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Inhibition of neurogenic plasma exudation and bronchoconstriction by a K+ channel activator, BRL 38227, in guinea pig airways in vivo
European Journal of Pharmacology, 239 (1993) 257-259
257
© 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00
EJP 21287
Short communication
Inhibition of neurogenic plasma exudation and bronchoconstriction by a K ÷ channel activator, BRL 38227, in guinea pig airways in vivo Y u - H o n g Lei, P e t e r J. Barnes and D u n c a n F. R o g e r s Department of Thoracic Medicine, National Heart & Lung Institute, Dovehouse Street, London SW3 6LY, UK Received 7 June 1993, accepted 8 June 1993
Intravenous administration of a K + channel activator, BRL 38227, inhibited cigarette smoke-induced plasma exudation in guinea pig airways in vivo in a dose-dependent manner with an approximate ED50 of 6/zg/kg. BRL 38227 also inhibited vagally induced plasma exudation and bronchoconstriction but did not inhibit substance P-induced plasma exudation or neurokinin A-induced bronchoconstriction. K + channels modulate neurotransmission in the airways and K + channel activators may have therapeutic potential in bronchial diseases including asthma and chronic bronchitis. Plasma exudation; Cigarette smoke; Vagus nerve stimulation; K + channel opener; Glibenclamide
1. Introduction
Cromakalim (BRL 34915), a selective and potent K ÷ channel activator, inhibits neurogenic airway smooth muscle contraction in vitro and in vivo (Black and Barnes, 1990). BRL 38227 (levcromakalim), the active enantiomer of cromakalim, also inhibits neurogenic airway smooth muscle contraction in vitro (Good et al., 1992) and goblet cell secretion in vivo (Kuo et al., 1992). The effects of cromakalim and BRL 38227 are inhibited by glibenclamide, a blocker of adenosine triphosphate (ATP)-sensitive K + channels (K~Tp). Activation of a population of sensory nerves sensitive to capsaicin induces release of neuropeptides including substance P (SP), neurokinin A (NKA) and calcitonin gene-related peptide. Activation of these nerves by cigarette smoke or by antidromic electrical stimulation of vagal sensory nerves induces plasma exudation and bronchoconstriction. We investigated the effect of BRL 38227 on neurogenic plasma exudation induced either by cigarette smoke or by electrical stimulation of the vagus nerves, as well as on vagaUy induced bronchoconstriction, in guinea pig airways in vivo. We used glibenclamide to confirm that K~,yp channels mediated the effect of BRL 38227.
Correspondence to: Duncan F. Rogers, Department of Thoracic Medicine, National Heart & Lung Institute, Dovehouse Street, London SW3 6LY, UK.
2. Materials and methods
Male Dunkin-Hartley guinea pigs (300-400 g) were anaesthetized with 2 g/ kg urethane and were mechanically ventilated via a tracheal cannula with 10 m l/k g room air at 60 breaths/min and changes in pulmonary insufflation pressure (PIP, a measure of airway tone) were measured via a side arm in the expiratory limb of the tubing connected to a pressure transducer (Sensortechnics, Rugby, UK). Blood pressure and heart rate were monitored via a catheter containing 2 units/ml heparin in 0.9% saline inserted into the left carotid artery and connected to a pressure transducer (PDCR75 S / N 1572, Druck, Groby, UK). Drugs were administered via the jugular veins. For vagal stimulation both cervical vagus nerves were sectioned below the nodose ganglia and placed across bipolar platinum electrodes (Subminiature electrode, Harvard Apparatus, Edenbridge, UK) 5 min before electrical stimulation (10 Hz, 5 V, 5 ms for 3 min) using a double-channel square wave stimulator (Model $88, Grass Instruments, Quincy, USA). Control animals were prepared as above but the vagi were not stimulated (sham stimulation). Cigarette smoke was administered by collecting smoke from a commercially available cigarette (UK Government Category 'Middle Tar' containing 1.2 mg nicotine and 11 mg carbon monoxide per cigarette) into a polyethylene syringe. Fifty tidal volumes of smoke from the syringe were blown into the airways via the tracheal cannula. Each volume of smoke was separated
258
from the next by 5 - 6 ventilated volumes of room air. Air drawn through an unlit cigarette was administered as a control procedure. Evans blue dye was used as a plasma marker (Rogers et al., 1989) and was injected into a jugular vein 1 min before administration of smoke or air, nerve or sham stimulation, or administration of SP or NKA. Fifteen minutes after exposure to smoke or air, or 5 min after nerve stimulation, or 10 min after administration of SP or NKA, the systemic circulation was perfused with saline at 100 mm Hg to remove intravascular dye. The main bronchi were removed, blotted dry with tissue and weighed. Evans blue was extracted from the bronchial tissue in 2 ml formamide (16 h at 40°C) and its concentration measured at 620 nm wavelength, interpolated on a standard curve and expressed as ng d y e / m g wet weight tissue. All animals for vagal stimulation or sham stimulation were pretreated with atropine and propranolol (1 m g / k g each, i.v. 30 min before stimulation). B R L 38227 (0.1-100 p,g/kg, i.v.: stock solution of 10 m g / m l in ethanol) or vehicle were injected 5 min before cigarette smoke or electrical stimulation of the vagus nerves. In certain animals morphine (1 m g / k g i.v.) was injected 10 min before cigarette smoke. Glibenclamide (Sigma; 3 0 / ~ m o l / k g in 2 ml 5% glucose: stock solution of 3 m m o l / m l in dimethyl sulphoxide) was infused i.v. 4 min before B R L 38227 ( 1 0 0 / z g / k g i.v.) or morphine (1 m g / k g i.v.). In separate animals, B R L 38227 (100 jzg/kg i.v.) or vehicle were injected 5 min before SP (Sigma; 1 n m o l / k g i.v.) or NKA (Sigma; 10 n m o l / k g i.v.). The Mann-Whitney U-test (two-tailed) was used to determine the significance of differences between groups. Data in Results are means + 1 S.E.M. The null hypothesis was rejected at P < 0.05.
200
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120 G)
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1 \\
m
\\
(u
//
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\\ \\
o AIR
CS
10 100 0.1 [BRL 38227] (~g kg -~)
100 [BRL]
+ gliben
Fig. 1. Inhibition by B R L 38227 (i.v.) of cigarette smoke (CS)-induced plasma exudation in guinea-pig main bronchi in vivo and reversal by glibenclamide (30 / x m o l / k g i.v.). Data are m e a n tissue content of the plasma marker Evans blue dye, ng dye per mg tissue; S.E. m e a n shown by vertical bars. n = 5 - 6 per group. * P < 0.05 compared with air.
PIP above baseline by 336%. B R L 38227 did not significantly inhibit SP-induced plasma exudation (increased by 8% above SP response, n = 5) or NKA-induced bronchoconstriction (reduced by 14%, n = 5). B R L 38227 (100 ~ g / k g ) alone had no significant effect on the Evans blue dye content in the main bronchi of air-exposed controls (reduced by 7%, n = 5). Glibenclamide alone had no significant effect on the Evans blue dye content in main bronchi of air-exposed controis (mean increase of 37%, n = 4). Morphine (1 m g / k g ) significantly (P < 0.05) inhibited cigarette
A
3. Results
Inhalation of cigarette smoke increased the Evans blue dye content of the main bronchi by 222% above air-exposed controls (fig. 1). B R L 38227 dose-dependently inhibited cigarette smoke-induced plasma exudation with an inhibition of 71% at a dose of 100 /zg/kg and an approximate EDs0 of 6 /xg/kg (fig. 1). Electrical stimulation of the vagus nerves increased the Evans blue content of the main bronchi by 184% above sham controls (fig. 2) and increased PIP by 230% above baseline (fig. 2). BRL 38227 (100/xg/kg) significantly inhibited vagally induced plasma exudation by 81% and bronchoconstriction by 80% (fig. 2). Glibenclamide completely reversed the inhibition by B R L 38227 of cigarette smoke-induced plasma exudation (by 127%, fig. 1) and vagally induced plasma exudation (by 80%) and bronchoconstriction (by 122%) (fig. 2). SP (1 n m o l / k g ) increased plasma exudation by 260% above saline controls and NKA (10 n m o l / k g ) increased
160
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_==
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~x
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BRL
Sham Stim
[
BRL
BRL
+gliben Stim - j
I
BRL +gliben
StJm- - . I
Fig. 2. Inhibition by B R L 38227 (100 /~g/kg i.v.) of vagus nerve stimulation (Stim) induced plasma exudation in main bronchi (A) and bronchoconstriction (B) in guinea-pigs in vivo and reversal by glibenclamide ( 3 0 / ~ m o l / k g i.v.). Data are m e a n tissue content of the plasma marker Evans blue dye, ng dye per mg tissue (A), or m e a n percent increases in pulmonary insufflation pressure (PIP), a measure of bronchial tone (B); S.E. m e a n shown by vertical bar. n = 4 - 6 per group. * P < 0.05 compared with sham stimulation (A) or vagally induced bronchoconstriction (B).
259
smoke-induced plasma exudation by 60.8% (n = 5), an effect which was not reversed by glibenclamide (n = 5). BRL 38227 alone dose-dependently decreased blood pressure with maximal decreases of 40.5 + 2.2% at 100 /zg/kg (n = 22) but did not affect heart rate. Glibenclamide alone increased blood pressure by 44.1 + 4.9% (n = 14). In the presence of glibenclamide, BRL 38227 decreased BP by 35.4 + 10.5% (n = 5). SP 1 nmol/kg and vagal stimulation decreased blood pressure by a further 41.2 + 4.8% (n = 5) and 39.4 + 4.9% (n = 5) respectively and in the presence of BRL 38227 caused additional decreases in blood pressure of 13.3% and 8.3% respectively (n = 5 each). In the presence of glibenclamide and BRL 38227, vagal stimulation decreased BP by 15.9% (n = 5).
BRL 38227 decreased blood pressure which may affect plasma exudation or change capillary transit time and alter clearance of administered drugs. However, in the present study both vagal stimulation and the tachykinins SP and NKA reduced blood pressure whereas only BRL 38227 inhibited vagally induced airway responses. Because of their dilator effects on airway smooth muscle, K + channel openers have been proposed as a new class of antiasthma drug. The importance of plasma exudation in the pathogenesis of asthma is increasingly recognized (Rogers and Evans, 1992). Thus, compounds such as BRL 38227 which reduced airway neurogenic inflammation may have therapeutic value in the treatment of asthma.
4. Discussion Acknowledgements In the present in vivo study, BRL 38227 inhibited cigarette smoke-induced or vagally induced plasma exudation, as well as vagally induced bronchoconstriction, but failed to affect SP induced-plasma exudation or NKA-induced bronchoconstriction. These data indicate that BRL 38227 has an action at a prejunctional site. Similarly, BRL 38227 inhibited both cholinergic and eNANC responses to electrical field stimulation but failed to prevent acetylcholine, SP or NKA induced tracheal smooth muscle contraction in vitro (Good et al., 1992; Stretton et al., 1992). Cromakalim opens K ÷ channels which are sensitive to the intracellular ATP concentration, an effect which is blocked by glibenclamide (Ashcroft et al., 1984). Glibenclamide reversed the inhibitions by BRL 38227 of cigarette smoke- or vagally induced plasma exudation and bronchoconstriction in the present study and has similar activity on airway smooth muscle contraction in vitro (Good et al., 1992; Stretton et al., 1992). Thus, BRL 38227 appears to activate K~TP channels. Morphine inhibits neurogenic plasma exudation in guinea pig airways (Belvisi et al., 1989) and in the present study inhibited cigarette smoke-induced plasma exudation. The inhibitory effects of morphine and opioid agonists are considered to be mediated via opening K ÷ channels (North, 1986). The failure of glibenclamide in the present study to reverse the inhibition by morphine of cigarette smoke-induced plasma exudation indicates that K + channels other than those sensitive to ATP are involved in morphine inhibition of neurotransmission in the airways (Stretton et al., 1992).
We thank Fisons plc for funding this study. BRL 38227 was kindly donated by SmithKline Beecham Pharmaceuticals (Chemotherapeutic Research Centre, Betchworth, UK). Y.-H.L. is a recipient of an Overseas Research Studentship (ORS award).
References Ashcroft, F.M., D.E. Harrison and S.J. Ashcroft, 1984, Glucose induces closure of single potassium channels in isolated rat pancreatic fl-cells, Nature 312, 446. Belvisi, M.G., D.F. Rogers and P.J. Barnes, 1989, Neurogenic plasma exudation: inhibition by morphine in guinea pig airways in vivo, J. Appl. Physiol. 66, 268. Black, J.L. and P.J. Barnes, 1990, Potassium channels and airway function, new therapeutic prospects, Thorax 45, 213. Good, D.M., J.C. Clapham and T.C. Hamilton, 1992, Effects of BRL 38227 on neurally-mediated responses in the guinea-pig isolated bronchus, Br. J. Pharmacol. 105, 933. Kuo, H.-P., J.A.L. Rohde, P.J. Barnes and D.F. Rogers, 1992, K + channel activator inhibition of neurogenic goblet cell secretion in guinea pig trachea, Eur. J. Pharmacol. 215, 297. North, R.A, 1986, Opioid receptor types and membrance ion channels, Trends Neurosci. 7, 114. Rogers, D.F. and T.W. Evans, 1992, Plasma exudation and oedema in asthma, Br. Med. Bull. 48, 120. Rogers, D.F., P. Boschetto and P.J. Barnes, 1989, Plasma exudation: correlation between Evans blue dye and radiolabelled albumin in guinea pig airways in vivo, J. Pharmacol. Methods 21,309. Stretton, D., M. Miura, M.G. Belvisi and P.J. Barnes, 1992, Calcium-activated potassium channels mediate prejunctional inhibition of peripheral sensory nerves, Proc. Natl. Acad. Sci. USA 89, 1325.
257
© 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00
EJP 21287
Short communication
Inhibition of neurogenic plasma exudation and bronchoconstriction by a K ÷ channel activator, BRL 38227, in guinea pig airways in vivo Y u - H o n g Lei, P e t e r J. Barnes and D u n c a n F. R o g e r s Department of Thoracic Medicine, National Heart & Lung Institute, Dovehouse Street, London SW3 6LY, UK Received 7 June 1993, accepted 8 June 1993
Intravenous administration of a K + channel activator, BRL 38227, inhibited cigarette smoke-induced plasma exudation in guinea pig airways in vivo in a dose-dependent manner with an approximate ED50 of 6/zg/kg. BRL 38227 also inhibited vagally induced plasma exudation and bronchoconstriction but did not inhibit substance P-induced plasma exudation or neurokinin A-induced bronchoconstriction. K + channels modulate neurotransmission in the airways and K + channel activators may have therapeutic potential in bronchial diseases including asthma and chronic bronchitis. Plasma exudation; Cigarette smoke; Vagus nerve stimulation; K + channel opener; Glibenclamide
1. Introduction
Cromakalim (BRL 34915), a selective and potent K ÷ channel activator, inhibits neurogenic airway smooth muscle contraction in vitro and in vivo (Black and Barnes, 1990). BRL 38227 (levcromakalim), the active enantiomer of cromakalim, also inhibits neurogenic airway smooth muscle contraction in vitro (Good et al., 1992) and goblet cell secretion in vivo (Kuo et al., 1992). The effects of cromakalim and BRL 38227 are inhibited by glibenclamide, a blocker of adenosine triphosphate (ATP)-sensitive K + channels (K~Tp). Activation of a population of sensory nerves sensitive to capsaicin induces release of neuropeptides including substance P (SP), neurokinin A (NKA) and calcitonin gene-related peptide. Activation of these nerves by cigarette smoke or by antidromic electrical stimulation of vagal sensory nerves induces plasma exudation and bronchoconstriction. We investigated the effect of BRL 38227 on neurogenic plasma exudation induced either by cigarette smoke or by electrical stimulation of the vagus nerves, as well as on vagaUy induced bronchoconstriction, in guinea pig airways in vivo. We used glibenclamide to confirm that K~,yp channels mediated the effect of BRL 38227.
Correspondence to: Duncan F. Rogers, Department of Thoracic Medicine, National Heart & Lung Institute, Dovehouse Street, London SW3 6LY, UK.
2. Materials and methods
Male Dunkin-Hartley guinea pigs (300-400 g) were anaesthetized with 2 g/ kg urethane and were mechanically ventilated via a tracheal cannula with 10 m l/k g room air at 60 breaths/min and changes in pulmonary insufflation pressure (PIP, a measure of airway tone) were measured via a side arm in the expiratory limb of the tubing connected to a pressure transducer (Sensortechnics, Rugby, UK). Blood pressure and heart rate were monitored via a catheter containing 2 units/ml heparin in 0.9% saline inserted into the left carotid artery and connected to a pressure transducer (PDCR75 S / N 1572, Druck, Groby, UK). Drugs were administered via the jugular veins. For vagal stimulation both cervical vagus nerves were sectioned below the nodose ganglia and placed across bipolar platinum electrodes (Subminiature electrode, Harvard Apparatus, Edenbridge, UK) 5 min before electrical stimulation (10 Hz, 5 V, 5 ms for 3 min) using a double-channel square wave stimulator (Model $88, Grass Instruments, Quincy, USA). Control animals were prepared as above but the vagi were not stimulated (sham stimulation). Cigarette smoke was administered by collecting smoke from a commercially available cigarette (UK Government Category 'Middle Tar' containing 1.2 mg nicotine and 11 mg carbon monoxide per cigarette) into a polyethylene syringe. Fifty tidal volumes of smoke from the syringe were blown into the airways via the tracheal cannula. Each volume of smoke was separated
258
from the next by 5 - 6 ventilated volumes of room air. Air drawn through an unlit cigarette was administered as a control procedure. Evans blue dye was used as a plasma marker (Rogers et al., 1989) and was injected into a jugular vein 1 min before administration of smoke or air, nerve or sham stimulation, or administration of SP or NKA. Fifteen minutes after exposure to smoke or air, or 5 min after nerve stimulation, or 10 min after administration of SP or NKA, the systemic circulation was perfused with saline at 100 mm Hg to remove intravascular dye. The main bronchi were removed, blotted dry with tissue and weighed. Evans blue was extracted from the bronchial tissue in 2 ml formamide (16 h at 40°C) and its concentration measured at 620 nm wavelength, interpolated on a standard curve and expressed as ng d y e / m g wet weight tissue. All animals for vagal stimulation or sham stimulation were pretreated with atropine and propranolol (1 m g / k g each, i.v. 30 min before stimulation). B R L 38227 (0.1-100 p,g/kg, i.v.: stock solution of 10 m g / m l in ethanol) or vehicle were injected 5 min before cigarette smoke or electrical stimulation of the vagus nerves. In certain animals morphine (1 m g / k g i.v.) was injected 10 min before cigarette smoke. Glibenclamide (Sigma; 3 0 / ~ m o l / k g in 2 ml 5% glucose: stock solution of 3 m m o l / m l in dimethyl sulphoxide) was infused i.v. 4 min before B R L 38227 ( 1 0 0 / z g / k g i.v.) or morphine (1 m g / k g i.v.). In separate animals, B R L 38227 (100 jzg/kg i.v.) or vehicle were injected 5 min before SP (Sigma; 1 n m o l / k g i.v.) or NKA (Sigma; 10 n m o l / k g i.v.). The Mann-Whitney U-test (two-tailed) was used to determine the significance of differences between groups. Data in Results are means + 1 S.E.M. The null hypothesis was rejected at P < 0.05.
200
E
r~
t"
120 G)
//
80
u
1 \\
m
\\
(u
//
kg
\\ \\
o AIR
CS
10 100 0.1 [BRL 38227] (~g kg -~)
100 [BRL]
+ gliben
Fig. 1. Inhibition by B R L 38227 (i.v.) of cigarette smoke (CS)-induced plasma exudation in guinea-pig main bronchi in vivo and reversal by glibenclamide (30 / x m o l / k g i.v.). Data are m e a n tissue content of the plasma marker Evans blue dye, ng dye per mg tissue; S.E. m e a n shown by vertical bars. n = 5 - 6 per group. * P < 0.05 compared with air.
PIP above baseline by 336%. B R L 38227 did not significantly inhibit SP-induced plasma exudation (increased by 8% above SP response, n = 5) or NKA-induced bronchoconstriction (reduced by 14%, n = 5). B R L 38227 (100 ~ g / k g ) alone had no significant effect on the Evans blue dye content in the main bronchi of air-exposed controls (reduced by 7%, n = 5). Glibenclamide alone had no significant effect on the Evans blue dye content in main bronchi of air-exposed controis (mean increase of 37%, n = 4). Morphine (1 m g / k g ) significantly (P < 0.05) inhibited cigarette
A
3. Results
Inhalation of cigarette smoke increased the Evans blue dye content of the main bronchi by 222% above air-exposed controls (fig. 1). B R L 38227 dose-dependently inhibited cigarette smoke-induced plasma exudation with an inhibition of 71% at a dose of 100 /zg/kg and an approximate EDs0 of 6 /xg/kg (fig. 1). Electrical stimulation of the vagus nerves increased the Evans blue content of the main bronchi by 184% above sham controls (fig. 2) and increased PIP by 230% above baseline (fig. 2). BRL 38227 (100/xg/kg) significantly inhibited vagally induced plasma exudation by 81% and bronchoconstriction by 80% (fig. 2). Glibenclamide completely reversed the inhibition by B R L 38227 of cigarette smoke-induced plasma exudation (by 127%, fig. 1) and vagally induced plasma exudation (by 80%) and bronchoconstriction (by 122%) (fig. 2). SP (1 n m o l / k g ) increased plasma exudation by 260% above saline controls and NKA (10 n m o l / k g ) increased
160
120 -
.~ G
g
90-
.t) ~a =,. 0
240
'1o
60
~
160
lot
E
_==
320
~x
D_
.~_
gO 30
88
u.I 0,
BRL
Sham Stim
[
BRL
BRL
+gliben Stim - j
I
BRL +gliben
StJm- - . I
Fig. 2. Inhibition by B R L 38227 (100 /~g/kg i.v.) of vagus nerve stimulation (Stim) induced plasma exudation in main bronchi (A) and bronchoconstriction (B) in guinea-pigs in vivo and reversal by glibenclamide ( 3 0 / ~ m o l / k g i.v.). Data are m e a n tissue content of the plasma marker Evans blue dye, ng dye per mg tissue (A), or m e a n percent increases in pulmonary insufflation pressure (PIP), a measure of bronchial tone (B); S.E. m e a n shown by vertical bar. n = 4 - 6 per group. * P < 0.05 compared with sham stimulation (A) or vagally induced bronchoconstriction (B).
259
smoke-induced plasma exudation by 60.8% (n = 5), an effect which was not reversed by glibenclamide (n = 5). BRL 38227 alone dose-dependently decreased blood pressure with maximal decreases of 40.5 + 2.2% at 100 /zg/kg (n = 22) but did not affect heart rate. Glibenclamide alone increased blood pressure by 44.1 + 4.9% (n = 14). In the presence of glibenclamide, BRL 38227 decreased BP by 35.4 + 10.5% (n = 5). SP 1 nmol/kg and vagal stimulation decreased blood pressure by a further 41.2 + 4.8% (n = 5) and 39.4 + 4.9% (n = 5) respectively and in the presence of BRL 38227 caused additional decreases in blood pressure of 13.3% and 8.3% respectively (n = 5 each). In the presence of glibenclamide and BRL 38227, vagal stimulation decreased BP by 15.9% (n = 5).
BRL 38227 decreased blood pressure which may affect plasma exudation or change capillary transit time and alter clearance of administered drugs. However, in the present study both vagal stimulation and the tachykinins SP and NKA reduced blood pressure whereas only BRL 38227 inhibited vagally induced airway responses. Because of their dilator effects on airway smooth muscle, K + channel openers have been proposed as a new class of antiasthma drug. The importance of plasma exudation in the pathogenesis of asthma is increasingly recognized (Rogers and Evans, 1992). Thus, compounds such as BRL 38227 which reduced airway neurogenic inflammation may have therapeutic value in the treatment of asthma.
4. Discussion Acknowledgements In the present in vivo study, BRL 38227 inhibited cigarette smoke-induced or vagally induced plasma exudation, as well as vagally induced bronchoconstriction, but failed to affect SP induced-plasma exudation or NKA-induced bronchoconstriction. These data indicate that BRL 38227 has an action at a prejunctional site. Similarly, BRL 38227 inhibited both cholinergic and eNANC responses to electrical field stimulation but failed to prevent acetylcholine, SP or NKA induced tracheal smooth muscle contraction in vitro (Good et al., 1992; Stretton et al., 1992). Cromakalim opens K ÷ channels which are sensitive to the intracellular ATP concentration, an effect which is blocked by glibenclamide (Ashcroft et al., 1984). Glibenclamide reversed the inhibitions by BRL 38227 of cigarette smoke- or vagally induced plasma exudation and bronchoconstriction in the present study and has similar activity on airway smooth muscle contraction in vitro (Good et al., 1992; Stretton et al., 1992). Thus, BRL 38227 appears to activate K~TP channels. Morphine inhibits neurogenic plasma exudation in guinea pig airways (Belvisi et al., 1989) and in the present study inhibited cigarette smoke-induced plasma exudation. The inhibitory effects of morphine and opioid agonists are considered to be mediated via opening K ÷ channels (North, 1986). The failure of glibenclamide in the present study to reverse the inhibition by morphine of cigarette smoke-induced plasma exudation indicates that K + channels other than those sensitive to ATP are involved in morphine inhibition of neurotransmission in the airways (Stretton et al., 1992).
We thank Fisons plc for funding this study. BRL 38227 was kindly donated by SmithKline Beecham Pharmaceuticals (Chemotherapeutic Research Centre, Betchworth, UK). Y.-H.L. is a recipient of an Overseas Research Studentship (ORS award).
References Ashcroft, F.M., D.E. Harrison and S.J. Ashcroft, 1984, Glucose induces closure of single potassium channels in isolated rat pancreatic fl-cells, Nature 312, 446. Belvisi, M.G., D.F. Rogers and P.J. Barnes, 1989, Neurogenic plasma exudation: inhibition by morphine in guinea pig airways in vivo, J. Appl. Physiol. 66, 268. Black, J.L. and P.J. Barnes, 1990, Potassium channels and airway function, new therapeutic prospects, Thorax 45, 213. Good, D.M., J.C. Clapham and T.C. Hamilton, 1992, Effects of BRL 38227 on neurally-mediated responses in the guinea-pig isolated bronchus, Br. J. Pharmacol. 105, 933. Kuo, H.-P., J.A.L. Rohde, P.J. Barnes and D.F. Rogers, 1992, K + channel activator inhibition of neurogenic goblet cell secretion in guinea pig trachea, Eur. J. Pharmacol. 215, 297. North, R.A, 1986, Opioid receptor types and membrance ion channels, Trends Neurosci. 7, 114. Rogers, D.F. and T.W. Evans, 1992, Plasma exudation and oedema in asthma, Br. Med. Bull. 48, 120. Rogers, D.F., P. Boschetto and P.J. Barnes, 1989, Plasma exudation: correlation between Evans blue dye and radiolabelled albumin in guinea pig airways in vivo, J. Pharmacol. Methods 21,309. Stretton, D., M. Miura, M.G. Belvisi and P.J. Barnes, 1992, Calcium-activated potassium channels mediate prejunctional inhibition of peripheral sensory nerves, Proc. Natl. Acad. Sci. USA 89, 1325.