Stimulation of afferent vagal endings in the intrapulmonary airways by prostaglandin endoperoxide analogues

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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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).

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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.

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Fig. 2. Effect of CE II on lung 'irritant' receptor activity (IR, impulses/see), recorded by a ratemeter, and on tracheal pressure (PTR, cm H20 ). The arrows in A and B indicate injections of 0.4 ~g/kg CE II into the right and left atrium, respectively. Aerosol of 5 ~g/ml CE II was begun at the arrow in C and maintained throughout C,D and E. In C and D, the respirator was turned off for approximately 15 sec, the lungs remaining moderately inflated (see tracheal pressure record). In E, the lungs were inflated with 4 tidal volumes by occluding the outflow tube between the tracheal cannula and the respirator. Interval between A and B is 15 min, C and D 6.5 min, and D and E 3 min. Fig. 2 depicts the time course and intensity of 'irritant' receptor stimulation and tracheal pressure change after injection into the right and left atrium (A and B) and aerosol administration (C) of CE II. 'Irritant' receptor activity did not start to increase until tracheal pressure had begun to rise, an observation which was consistently made following administration of CE I and CE II. Subsequently, fiber activity declined although tracheal pressure remained elevated, reflecting adaptation to the stimulus; this was particularly obvious during aerosol administration. Fig. 2 (A and B) also illustrates that right atrial injection of CE II led to a much more pronounced increase in 'irritant' receptor activity with a shorter latency of onset than did left atrial injection. For the group as a whole (5 trials on 3 fibers), the effects of left atrial injection of cyclic ethers on 'irritant' receptors were relatively weak and delayed, impulse activity increasing from 1.9 ± 0.6 to 6.3 ± 0.7 impulses/sec with latencies of onset ranging from 12 to 60 sec.

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Thus, CE I and CE II behaved similarly to PGF 2 which also exerted its main stimulant effect on 'irritant' receptors following injection into the right atrium, i.e., via the pulmonary circulation (4). If the majority of 'irritant' receptors are situated in the larger airways which are supplied by the bronchial circulation (14), then CE I and II, as PGF2~ , stimulate 'irritant' receptors not by a direct action upon the nerve ending, but as a consequence of their effects on lung compliance and resistance which are most prominent when the drugs are injected into the pulmonary circulation. The hypothesis that 'irritant' receptor stimulation depends predominantly on the drug-induced changes in lung mechanics is strengthened by the observation that firing evoked by administration of aerosol was either decreased (Fig. 2C) or arrested (Fig. 2D) when the respirator was stopped. In the first case (2C), the receptor fired throughout the respiratory cycle with peak activity in inspiration; in the second (2D), firing was limited to the inspiratory phase. Similarly, when the lungs were inflated with more than one tidal volume to produce a subsequent decrease in tracheal pressure, fiber activity was also silenced (Fig. 2E). That the stimulation of 'irritant' receptors by P G ~ is secondary to mechanical changes was supported by the finding that isoproterenol significantly reduced the effect of P G ~ on both receptor activity and tracheal pressure (4). In the present study it was found that isoproterenol, injected into the right atrium (10-20 ~g/kg) or given as an aerosol (0.2 mg/ml), decreased the cyclic ether-induced changes in 'irritant' receptor firing by 54-85%, with corresponding reductions in changes in R L and CL. The cyclic ethers produced powerful stimulation of intrapulmonary bronchial C-fibers. In 7 trials on 3 fibers, activity was increased by 2-4 times after right atrial injection and by 4-6 times after left atrial injection of I ~g/kg CE I and 0.4 ~g/kg CE II. These doses were less than 1/20 of those required to produce equivalent C-fiber stimulation with PGE 2. The latencies of onset varied from 8-13 sec after right atrial injection and 4-6 sec after left atrial injection. In 3 trials on 2 fibers, aerosols in concentrations of 20 ~g/ml (CE I) and 5 ~g/ml (CE II) increased C-fiber activity by 2-4 times. These effects are illustrated in Fig. 3 (A,B, and C). The absence of changes in tracheal pressure, C L and R L at the peak of C-fiber stimulation following left atrial injection (B) indicates that the effect is not secondary to changes in lung mechanics. Thus, in marked contrast to the response of 'irritant' receptors, C-fibers were more effectively and more rapidly stimulated when the drugs were injected into the left atrium than after right atrial injection. This suggests that cyclic ethers gain access to bronchial C-fiber endings via the bronchial circulation, and produce stimulation by a direct effect on the endings. Our findings indicate that the cyclic ether analogues of prostaglandin endoperoxide PGH2, a precursor intermediate of both P G ~

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and PGEs, combine the sensory stimulating properties of these prostaglandins and excite both 'irritant' receptors and bronchial C-fiber endings. 'Irritant' receptors appear to be stimulated largely as a consequence of the mechanical changes which are induced in the lung by the cyclic ethers, whereas bronchial C-fibers appear to be stimulated by a direct action on the nerve endings. The cyclic ethers are the most powerful known stimulants of lung airway afferents, their potency greatly exceeding that of PGF2~ and PGEs, so that the reflex effects arising from their action may well prove to have profound pathophysiological significance. The results of this study, in conjunction with a recent report that an endoperoxide analogue (CE II) surpassed PGF2_ and PGE 2 also in pulmonary vasoconstrictor potency (15), a fin~ing corroborated by our own data, supports the contention that the endoperoxides do not solely act through their degradation products but constitute a highly active form of the prostaglandins in the lung (7,15,16).

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PROSTAGLANDINS REFERENCES i)

Fanburg, B.L~ Prostaglandins and the Lung. Dis. 108:482-489, 1973.

2)

Wasserman, M.A. Bronchopulmonary Responses to Prostaglandin F2e , Histamine and Acetylcholine in the Dog. Eur. J. Pharmacol. 32:146-155, 1975.

3)

Parker, C.W. and D.E. Snider. Prostaglandins and Asthma. Intern. Med. 78:963-965, 1973.

4)

Coleridge, H.M., JoC.Go Coleridge, KoHo Ginzel, D.G. Baker, RoB. Banzett, and M.A. Morrison. Stimulation of 'Irritant' Receptors and Afferent C-Fibers in the Lungs by Prostaglandins. Nature 264:451-453, 1976.

5)

Hamberg, M. and B. gamuelsson. Detection and Isolation of an Endoperoxide Intermediate in Prostaglandin Biosynthesis. Proc. Nat. Acad. Sei. (USA) 70:899-903, 1973.

6)

Namberg, M., J. Svensson, T. Wakabayashi, and B. Samuelsson. Isolation and Structure of Two Prostaglandin Endoperoxides That Cause Platelet Aggregation. Proc. Nat. Acad. Sci. (USA) 71: 345-349, 1974.

7)

Hamberg, M., P. Hedqvist, K. Strandberg, J. Svensson, and B. Samuelsson. Prostaglandin Endoperoxides IV. Effects on Smooth Muscle. Life Sciences 16:451-462, 1975o

8)

Wasserman, M.A. Bronchopulmonary Pharmacology of Some Prostaglandin Endoperoxide Analogs in the Dog. Eur. J. Pharmacol. 36:103-114, 1976.

9)

Bundy, G.L. The Synthesis of Prostaglandin Endoperoxide Analogs. Tetrahedron Lett. 24, 1957.

i0)

Mead, J. and J.L. Whittenberger. Physical Properties of Human Lungs Measured During Spontaneous Respiration. J. Appl. Physiol. 5:779-796, 1953.

ii)

Mills, JOE., H. Selliek, and J.Go Widdicombe. Activity of Lung'Irritant' Receptors in Pulmonary Micro-Embolism, Anaphylaxis and Drug-Induced Bronchoconstrictions. J. Physiol., London 203:337-357, 1969.

12)

Sampson, S.R. and E.H, Vidruk. Properties of 'Irritant' Receptors in Canine Lung. Respir. Physiol. 25:9-22, 1975.

13)

Coleridge, H.M. and J.C°G. Coleridge. Impulse Activity in Afferent Vagal C-Fibers with Endings in the Intrapulmonary Airways of Dogs. Respir. Physiolo 29:125-142, 1977.

14)

Fillenz, M. and J.G. Widdicombe. Receptors of the Lungs and Airways. In: Enteroceptors. (E. Nell, ed.) gpringer-Verlag, New York, 1972, p. 81o

15)

Kadowitz, P,J. and A.L. Hyman. Influence of a Prostaglandin Endoperoxide Analogue on the Canine Pulmonary Vascular Bed. Circul. Res. 40:282-287, 1977.

16)

Hamberg, M. and B. Samuelsson. Prostaglandin Endoperoxides VII. Novel Transformations of Arachidonic Acid in Guinea Pig Lung. Biochem. Biophys. Res. Comm. 61:942-949, 1974.

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