylephrine, but not the alpha 2-adrenoceptor agonist, xylazine (up to 10 microM), elicited concentration-dependent contractions from preparations of cauda epididymis (EC50 3.4 microM). The L-type Ca2+ channel antagonist, nifedipine (10 microM), reduced the maximal response to phenylephrine (by 77%). Preincubation of tissues with the alpha 1B-adrenoceptor-alkylating agent, chloroethylclonidine (50 microM, 30 min), shifted phenylephrine concentration-response curves to the right (4 fold) only when the alpha 2-adrenoceptor antagonist idazoxan (100 nM) was included during the pre-incubation with chloroethylclonidine. 2. Xylazine (1 microM) significantly shifted phenylephrine concentration-response curves to the left (3 fold); this effect was attenuated by idazoxan (100 nM). Both the incubation of preparations with nifedipine (10 microM) and the pre-incubation of preparations with chloroethylclonidine (50 microM, 30 min) attenuated the potentiating effects of xylazine (1 microM). Protection of alpha 2-adrenoceptors with idazoxan (100 nM) during the chloroethylclonidine (50 microM, 30 min) incubation restored the xylazine-mediated enhancement of phenylephrine concentration-response curves. Pertussis toxin (200 ng ml-1, 24 h) attenuated the xylazine (1 microM)-mediated potentiation of phenylephrine concentration-response curves. 3. Following the pre-incubation of preparations with chloroethylclonidine (50 microM, 30 min) 5-methylurapidil (10 nM to 3 microM) shifted phenylephrine concentration-response curves, in parallel, to the right with mean pKB values in the range of 8.27 (at 10 nM 5-methylurapidil) to 7.76 (at 3 microM 5-methylurapidil), the addition of idazoxan (100 nM) to the incubation medium did not significantly affect the 5-methylurapidil (10 to 300 nM) pKB values (8.41 to 7.64, respectively). In the presence of both idazoxan (100 nM) and nifedipine (10 microM), and following the pre-incubation with chloroethylclonidine (50 microM, 30 min), 5-methylurapidil (30 to 300 nM) still shifted phenylephrine concentration-response curves to the right (pKB values 7.77 to 7.36, respectively). 4. Phenylephrine (1 microM to 1 mM) increased the accumulation of [3H]-inositol phosphates (10 fold) in preparations of cauda epididymis (EC50 12 microM). This effect was sensitive to chloroethylclonidine pretreatment (50 microM, 30 min), antagonized with low affinity by 5-methylurapidil (- log pKi 7.8), but not potentiated by xylazine (1 microM). Xylazine (10 nM - 100 microM) reversed the forskolin (10 or 30 microM) stimulated accumulation of [3H]-adenosine 3':5'-cylic monophosphate (cyclic AMP) in preparations of cauda epididymis (by approximately 45%). Incubation of tissues with both pertussis toxin (200 ng ml-1, 24 h) and pertussis toxin vehicle increased the basal activity of adenylate cyclase (3 fold) but did not increase the capacity of forskolin (30 microM) to stimulate the accumulation of [3H]-cyclic AMP in these tissues. Xylazine did not significantly inhibit the forskolin-stimulated accumulation of [3H]-cyclic AMP in either vehicle or pertussis toxin treated tissues. 5. These studies indicate that the epididymis of the guinea-pig contains alpha 1- and alpha 2-adrenoceptors. On the basis of the actions of chloroethylclonidine and 5-methylurapidil the alpha 1-adrenoceptors of this tissue may be of the alpha 1A- and alpha 1B-subtypes and are linked to both the influx of extracellular Ca2+ and to phospholipase C. The alpha 2-adrenoceptors of this tissue are negatively coupled to adenylate cyclase, sensitive to pertussis toxin, but do not amplify phenylephrine-stimulated [3H]-inositol phosphate accumulation. Stimulation of the alpha 2-adrenoceptors of this tissue may selectively potentiate the influx of extracellular Ca2+.

Regulation of histamine H1 receptor coupling by dexamethasone in human cultured airway smooth muscle

1. The regulation of histamine-induced [3H]-inositol phosphate and intracellular calcium responses in human cultured airway smooth muscle cells was studied. 2. Histamine induced concentration-dependent [3H]-inositol phosphate formation (EC50 4 microM). This response was inhibited by a range of selective H1 receptor antagonists but not by the H2-selective antagonist, tiotidone or the H3 receptor-selective antagonist, thioperamide, indicating that an H1 receptor is involved in this response in human cultured airway smooth muscle cells. 3. Preincubation of human cultured airway smooth muscle cells with concentrations of dexamethasone > 10 nM for 22 h produced concentration-dependent inhibition of histamine-induced inositol phosphate formation. The maximum inhibition observed was 45% of the response in control cells. The inhibitory effect of dexamethasone was itself reversed by prior exposure to the glucocorticoid receptor antagonist, RU38486 (10 microM). Preincubation for 22 h with 1 microM dexamethasone produced inhibition of the inositol phosphate response to histamine to all concentrations of histamine inducing significant inositol phosphate formation in these cells. In contrast, the response to the G protein activator, NaF (0.1-20 mM) was unaltered by preincubation with dexamethasone. 4. Preincubation of human airway smooth muscle cells with 1 microM dexamethasone for time periods of < 6 h failed to inhibit histamine-induced inositol phosphate formation in human airway smooth muscle cells. 5. Histamine also induced concentration-dependent elevation of intracellular calcium levels in Fura 2-loaded human airway smooth muscle cells. This response was inhibited by preincubation with 1 microM dexamethasone. 6. We conclude that signal transduction through the H1 receptor in human airway smooth muscle is subject to regulation by dexamethasone and that this may in part account for the protective effect of dexamethasone against spasmogen-induced contractile responses in the airways.

Modulation of agonist-induced phosphoinositide metabolism, Ca2+ signalling and contraction of airway smooth muscle by cyclic AMP-dependent mechanisms

1. The effects of increased cellular cyclic AMP levels induced by isoprenaline, forskolin and 8-bromoadenosine 3':5'-cyclic monophosphate (8-Br-cyclic AMP) on phosphoinositide metabolism and changes in intracellular Ca2+ elicited by methacholine and histamine were examined in bovine isolated tracheal smooth muscle (BTSM) cells. 2. Isoprenaline (pD2 (-log10 EC50) = 6.32 +/- 0.24) and forskolin (pD2 = 5.6 +/- 0.05) enhanced cyclic AMP levels in a concentration-dependent fashion in these cells, while methacholine (pD2 = 5.64 +/- 0.12) and histamine (pD2 = 4.90 +/- 0.04) caused a concentration-related increase in [3H]-inositol phosphates (IP) accumulation in the presence of 10 mM LiCl. 3. Preincubation of the cells (5 min, 37 degrees C) with isoprenaline (1 microM), forskolin (10 microM) and 8-Br-cyclic AMP (1 mM) did not affect the IP accumulation induced by methacholine, but significantly reduced the maximal IP production by histamine (1 mM). However, the effect of isoprenaline was small (15.0 +/- 0.6% inhibition) and insignificant at histamine concentrations between 0.1 and 100 microM. 4. Both methacholine and histamine induced a fast (max. in 0.5-2 s) and transient increase of intracellular Ca2+ concentration ([Ca2+]i) followed by a sustained phase lasting several minutes. EGTA (5 mM) attenuated the sustained phase, indicating that this phase depends on extracellular Ca2+. 5. Preincubation of the cells (5 min, 37 degrees C) with isoprenaline (1 microM), forskolin (10 microM) and 8-Br-cyclic AMP (1 microM) significantly attenuated both the Ca(2+)-transient and the sustained phase generated at equipotent IP producing concentrations of 1 microM methacholine and 100 microM histamine (approx. 40% of maximal methacholine-induced IP response), but did not affect changes in [Ca2+]i induced by 100 microM methacholine (95.2 +/- 3.5% of maximal methacholine-induced IP response). 6. Significant correlations were found between the isoprenaline-induced inhibition of BTSM contraction and inhibition of Ca2+ mobilization or influx induced by methacholine and histamine, that were similar for each contractile agonist. 7. These data indicate that (a) cyclic AMP-dependent inhibition of Ca2+ mobilization in BTSM cells is not primarily caused by attenuation of IP production, suggesting that cyclic AMP induced protein kinase A (PKA) activation is effective at a different level in the [Ca2+]i homeostasis, (b) that attenuation of intracellular Ca2+ concentration plays a major role in beta-adrenoceptor-mediated relaxation of methacholine- and histamine-induced airway smooth muscle contraction, and (c) that the relative resistance of the muscarinic agonist-induced contraction to beta-adrenoceptor agonists, especially at (supra) maximal contractile concentrations is largely determined by its higher potency in inducing intracellular Ca2+ changes.

Inhibition of muscarinic stimulated phosphoinositide hydrolysis in the rat parotid gland by cAMP

The ability of agents that increase or mimic cAMP to affect muscarinic receptor mediated phosphoinositide hydrolysis was investigated in the rat parotid gland. Forskolin (10 microM) and isoproterenol (10 microM) elevated cAMP in the parotid gland by 2-fold and 7-fold, respectively, and these agents also inhibited oxotremorine-M (3 microM) mediated phosphoinositide hydrolysis by 14% and 26%, respectively. Forskolin (1, 4.3, 18, and 75 microM) increased cAMP accumulation and inhibited PIP2 hydrolysis in a concentration-dependent manner. Forskolin (75 micrometers) shifted the concentration-response curve for the full agonist oxotremorine-M rightward by 4.2-fold. Pre-treatment with the phosphodiesterase inhibitor isobutylmethylxanthine (1 mM) reduced the maximum effect of oxotremorine-M by 31%. The inhibitory effect of isoproterenol and forskolin on muscarinic receptor-mediated phosphoinositide hydrolysis was unaffected by the removal of extracellular Ca2+. Moreover, isoproterenol and forskolin dampened sodium fluoride and oxotremorine-M mediated phosphoinositide hydrolysis to the same extent suggesting that the inhibitory effect of cAMP is downstream from the muscarinic receptor.

Correlation of cyclic AMP accumulation and relaxant actions of salmeterol and salbutamol in bovine tracheal smooth muscle

1. The ability of salmeterol to stimulate cyclic AMP accumulation and relaxation has been compared with that of salbutamol in bovine tracheal smooth muscle. In addition, the anti-spasmogenic effects of these agents and their abilities to modulate histamine-stimulated [3H]-inositol phosphate accumulation have also been investigated. 2. In tissue strips, a close temporal correlation was found to exist between salmeterol (0.1 microM)-induced relaxation of methacholine (500 nM)-induced tone and cyclic AMP accumulation, both maximal reversal of induced tone (26.2 +/- 6.0%) and maximal levels of cyclic AMP accumulation being achieved after 30-40 min. In contrast to salmeterol, salbutamol exerted greater and more rapid effects on both parameters. Maximal reversal of methacholine-induced tone (79.3 +/- 14.0%) and maximal levels of cyclic AMP accumulation were produced within 5 min. 3. Salmeterol-induced cyclic AMP accumulation (EC50 = 5.3 [1.8 - 15.2] nM) and inhibition of histamine (0.1 mM)-stimulated [3H]-inositol phosphate accumulation (IC50 = 1.4 [0.3-6.3] nM) were both more potent than those induced by salbutamol (EC50 = 169 [99 - 290] nM; IC50 = 13.8 [7.0 - 27.4] nM). However, maximal effects exerted by each of these agents were similar in magnitude. 4. Anti-spasmogenic effects were examined by beta-adrenoceptor agonist application to tissue strips prior to construction of spasmogen concentration-effect curves. Both salmeterol and salbutamol exerted more marked inhibition of the contractile response induced by histamine than that induced by methacholine, salmeterol being the more potent agent, while salbutamol produced a greater maximal inhibitory effect. 5. The results demonstrate that salmeterol is a more potent agent than salbutamol and have highlighted a close temporal correlation between promotion of cyclic AMP accumulation and tissue relaxation stimulated by each agent when both parameters are measured under identical conditions.

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.

Distribution of receptors mediating phosphoinositide hydrolysis in cultured human umbilical artery smooth muscle and endothelial cells

Cultures of human umbilical artery smooth muscle and endothelial cells have been established and the effect of a range of calcium-mobilizing receptor agonists on inositol phospholipid hydrolysis has been compared in the two cell types. In human umbilical artery endothelial cells, histamine (EC50 20 microM), ATP (EC50 6.7 microM), sodium fluoride (20 mM) and thrombin (1 U/mL) produced marked increases in [3H]inositol phosphate accumulation. In contrast, bradykinin (1 microM), 5-hydroxytryptamine (5-HT) (0.1 mM) and carbachol (1 mM) produced only a small (< 1% of the response to 1 mM histamine) effect on [3H]inositol phosphate accumulation in these cells. In human umbilical artery smooth muscle cells, histamine (EC50 16 microM), bradykinin (EC50 4.5 nM), 5-HT (EC50 0.7 microM) and carbachol (EC50 21 microM) produced substantial effects (> 20% of the response to 1 mM histamine) on inositol phospholipid hydrolysis while ATP (1 mM) and thrombin (1 U/mL) were much less effective. The response to histamine in both smooth muscle and endothelial cells was antagonized by 50 nM mepyramine (apparent Kd = 5.6 and 2.9 nM in the two cell types, respectively). The response to 5-HT in smooth muscle cells was antagonized by 50 nM ketanserin (apparent Kd = 4.5 nM). In human umbilical artery smooth muscle cells the inositol phosphate response to carbachol was antagonized by 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; pKd = 9.3), atropine (pKd = 9.7), pirenzepine (pKd = 6.7) and methoctramine (pKd = 6.9). These data are consistent with the involvement of an M3-muscarinic receptor in this response. These studies suggest that receptors mediating inositol phospholipid hydrolysis are differentially distributed between human umbilical artery endothelial and smooth muscle cells.

Endogenous expression of histamine H1 receptors functionally coupled to phosphoinositide hydrolysis in C6 glioma cells: regulation by cyclic AMP

1. The effects of histamine receptor agonists and antagonists on phospholipid hydrolysis in rat-derived C6 glioma cells have been investigated. 2. Histamine H1 receptor-stimulation caused a concentration-dependent increase in the accumulation of total [3H]-inositol phosphates in cells prelabelled with [3H]-myo-inositol. The rank order of agonist potencies was histamine (EC50 = 24 microM) > N alpha-methylhistamine (EC50 = 31 microM) > 2-thiazolylethylamine (EC50 = 91 microM). 3. The response to 0.1 mM histamine was antagonized in a concentration-dependent manner by the H1-antagonists, mepyramine (apparent Kd = 1 nM) and (+)-chlorpheniramine (apparent Kd = 4 nM). In addition, (-)-chlorpheniramine was more than two orders of magnitude less potent than its (+)-stereoisomer. 4. Elevation of intracellular cyclic AMP accumulation with forskolin (10 microM, EC50 = 0.3 microM), isoprenaline (1 microM, EC50 = 4 nM) or rolipram (0.5 mM), significantly reduced the histamine-mediated (0.1 mM) inositol phosphate response by 37%, 43% and 26% respectively. In contrast, 1,9-dideoxyforskolin did not increase cyclic AMP accumulation and had no effect on the phosphoinositide response to histamine. 5. These data indicate the presence of functionally coupled, endogenous histamine H1 receptors in C6 glioma cells. Furthermore, the results also indicate that H1 receptor-mediated phospholipid hydrolysis is inhibited by the elevation of cyclic AMP levels in these cells.

Effect of temperature on muscarinic cholinoceptor-mediated phosphoinositide metabolism and tension generation in bovine tracheal smooth muscle

The effect of decreased temperature on phosphoinositide metabolism was studied in flurbiprofen pretreated bovine tracheal smooth muscle (BTSM) by investigating the consequences of cooling on muscarinic-cholinoceptor-mediated [3H]inositol phosphate ([3H]InsP) and inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) accumulation, basal phosphoinositidase C (PIC) activity and airways smooth muscle (ASM) tone. Cooling of [3H]Ins labelled BTSM slices from 37 degrees C to 27 degrees C for 20 min prior to the addition of agonist caused a substantial (73.0 +/- 2.5%) inhibition of carbachol (100 microM, 30 min)-stimulated [3H]InsP accumulation compared to values measured at 37 degrees C. The degree of inhibition of [3H]InsP accumulation was similar at all agonist time points (2-30 min) studied. In parallel experiments, cooling of unlabelled BTSM slices from 37 degrees C to 27 degrees C resulted in a 34% reduction in basal Ins(1,4,5)P3 mass (37 degrees C, 13.1 +/- 0.6 pmol mg-1 protein; 27 degrees C, 8.9 +/- 0.9 pmol mg-1 protein; P < 0.02) and markedly attenuated carbachol (100 microM)-stimulated increases in Ins(1,4,5)P3 accumulation. Basal PIC activity in the soluble fraction of BTSM homogenates, measured using a [3H]phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) /deoxycholate assay system, was also significantly lower at 27 degrees C compared to 37 degrees C (initial velocities of PtdIns(4,5)P2 hydrolysis of 853 +/- 167 (37 degrees C) and 418 +/- 119 (27 degrees C) pmol min-1 ml-1 (1/400 diluted) BTSM cytosol; p < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of membrane depolarization and changes in intra- and extracellular calcium concentration on phosphoinositide hydrolysis in bovine tracheal smooth muscle

Agonist-stimulated phosphoinositide metabolism plays a central role in pharmacomechanical coupling in airways smooth muscle (ASM). In many other tissues and cells, most noteably excitable cells, membrane depolarization or an increase in intracellular Ca2+ ([Ca2+]i) generated by inositol 1,4,5-trisphosphate (Ins(1,4,5)P3)-induced Ca2+ release or agonist-mediated Ca2+ influx is able to trigger or augment phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) hydrolysis and/or initiate PtdIns4P/PtdIns hydrolysis by direct stimulation of PIC. To assess the importance of these mechanisms in ASM the effects of KCl-induced membrane depolarization, extracellular Ca2+ ([Ca2+]e) chelation, and addition of ionomycin to elevate [Ca2+]i on basal and agonist-stimulated Ins(1,4,5)P3 concentration and [3H]-InsPx accumulation have been examined. Reducing [Ca2+]e from 1.8 mM to 6 or 0.8 microM caused a progressive inhibition of agonist-stimulated [3H]inositol polyphosphate accumulation over 30 min with the histamine-stimulated response being significantly more sensitive to [Ca2+]e chelation than the response to carbachol. In contrast, the initial accumulation of Ins(1,4,5)P3 was completely unaffected by such reductions in [Ca2+]e. Incubation of [3H]inositol-prelabelled BTSM slices with buffer containing 80 mM KCl failed to stimulate [3H]InsPx accumulation, causing instead a small inhibition of carbachol-stimulated [3H]InsPx accumulation with a similar effect seen with respect to Ins(1,4,5)P3 accumulation. Addition of 5 microM ionomycin to BTSM slices similarly did not stimulate Ins(1,4,5)P3 generation and only increased [3H]InsPx accumulation after prolonged stimulation in the presence of high (mM) [Ca2+]e. These data indicated that in ASM, membrane depolarization or physiological increases in [Ca2+]i did not result in either independent activation of PIC or augmentation of initial agonist-stimulated PtdIns(4,5)P2 hydrolysis. However, while the initial agonist-stimulated generation of Ins(1,4,5)P3 was not dependent on [Ca2+]e, a normal plasmalemmal Ca2+ gradient was required to sustain maximal rates of agonist-stimulated PtdIns(4,5)P2 hydrolysis.

Phosphoinositide metabolism in airway smooth muscle

Agonist-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate, which generates inositol 1,4,5-trisphosphate and sn-1,2-diacylglycerol, is thought to be one of the major mechanisms underlying pharmacomechanical coupling in airway smooth muscle. This article is a review of the currently available information on phosphoinositide and inositol 1,4,5-trisphosphate metabolism in this tissue and includes data on inositol 1,4,5-trisphosphate-induced Ca2+ release and the receptor mediating this effect. The final section outlines the potential mechanisms underlying physiological regulation of phosphoinositide metabolism by other second-messenger pathways operative in this tissue.

Expression of muscarinic M2 receptors in cultured human airway smooth muscle cells

[3H]-inositol phosphate formation and the inhibition of isoproterenol-induced [3H]-cyclic adenosine monophosphate (AMP) formation in response to the muscarinic agonist carbachol were studied in cultured human airway smooth muscle cells. Stimulation with carbachol produced concentration-dependent inhibition of isoproterenol (1 microM)-induced cyclic AMP formation (EC50, 0.15 microM; maximal inhibition, 60%). This response was itself reversed by pertussis toxin (100 ng/ml) and was competitively inhibited by the muscarinic antagonists pirenzepine (pA2, 6.5), methoctramine (pA2, 8.0), 4-diphenylacetoxy-N-methyl-piperidine (pA2, 8.0), and parafluorohexahydrosiladifenidol (pA2, 6.5), indicating that the M2 receptor subtype was mediating this response. In addition, carbachol also induced [3H]-inositol phosphate formation in these cells (EC50, 11 microM; 2.1-fold stimulation over basal), although the response observed was markedly down-regulated compared with the response seen in noncultured airway smooth muscle preparations. Growth arrest of cells failed to increase the magnitude of the inositol phosphate response to carbachol. These results demonstrate that cultured human airway smooth muscle cells express functionally coupled M2 receptors and probably also low levels of coupled M3 receptors.

Salmeterol, a long-acting beta 2-adrenoceptor agonist mediating cyclic AMP accumulation in a neuronal cell line

1. The accumulation of cyclic AMP stimulated by salmeterol, a long-acting beta 2-adrenoceptor agonist and by isoprenaline, a non-selective beta-adrenoceptor agonist have been compared in the B50 neuroblastoma cell line. 2. Salmeterol produced a concentration-dependent increase in the accumulation of total [3H]-cyclic AMP in B50 cells yielding an EC50 value of 37 nM which was lower than that obtained with isoprenaline (294 nM). The maximum response to salmeterol was only 46% of that obtained with isoprenaline. 3. The beta 2-adrenoceptor antagonist, ICI 118551, inhibited the responses to both salmeterol (apparent KD 2.2 nM) and isoprenaline (apparent KD 1.6 nM). However, the beta 1-adrenoceptor antagonist, atenolol, produced no significant effect at concentrations up to 100 microM. 4. Salmeterol (1 microM) changed the concentration-response curve of isoprenaline in the manner of a partial agonist interacting with a full agonist. The KD of salmeterol obtained from the interaction was 55.6 nM. 5. Whereas salmeterol has a slow onset of action in airway smooth muscle compared to other beta 2-adrenoceptor agonists, in B50 monolayers both salmeterol and isoprenaline produced a rapid increase in cyclic AMP accumulation (t1/2 1.1 min and 0.4 min respectively). 6. Despite the existence of cyclic AMP efflux mechanisms that exist in this cell line it was possible to investigate the duration of agonist action by measuring intracellular levels of the second messenger. Replacement of drug-containing medium with fresh buffer led to a rapid reduction in intracellular levels of cyclic AMP in isoprenaline-stimulated cells whereas cyclic AMP accumulation was sustained for much longer periods in salmeterol-stimulated cells. However, the persistent action of salmeterol could be reversed by the addition of a beta2-selective antagonist.7. These results confirm that salmeterol has a high affinity, but low efficacy (relative to isoprenaline) for beta2-adrenoceptors coupled to cyclic AMP accumulation and that the drug persists at its site of action for long periods in the B50 neuronal cell line.

Control of histamine induced inositol phospholipid hydrolysis in cultured human tracheal smooth muscle cells

[3H]Inositol phosphate responses to histamine and a range of other agonists were studied in cultured human tracheal smooth muscle cells. Histamine (EC50 6.5 microM), bradykinin (EC50 9.7 nM), carbachol (EC50 10 microM), substance P and NaF all produced concentration dependent [3H]inositol phosphate formation in these cells. The response to histamine was inhibited by mepyramine (KA 4.3 x 10(9) M-1), indicating the involvement of the histamine H1 receptor in this response. The inositol phosphate response to histamine was apparently desensitized following prolonged agonist exposure. The response to histamine was inhibited by phorbol dibutyrate (IC50 6 nM), and this inhibitory effect was reversed by staurosporine (150 nM). Isoprenaline (1 microM), rolipram (0.1-100 microM) and 3-isobutyl-1-methylxanthine (0.1 mM) all produced small inhibitory effects upon histamine induced inositol phosphate formation. These results demonstrate that cultured human tracheal smooth muscle cells express histamine H1 receptors coupled to phosphoinositidase C and suggest that the inositol phosphate response induced by stimulation of this receptor subtype is inhibited by activation of protein kinase C and, to a lesser extent, by elevation of cell cyclic AMP content.

The effects of elevated cyclic AMP levels on histamine-H1-receptor-stimulated inositol phospholipid hydrolysis and calcium mobilization in the smooth-muscle cell line DDT1MF-2

The effects of raising cyclic AMP levels, by forskolin stimulation, beta-adrenoceptor activation or cyclic AMP phosphodiesterase inhibition, on inositol phospholipid hydrolysis and increases in intracellular free [Ca2+] ([Ca2+]i) elicited by a range of agonists have been investigated in the hamster vas deferens smooth-muscle cell line DDT1MF-2. Isoprenaline (log [EC50 (M)] = -7.7 +/- 0.2), forskolin and the type IV cyclic AMP phosphodiesterase inhibitor rolipram elicited significant increases in the accumulation of cyclic [3H]AMP. Pretreatment with forskolin (10 microM) attenuated histamine (100 microM)- and N6-cyclopentyladenosine (CPA; 300 nM)-induced release of intracellular Ca2+, observed when cells are stimulated in Ca(2+)-free buffer containing 0.1 mM EGTA. Forskolin had no effect on ATP (100 microM)- or bradykinin (1 microM)-stimulated release of intracellular Ca2+. Histamine-induced intracellular Ca2+ release was also inhibited by pretreatment with rolipram (100 microM) or the membrane-permeant cyclic AMP analogue (Sp)-adenosine 3',5'-monophosphothioate (100 microM). Isoprenaline (1 microM) pretreatment (in the presence of 10 microM rolipram, a concentration which on its own did not decrease the histamine response) attenuated histamine-induced intracellular Ca2+ release. Forskolin inhibited histamine (100 microM)- and CPA (100 nM) stimulated accumulation of [3H]-inositol phosphates, but was without effect on ATP or bradykinin responses. Addition of forskolin (in the presence of 100 microM rolipram) after the cells had been stimulated with histamine (in experiments initiated in Ca(2+)-free buffer) inhibited the rise in [Ca2+]i observed when extracellular Ca2+ (2 mM) was re-applied (owing to receptor-mediated Ca2+ influx). Finally, the refilling of intracellular Ca2+ stores (after receptor-mediated Ca2+ influx is blocked by mepyramine) can be demonstrated in the presence of raised cyclic AMP levels.

Muscarinic blockade of beta-adrenoceptor-stimulated adenylyl cyclase: the role of stimulatory and inhibitory guanine-nucleotide binding regulatory proteins (Gs and Gi)

1. The functional antagonism that exists between muscarinic and beta-adrenoceptor function in guinea-pig tracheal smooth muscle was investigated by assessing Gs and Gi regulated adenylyl cyclase activity in isolated membranes. 2. Membranes from guinea-pig tracheal smooth muscle contain both Gi alpha and Gs alpha as assessed by Western blots with anti-G-protein antibodies. 3. GppNHp, a non-hydrolysable analogue of guanosine 5'-triphosphate (GTP), was shown to stimulate adenylyl cyclase activity at high concentrations (10(-6)-10(-4) M). GppNHp also produced a concentration-dependent reduction in pertussis toxin-catalysed adenosine diphosphate (ADP)-ribosylation of Gi alpha. 4. Pretreatment of tracheal smooth muscle slices with methacholine (10(-6) M) provoked a blockade of isoprenaline plus GTP, GppNHp- and GTP-stimulated adenylyl cyclase. 5. Addition of methacholine to membranes did not trigger inhibition of GTP-stimulated adenylyl cyclase activity but did block the isoprenaline-mediated augmentation of GTP-stimulated adenylyl cyclase activity. 6. Pretreatment of tracheal smooth muscle with methacholine (10(-6) M) provoked a blockade of cholera toxin-catalysed NAD(+)-dependent ADP-ribosylation of Gs alpha. 7. Phorbol-12-myristate 13-acetate (PMA)-treatment of tracheal smooth muscle slices actually enhanced GppNHp-stimulated adenylyl cyclase activity in subsequently prepared membranes. 8. We suggest that methacholine in addition to inhibiting adenylyl cyclase via a Gi-dependent mechanism induces a functional inactivation of Gs activity. These results together may explain the functional antagonism that exists between increased muscarinic tone and the ability of beta-adrenoceptor agonists to provoke excitation-contraction uncoupling.

Beta-adrenoceptors on smooth muscle, nerves and inflammatory cells

Although the bronchodilator action of beta 2-adrenoceptor agonists in asthma is largely due to relaxation of airway smooth muscle, these agents have other effects which may contribute to their anti-asthma action. Human airway smooth muscle contains only beta 2-receptors which, when stimulated, stimulate a rise in intracellular cAMP and activation of PKA (protein kinase A), which in turn phosphorylates several cellular proteins, resulting in relaxation. However, beta-agonists also influence membrane K+ channels and induce smooth muscle relaxation without a rise in cAMP, and this mechanism appears to be the major feature of bronchodilatation in asthma. There is also evidence that beta-agonists may modulate neurotransmission in airways via prejunctional receptors on airway nerves, both sensory and motor. Blockade of prejunctional beta 2-receptors in asthma patients may lead to marked rise in acetylcholine release, with severe bronchoconstriction. Although beta-agonists have little or no effect on the chronic inflammatory response which underlies chronic airway hyper-responsiveness, they do inhibit the release of histamine from mast cells in vitro. The presence of beta-receptors has also been detected not only on mast cells but also on eosinophils, macrophages, lymphocytes and neutrophils, but beta-agonists have little or no inhibitory action on the activities of all these cells due to rapid tachyphylaxis.

Increased contraction and inositol phosphate formation of tracheal smooth muscle from hyperresponsive guinea pigs

Tracheal smooth muscle from guinea pigs with documented airway hyperresponsiveness in vivo after multiple antigen challenges produced 30% to 50% greater force than tracheas from control guinea pigs, when stimulated with carbachol, histamine, or leukotriene D4. When cultured smooth muscle cells were incubated with myo[2-3H]inositol, basal uptake of [3H]inositol was similar in cells from normal and hyperresponsive guinea pigs, but when these cells were stimulated with contractile agonists, there was increased uptake of inositol in hyperresponsive cells. Analysis of inositol phosphates by column chromatography and high-performance liquid chromatography revealed the presence of inositol-1,4,5-triphosphate, inositol-1,3,4-trisphosphate, inositol-1,4-bisphosphate, and inositol-1-monophosphate. The release of inositol-1,4,5-triphosphate, inositol-1,4-biphosphate, and inositol-1-monophosphate by smooth muscle cells stimulated with carbachol, leukotriene D4, or histamine was 20% to 40% greater in cells derived from hyperresponsive animals than cells from normal animals. These data demonstrate that the increased muscle contraction of hyperresponsive guinea pig tracheas is associated with increased inositol phosphate metabolism in these cells. Delineating the mechanisms of airway smooth muscle contraction should provide new pharmacologic targets for the inhibition of bronchoconstriction in asthma.

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.

Bradykinin B2 receptor-mediated phosphoinositide hydrolysis in bovine cultured tracheal smooth muscle cells

1. Bovine tracheal smooth muscle cells were established in culture to study agonist-induced phosphoinositide (PI) hydrolysis in this tissue. 2. Bradykinin (0.1 nM-10 microM) evoked a concentration-dependent increase (log EC50 (M) = -9.4 +/- 0.2; n = 8) in the accumulation of total [3H]-inositol phosphates in cultured tracheal smooth muscle cells whereas the selective B1 receptor agonist des-Arg9-bradykinin (10 microM) was significantly less effective (16% of bradykinin maximal response; relative potency = 0.2 with respect to bradykinin = 100). 3. The bradykinin-induced increase in PI hydrolysis was unaffected by the B1 receptor antagonist des-Arg9[Leu8]-bradykinin (1 nM-1 microM) but showed marked attenuation in the presence of the B2 receptor antagonists D-Arg,[Hyp3,D-Phe7]-bradykinin (10 nM-10 microM) or D-Arg[Hyp3,Thi5,8,D-Phe7]-bradykinin (10 nM-10 microM). The estimated KB values obtained for these two compounds, assuming competitive antagonism, were 40 +/- 14 nM and 8.6 +/- 2.8 nM for D-Arg,[Hyp3,D-Phe7]-bradykinin and D-Arg[Hyp3,Thi5,8,D-Phe7]-bradykinin respectively. 4. We conclude that bradykinin B2 receptors are expressed in cultured bovine tracheal smooth muscle cells and are coupled to PI hydrolysis mechanisms.

Control of cyclic AMP levels in primary cultures of human tracheal smooth muscle cells

1. [3H]-adenosine 3':5'-cyclic monophosphate ([3H]-cyclic AMP) responses were studied in primary cultures of human tracheal smooth muscle cells derived from explants of human trachealis muscle and in short term cultures of acutely dissociated trachealis cells. 2. Isoprenaline induced concentration-dependent [3H]-cyclic AMP formation with an EC50 of 0.2 microM. The response to 10 microM isoprenaline reached a maximum after 5-10 min stimulation and remained stable for periods of up to 1 h. After 10 min stimulation, 1 microM isoprenaline produced a 9.5 fold increase over basal [3H]-cyclic AMP levels. The response to isoprenaline was inhibited by ICI 118551 (10 nM), (apparent KA 1.9 x 10(9) M-1) indicating the probable involvement of a beta 2-adrenoceptor in this response in human cultured tracheal smooth muscle cells. However, with 50 nM ICI 118551 there was a reduction in the maximum response to isoprenaline. Prostaglandin E2 also produced concentration-dependent [3H]-cyclic AMP formation (EC50 0.7 microM, response to 1 microM PGE2 6.4 fold over basal). 3. Forskolin (1 nM - 100 microM) induced concentration-dependent [3H]-cyclic AMP formation in these cells. A 1.6 fold (over basal) response was also observed following stimulation with NaF (10 mM). 4. The nonselective phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) (0.1 mM) and the type IV, cyclic AMP selective, phosphodiesterase inhibitor rolipram (0.1 mM) both elevated basal [3H]-cyclic AMP levels by 1.8 and 1.5 fold respectively. IBMX (1-100 microM) and low concentrations of rolipram (< 10 microM), also potentiated the response to 1 microM isoprenaline. Inhibitors of the type III phosphodiesterase isoenzyme (SK&F 94120 and SK&F 94836) were without effect upon basal or isoprenaline-stimulated cyclic AMP responses in these cells.5. Carbachol (1 nM-I 00 microM) produced concentration-dependent inhibition of the [3H]-cyclic AMP response to 1 microM isoprenaline in human cultured tracheal smooth muscle cells (IC50 0.24 JM). Carbachol(1 JM) inhibited the [3H]-cyclic AMP response to 1 JM isoprenaline by 60%. This effect of carbachol was itself inhibited by atropine (50 nM) (KA 2.3 x 109 M-') indicating the involvement of a muscarinic receptor.6. These results show that primary cultures of human tracheal smooth muscle cells demonstrate cyclic AMP responses to direct receptor stimulation, adenylyl cyclase activation and inhibition with nonselective and type IV-selective cyclic AMP phosphodiesterase isoenzyme inhibitors, and that the cyclic AMP response to isoprenaline can be inhibited by muscarinic receptor stimulation.

The effects of noradrenaline and adenosine 5'-triphosphate on polyphosphoinositide and phosphatidylcholine hydrolysis in arterial smooth muscle

1. The effects of noradrenaline and alpha,beta,methylene adenosine 5'-triphosphate (alpha,beta,methylene ATP) on polyphosphoinositide metabolism, phosphatidylcholine hydrolysis and contraction in rabbit saphenous arteries were investigated. The effect of noradrenaline upon polyphosphoinositide metabolism was also investigated in the rat tail artery. 2. Noradrenaline (10(-7)-10(-4) M) evoked a concentration-dependent increase in total inositol phosphate accumulation in the rat tail but not in the rabbit saphenous artery. Propranolol (3 x 10(-6) M) did not alter this result in the rabbit saphenous artery. In addition, alpha,beta,methylene ATP (10(-6) M) significantly increased total inositol phosphate accumulation in the rabbit saphenous artery, while potassium chloride (8 x 10(-2) M) was ineffective. 3. Phorbol 1,2-myristate 1,3-acetate (3 x 10(-8) M) enhanced noradrenaline (10(-2)-10(-4) M)-evoked contractions in rabbit saphenous artery. The contractile responses to potassium chloride (1- 16 x 10(-2) M) in tissues treated with 6-hydroxydopamine (5 x 10(-4) M), in vitro, were unaffected by these concentrations of the phorbol ester. 4. Noradrenaline (10(-6)-10(-4) M) evoked a concentration-dependent increase in the levels of choline and choline phosphate, but not in those of glycerophosphocholine, in the rabbit saphenous artery. Choline levels increased significantly over the first 15-30 s then declined to control levels within 2 min of addition of noradrenaline (10(-5) M). A smaller initial rise in choline phosphate levels (15-30 s) was followed by a larger secondary rise at 2-4 min.5. alpha, beta, methylene ATP (10-1_ 0-4 M) also evoked a concentration-dependent increase in the levels of both choline and choline phosphate, but not those of glycerophosphocholine, in the rabbit saphenous artery. alpha, beta, methylene ATP (10-4 M) significantly increased levels of both of these products within the first 15-30 s of addition of the drug; these levels reached a stable plateau 1 min after addition.6. The maximum accumulation of choline or choline phosphate evoked by either noradrenaline or alpha, beta, methylene ATP, acting alone or in combination, was not significantly different. No evidence of synergism between noradrenaline and alpha, beta, methylene ATP was observed.7. This study demonstrates that each of the co-transmitters in the rabbit saphenous artery, noradrenaline and adenosine 5'-triphosphate (ATP), promote phosphatidylcholine hydrolysis. Noradrenaline seems to rely on phosphatidylcholine hydrolysis to mediate its contractile effects, whilst ATP promotes both polyphosphoinositide and phosphatidylcholine metabolism suggesting that multiple signal-transduction mechanisms are involved in stimulus-contraction coupling in this artery.

Human bronchial cyclic nucleotide phosphodiesterase isoenzymes: biochemical and pharmacological analysis using selective inhibitors

1 The aims of the present study were to characterize the cyclic nucleotide phosphodiesterase (PDE) isoenzyme activities present in human bronchi and to examine the ability of selective isoenzyme inhibitors to relax histamine and methacholine precontracted preparations of human bronchi. 2 Three separations of pooled human bronchial tissue samples were performed. Ion-exchange chromatography showed that the soluble fraction of human bronchial preparations contains PDE I, II, III, IV and V isoenzyme activities. Multiple forms of PDE I and PDE IV were observed and PDE IV was the main cyclic AMP hydrolytic activity. 3 3-Isobutyl-l-methylxanthine (IBMX) non-selectively inhibited all separated isoenzyme activities. Zaprinast selectively inhibited PDE V, but also effectively inhibited one of the two PDE I isoforms identified. The PDE IV selective inhibitors rolipram and RO-201724, inhibited the PDE IV activities as did the dual PDE III/IV inhibitor, Org 30029. Org 9935, a PDE III selective inhibitor, potently attenuated part of the PDE IV activity peak in one of three separations performed, indicating that some PDE III activity may co-elute with PDE IV under the experimental conditions employed. 4 PDE IV-selective (rolipram), PDE III-selective (Org 9935) and dual PDE III/IV (Org 30029) inhibitors were effective relaxants of human bronchial smooth muscle. The PDE V/PDE I inhibitor, zaprinast was relatively ineffective. 5 The present study demonstrates in human bronchi, as in animal airways smooth muscle, that inhibitors of PDE III, PDEIV and dual PDE III/IV have potentially useful bronchodilator activity and are worthy of further consideration as anti-asthma drugs.

Formoterol, fenoterol, and salbutamol as partial agonists for relaxation of maximally contracted guinea pig tracheae: comparison of relaxation with receptor binding

In severe asthma attacks beta 2-sympathomimetics lose part of their therapeutic efficiency. To elucidate this loss of efficiency in an experimental model we compared the relaxant potency of salbutamol (SAL), fenoterol (FEN), formoterol (FOR), and (-)-isoprenaline (ISO) in guinea pig tracheae partially and maximally precontracted by 0.1 and 60 mumol/L carbachol, respectively. In partially precontracted tracheae the beta 2-sympathomimetics exerted maximum relaxation in comparison with ISO and low EC50S (nmol/L) for relaxation (SAL, 20; FEN, 5.6; FOR, 0.28; and ISO, 2.5). In maximally precontracted tracheae, however, the beta 2-sympathomimetics were only partial agonists for relaxation with reduced intrinsic activities (%) in comparison to ISO (SAL, 59%; FEN, 61%; FOR, 76%) and significantly increased EC50S (nmol/L) for relaxation (SAL, 130; FEN, 57; FOR, 3.0; ISO, 37). To investigate if the high relaxant potency of FOR is correlated with a higher binding affinity and/or a higher intrinsic activity for adenylate cyclase stimulation than for FEN and SAL, we performed experiments in receptor membranes from guinea pig lung. Binding competition of SAL, FEN, and FOR with [3H]ICI 118,551 for lung beta 2-adrenoceptors yielded dissociation constants (KD) of 320 (SAL), 120 (FEN), and 7.6 (FOR) nmol/L, which exhibited the same ranking as EC50S for relaxation. Concentrations of SAL, FEN, and FOR equivalent to 100 KD of the respective dissociation constants stimulated beta 2-adrenoceptor-coupled adenylate cyclase with different intrinsic activities (%) incomparison to ISO (SAL, 61%; FEN, 63%; FOR, 89%) matching intrinsic activities for relaxation. From these experiments it may be concluded that FOR might improve drug therapy of severe asthma not only due to its long mode of action discovered in clinical studies but also due to its high intrinsic activity and receptor affinity.

Comparative effects of BRL 38227, nitrendipine and isoprenaline on carbachol- and histamine-stimulated phosphoinositide metabolism in airway smooth muscle

1. The ability of BRL 38227 and nitrendipine to affect muscarinic agonist and histamine-stimulated [3H]-inositol phosphate accumulation in slices of bovine tracheal smooth muscle has been studied and compared with the established inhibitory effects of isoprenaline on this pathway. 2. Pre-addition of BRL 38227 (5 microM), nitrendipine (1 microM) or isoprenaline (10 microM) significantly inhibited the subsequent inositol phosphate response to histamine at all concentrations studied (10- 1000 microM). BRL 38227 and nitrendipine also significantly inhibited the [3H]-inositol phosphate response to low (1 microM), but not high (100 microM) concentrations of carbachol. Isoprenaline had no effect at any concentration of carbachol studied. 3. Nitrendipine (IC50 = 95 nM) and BRL 38227 (IC50 = 322 nM) caused concentration-related inhibitions of the inositol phosphate response to histamine (100 microM). Similar maximal inhibitions were caused by each agent (55-58%). Inhibitory effect of BRL 38227 was reduced in potency (IC50 = 5.5 microM), but not magnitude, in the presence of glibenclamide (0.5 microM). 4. Time-course studies comparing the effects of BRL 38227 addition 15 min before, and 10 min after histamine challenge showed that for pre-addition a distinct (less than 2 min) lag occurred following histamine addition before the inhibitory effect of BRL 38227 was manifest. In contrast, when BRL 38227 was added 10 min after histamine, an inhibitory effect was immediately apparent. 5. Further evidence for an initial, 'protected' phase of inositol phosphate accumulation was provided by the finding that BRL 38227 pre-addition had no effect on the early (0-300 s) time-course of inositol 1,4,5-trisphosphate mass accumulation. 6. The inhibitory effect of BRL 38227, but not that of nitrendipine or isoprenaline, on histaminestimulated [3H]-inositol phosphate accumulation was completely prevented in the presence of an elevated extracellular K+ (65 mM) concentration. 7. The results demonstrate that membrane hyperpolarization, and/or blockade of voltage-operated Ca2"-channels can regulate agonist-stimulated phosphoinositide metabolism in airway smooth muscle. The possible contribution of this regulatory mechanism to the relaxant properties of these agents is discussed.