PROSTAGLANDINS STIMULATION OF AFFERENT VAGAL ENDINGS IN THE INTRAPULMONARY AIRWAYS BY PROSTAGLANDIN ENDOPEROXIDE ANALOGUES 1 K.H. Ginzel 2, M.A. Morrison 2, D.G. Baker, H.M. Coleridge, and J.C.G. Coleridge Cardiovascular Research Institute, University of California San Francisco, San Francisco, California 94143 ABSTRACT Two cyclic ether (CE) analogues of the prostaglandin endoperoxide PGH2, CE I and C E I I , have been found to exert powerful stimulant effects on lung 'irritant' receptors and bronchial C-fiber endings after intravascular or aerosol administration in open-chest dogs under Dial-pentobarbital anesthesia. 'Irritant' receptors responded to a dose as small as 0.i ~g/kg C E I I , injected into the right atrium. C E I I was twice as effective as CE I and 10-20 times more potent than PGF2~ . As an aerosol, it exceeded histamine in potency by more than 800 times. 'Irritant'receptor stimulation was always associated with decrease in lung compliance and increase in lung resistance. Isoproterenol which reduced the latter effects also diminished the response of 'irritant' receptors. Left atrial injection of CE s had only weak and delayed effects. CE-induced 'irritant' receptor firing declined or ceased during ventilatory arrest in expiration and following hyperinflation of the lungs. In contrast to 'irritant' receptors, C-fibers responded more effectively and more rapidly, and in the absence of mechanical changes, when the drugs were injected into the left atrium as compared to right atrial injection. These findings suggest that CE-induced 'irritant' receptor stimulation is secondary to changes in lung mechanics, whereas C-fiber stimulation is a direct effect upon the nerve ending. INTRODUCTION Prostaglandins (PGs) F2~ , E 1 and E 2 have powerful effects on airway smooth muscle and have been implicated in the regulation of bronchial tone in health and disease (1,2,3). PGF2~ , which constricts the airways, stimulates rapidly-adapting lung stretch iWe thank Dr. John Pike of the Upjohn Company for the prostaglandins used in our experiments, and Mr. A. Dangel and Ms. S. Montgomery for technical assistance. This work was supported in part by U.S. Public Health Service Program Project Grant HL-06285 from the National Heart, Lung and Blood Institute. 2Department of Pharmacology, University of Arkansas Sciences, Little Rock, Arkansas 72201.
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131
PROSTAGLANDINS ('irritant') receptors, and has less pronounced stimulant effects on lung afferent C-fiber endings (4). PGEs, which dilate the airways, cause conspicuous and prolonged excitation of lung C-fiber endings but have little or no effect on 'irritant' receptors (4). Two intermediates in the biosynthesis of PGs have recently attracted attention, namely the cyclic endoperoxides PGG 2 and PGH 2 (5,6). These compounds have greater biological activity than the PGs proper (7,8). We have examined the effect of two stable analogues of PGH2, the cyclic ethers (CE) I and II (9) on firing frequency of 'irritant' receptors and bronchial C-fiber endings in the intrapulmonary airways of dogs. We found that these agents strongly excited both types of sensory airway ending, their potency greatly exceeding that of PGF2~ and PGEs. METHODS Adult mongrel dogs of both sexes (10-24 kg), which had received promazine hydrochloride (Sparine, Wyeth Laboratories, 50 mg i.m.) one hour earlier, were anesthetized with 0.25 ml/kg i.v. of a i:i mixture of Dial Compound (allobarbital i00 mg/ml, urethane 400 mg/ml; Ciba) and sodium pentobarbital (50 mg/ml). The chest was opened in the midsternal line and the lungs were ventilated with 50% oxygen in nitrogen by a Harvard constant volume positive pressure respirator; the expiratory outlet from the pump was placed under 3-5 cm of water. Tidal carbon dioxide was monitored continuously with a Beckman (LB-I) CO 2 analyzer and maintained within normal limits. Sodium bicarbonate was injected periodically to maintain normal blood pH. Tracheal pressure was recorded with a strain gauge (Statham P23Gb) connected to a side arm on the tracheal cannula. Air flow was recorded with a pneumotachograph (Fleisch Type i) and a differential pressure transducer (Statham PMISTC) whose signal was integrated (Grass Polygraph 7PIOA integrator) to provide a record of tidal volume. Lung compliance (CL) was calculated by dividing tidal volume by the difference between end deflation and end inflation tracheal pressure (measured at times of zero flow). Total lung resistance (RL) was measured by the 'subtractor' method of Mead and Whittenberger (i0). Pulmonary arterial pressure was recorded from the central end of a lobar branch of the right pulmonary artery. The lobe which was deprived of its circulation was ligated. Systemic arterial blood pressure was recorded from a femoral artery. Pressures were recorded with Statham P23Gb strain gauges. We recorded afferent impulses from fine strands of the left cervical vagus nerve. The two types of lung afferent, 'irritant' receptors and bronchial C-fiber endings, were identified by their pattern of discharge, their response to various procedures (e.g., hyperinflation of the lungs and injection of chemicals), and their conduction velocity (11,12,13). We confirmed that all endings were located in the lung (13).
132
JANUARY 1978 VOL. 15 NO. 1
PROSTAGLANDINS
After suitable amplification, action potentials, a voltage analogue of impulse frequency, pressures, airway C02, electrocardiogram, and other variables were recorded by a direct-writing ultraviolet light recorder (S.E. Laboratories Model 2000) and also on magnetic tape (Ampex FR 1300 tape recorder), and all but the action potentials were recorded by a multi-channel pen writer (Grass Model 7 Polygraph). Drugs were injected in 0.5 ml saline into the right or left atrium and washed in with 1.0 ml saline. Injection was completed in about i sec. Aerosols were generated by an ultrasonic nebulizer (DeVilbis). For quantitative assessment of drug effects on impulse activity, C L and RL, the values measured during one ventilatory cycle at the peak of the response were compared with those averaged from three preceding control ventilatory cycles. The action potentials were counted from the u.v. record for each entire ventilatory cycle and expressed as impulses/sec. RESULTS AND DISCUSSION The most striking feature of the effect of cyclic ethers on afferent nerve endings in the intrapulmonary airways was the small dose required to evoke a conspicuous increase in firing. We were able to make repeated observations over an extended period on three of the 'irritant' receptors, and the results (Fig. i) provide a dose-response relationship that illustrates the potency of the cyclic ethers, both after injection into the right atrium and administration in form of an aerosol. For comparison, submaximal responses to right atrial injection• of . PGFo (~' and to PGF~ and histamine given as an aerosol are shown in Fig. i. CE II was ~ far the most potent compound, being twice as effective as CE I and 10-20 times stronger than PGF~ . As an aerosol, CE II was more than 800 times stronger than hlslammne, and is thus the most powerful known stimulant of airway nerve endings. Fig. i also shows that drug-induced changes in 'irritant' receptor activity were accompanied by increases in R L and decreases in CL in a dose-dependent fashion. Cyclic ethers were injected into the right atrium in doses of 0.i - 2.0 ~g/kg (CE II) and 0.8 - 5.0 ~g/kg (CE I) in a total of 30 trials on 8 'irritant' receptors. When all results were pooled, impulse activity increased from 2.0 ± 0.3 to 12.5 ± 1.9 impulses/sec (mean ± s.e.), with peak increments of as much as 50-60 impulses/sec. Maximum instantaneous firing rates as seen during inspiration were higher than the above values which were averaged over the whole ventilatory cycle. The latency of response ranged from 5 to 35 sec (13.2 i 1.4) and firing remained above control for 15 to 130 sec. Tracheal pressure increased by 6.6 ± I.i cm H20 , and ~. by 458 ± 97%. Systemic and pulmonary arterial pressures rose by 2 5 - 6 O m m H g and 5-40 mm Hg, respectively. In 6 trials on 3 'irritant' receptors, CE II aerosol (2 and 5 ~g/ml) increased impulse activity from 1.4 ± 0.5 to I0.i ± 2.6 impulses/sec.
JANUARY 1978 VOL. 15 NO. 1
133
z o
O
z
>
0
2
I0
O.l~g/Kg (n=l)
'IR' RLCL
9.4ag/Kg
(n=5)
0.2JJg/Kg
(n=6)
#
i
I_
T
3yclic EtherZ
(n=l)
0.Sag/Kg (n=l)
0.Sag/Kg
:!:!
!:i:l :i:l
1
(n=l)
1.6ag/Kg
3yclic Ether I
(n=3)
4,0ag/Kg
=GFz,~
2~g/ml (n=l)
(n=l)
5,ug/ml
(n=l)
IO0~g/ml
i:~:1 !!~!1
iiil
AEROSOLS Cyclic Ether~ PG~
(n=l)
4mg/ml
Histamine
E
0.000
0.002
.I
0.005 z
J
O.OLO ~
0.020
0.040
Fig. i. Comparison of effects of CE I and CE II on lung 'irritant' receptor activity (IR), lung resistance (RL) and lung compliance (CL) with those of submaximal doses of PGF~z ~ and histamine acid phosphate. Drugs were administered by injection into the right atrium (RA) or as aerosols. Doses, concentrations and n values are indicated at the foot of the columns. Data are derived from experiments on 3 'irritant' receptors in 2 dogs. Drugs were administered at intervals of at least 15 min.
-
z
w
JANUARY 1 9 7 8 VOL. 15 NO. 1
for Medical
131
PROSTAGLANDINS ('irritant') receptors, and has less pronounced stimulant effects on lung afferent C-fiber endings (4). PGEs, which dilate the airways, cause conspicuous and prolonged excitation of lung C-fiber endings but have little or no effect on 'irritant' receptors (4). Two intermediates in the biosynthesis of PGs have recently attracted attention, namely the cyclic endoperoxides PGG 2 and PGH 2 (5,6). These compounds have greater biological activity than the PGs proper (7,8). We have examined the effect of two stable analogues of PGH2, the cyclic ethers (CE) I and II (9) on firing frequency of 'irritant' receptors and bronchial C-fiber endings in the intrapulmonary airways of dogs. We found that these agents strongly excited both types of sensory airway ending, their potency greatly exceeding that of PGF2~ and PGEs. METHODS Adult mongrel dogs of both sexes (10-24 kg), which had received promazine hydrochloride (Sparine, Wyeth Laboratories, 50 mg i.m.) one hour earlier, were anesthetized with 0.25 ml/kg i.v. of a i:i mixture of Dial Compound (allobarbital i00 mg/ml, urethane 400 mg/ml; Ciba) and sodium pentobarbital (50 mg/ml). The chest was opened in the midsternal line and the lungs were ventilated with 50% oxygen in nitrogen by a Harvard constant volume positive pressure respirator; the expiratory outlet from the pump was placed under 3-5 cm of water. Tidal carbon dioxide was monitored continuously with a Beckman (LB-I) CO 2 analyzer and maintained within normal limits. Sodium bicarbonate was injected periodically to maintain normal blood pH. Tracheal pressure was recorded with a strain gauge (Statham P23Gb) connected to a side arm on the tracheal cannula. Air flow was recorded with a pneumotachograph (Fleisch Type i) and a differential pressure transducer (Statham PMISTC) whose signal was integrated (Grass Polygraph 7PIOA integrator) to provide a record of tidal volume. Lung compliance (CL) was calculated by dividing tidal volume by the difference between end deflation and end inflation tracheal pressure (measured at times of zero flow). Total lung resistance (RL) was measured by the 'subtractor' method of Mead and Whittenberger (i0). Pulmonary arterial pressure was recorded from the central end of a lobar branch of the right pulmonary artery. The lobe which was deprived of its circulation was ligated. Systemic arterial blood pressure was recorded from a femoral artery. Pressures were recorded with Statham P23Gb strain gauges. We recorded afferent impulses from fine strands of the left cervical vagus nerve. The two types of lung afferent, 'irritant' receptors and bronchial C-fiber endings, were identified by their pattern of discharge, their response to various procedures (e.g., hyperinflation of the lungs and injection of chemicals), and their conduction velocity (11,12,13). We confirmed that all endings were located in the lung (13).
132
JANUARY 1978 VOL. 15 NO. 1
PROSTAGLANDINS
After suitable amplification, action potentials, a voltage analogue of impulse frequency, pressures, airway C02, electrocardiogram, and other variables were recorded by a direct-writing ultraviolet light recorder (S.E. Laboratories Model 2000) and also on magnetic tape (Ampex FR 1300 tape recorder), and all but the action potentials were recorded by a multi-channel pen writer (Grass Model 7 Polygraph). Drugs were injected in 0.5 ml saline into the right or left atrium and washed in with 1.0 ml saline. Injection was completed in about i sec. Aerosols were generated by an ultrasonic nebulizer (DeVilbis). For quantitative assessment of drug effects on impulse activity, C L and RL, the values measured during one ventilatory cycle at the peak of the response were compared with those averaged from three preceding control ventilatory cycles. The action potentials were counted from the u.v. record for each entire ventilatory cycle and expressed as impulses/sec. RESULTS AND DISCUSSION The most striking feature of the effect of cyclic ethers on afferent nerve endings in the intrapulmonary airways was the small dose required to evoke a conspicuous increase in firing. We were able to make repeated observations over an extended period on three of the 'irritant' receptors, and the results (Fig. i) provide a dose-response relationship that illustrates the potency of the cyclic ethers, both after injection into the right atrium and administration in form of an aerosol. For comparison, submaximal responses to right atrial injection• of . PGFo (~' and to PGF~ and histamine given as an aerosol are shown in Fig. i. CE II was ~ far the most potent compound, being twice as effective as CE I and 10-20 times stronger than PGF~ . As an aerosol, CE II was more than 800 times stronger than hlslammne, and is thus the most powerful known stimulant of airway nerve endings. Fig. i also shows that drug-induced changes in 'irritant' receptor activity were accompanied by increases in R L and decreases in CL in a dose-dependent fashion. Cyclic ethers were injected into the right atrium in doses of 0.i - 2.0 ~g/kg (CE II) and 0.8 - 5.0 ~g/kg (CE I) in a total of 30 trials on 8 'irritant' receptors. When all results were pooled, impulse activity increased from 2.0 ± 0.3 to 12.5 ± 1.9 impulses/sec (mean ± s.e.), with peak increments of as much as 50-60 impulses/sec. Maximum instantaneous firing rates as seen during inspiration were higher than the above values which were averaged over the whole ventilatory cycle. The latency of response ranged from 5 to 35 sec (13.2 i 1.4) and firing remained above control for 15 to 130 sec. Tracheal pressure increased by 6.6 ± I.i cm H20 , and ~. by 458 ± 97%. Systemic and pulmonary arterial pressures rose by 2 5 - 6 O m m H g and 5-40 mm Hg, respectively. In 6 trials on 3 'irritant' receptors, CE II aerosol (2 and 5 ~g/ml) increased impulse activity from 1.4 ± 0.5 to I0.i ± 2.6 impulses/sec.
JANUARY 1978 VOL. 15 NO. 1
133
z o
O
z
>
0
2
I0
O.l~g/Kg (n=l)
'IR' RLCL
9.4ag/Kg
(n=5)
0.2JJg/Kg
(n=6)
#
i
I_
T
3yclic EtherZ
(n=l)
0.Sag/Kg (n=l)
0.Sag/Kg
:!:!
!:i:l :i:l
1
(n=l)
1.6ag/Kg
3yclic Ether I
(n=3)
4,0ag/Kg
=GFz,~
2~g/ml (n=l)
(n=l)
5,ug/ml
(n=l)
IO0~g/ml
i:~:1 !!~!1
iiil
AEROSOLS Cyclic Ether~ PG~
(n=l)
4mg/ml
Histamine
E
0.000
0.002
.I
0.005 z
J
O.OLO ~
0.020
0.040
Fig. i. Comparison of effects of CE I and CE II on lung 'irritant' receptor activity (IR), lung resistance (RL) and lung compliance (CL) with those of submaximal doses of PGF~z ~ and histamine acid phosphate. Drugs were administered by injection into the right atrium (RA) or as aerosols. Doses, concentrations and n values are indicated at the foot of the columns. Data are derived from experiments on 3 'irritant' receptors in 2 dogs. Drugs were administered at intervals of at least 15 min.
-
z
w
