LUNG REFLEXES AFFECTING THE LARYNX IN THE PIG, AND THE EFFECT OF PULMONARY MICROEMBOLISM

Transcription

1 Quarterly Journal of Experimental Physiology (1987) 72, Printed in Great Britain LUNG REFLEXES AFFECTING THE LARYNX IN THE PIG, AND THE EFFECT OF PULMONARY MICROEMBOLISM G. AGUGGINI, M. G. CLEMENT AND J. G. WIDDICOMBE* Istituto di Fisiologia Veterinaria e Biochimica, Universita di Milano, Via Celoria 1, 2233 Milano, Italy (RECEIVED FOR PUBLICATION 2 MARCH 1986) SUMMARY Laryngeal airflow resistance was measured in anaesthetized pigs during stimulation of lung vagal reflexes by injection of phenylbiguanide and of capsaicin, before and during pulmonary microembolism due to intravenous injection of hardened red blood cells; the microembolism caused pulmonary hypertension. The Breuer-Hering reflex was also assessed before and after pulmonary microembolism. Phenylbiguanide and capsaicin caused apnoea followed by rapid shallow breathing, hypertension, bradycardia and laryngeal constriction in inspiration and expiration. Most of these effects were abolished by bilateral cervical vagotomy. Pulmonary microembolism caused only small changes in breathing pattern, mainly a decrease in inspiratory time. After microembolism the Breuer-Hering reflex was enhanced, and injections of phenylbiguanide and capsaicin caused longer apnoeas by a vagal reflex. The changes in pattern of breathing and in lung reflexes after lung microembolism and during the associated pulmonary hypertension are consistent with an enhancement of pulmonary stretch receptor activity in this condition, but do not indicate any important role for C fibre reflexes. INTRODUCTION The calibre of the larynx shows respiratory variations and is also affected by lung reflexes that change the discharge of inspiratory and expiratory laryngeal motoneurones, as shown for the cat, rabbit and dog (Stransky, Szereda-Przestaszewska & Widdicombe, 1973; Dixon, Szereda-Przestaszewska, Widdicombe & Wise, 1974; Glogowska, Stransky & Widdicombe, 1974; Jammes, Davies & Widdicombe, 1985). Furthermore, Bartlett and co-workers (Bartlett, Remmers & Gautier, 1973; Gautier, Remmers & Bartlett, 1973; Remmers & Bartlett, 1977) showed that the larynx can control breathing pattern by regulating expiratory time and flow. The complex nervous mechanism which controls laryngeal lumen involves reflexes from the lungs and from the larynx (Stransky et al. 1973; Glogowska et al. 1974; Sant'Ambrogio, Mathew, Fisher & Sant'Ambrogio, 1983; Widdicombe, 1986) and may be involved in pathological conditions; lung microembolism changes the pattern of breathing by stimulating vagal lung receptors (Whitteridge, 195), which might also modify the calibre of the larynx, thereby affecting breathing. Lung microembolism also causes pulmonary hypertension (Clement, Aguggini & Miserocchi, 1981; Clement, Damasio, Parravivini, Russo & Aguggini, 1983). We have studied the control of laryngeal calibre in the pig after lung microembolism caused by i.v. injection of hardened red blood cells (Clement et al. 1983). We have also studied the reflexes activated in the pig by stimulation of lung vagal receptors by lung inflation and by phenylbiguanide (PBG) and by capsaicin injected i.v. before and after lung microembolism in an attempt to analyse the lung reflexes that might be involved in the responses. * Address for correspondence: Department of Physiology, St George's Hospital Medical School, Cranmer Terrace, London SW17 ORE.

2 96 G. AGUGGINI, M. G. CLEMENT AND J. G. WIDDICOMBE METHODS Experiments were performed with ten pigs of either sex weighing 16-2 kg that were anaesthetized with sodium thiopentone (3-4 mg.kg-1), initially injected into the marginal vein of an ear. The animals were pre-medicated with 1% proprionylpromazine (5 ml.kg-') intramuscularly. During experiments the level of anaesthesia was maintained by infusion of sodium pentobarbitone (3-6 mg.kg-.h'-) given through a catheter inserted into a femoral vein. The pigs were tied supine and body temperature was measured with a rectal probe and maintained at 'C by an electric blanket. The animals breathed through a tracheal cannula inserted into the lower cervical trachea, directed caudally. A second tracheal cannula was inserted just below the cricoid cartilage, directed rostrally. Care was taken not to damage the recurrent laryngeal nerves. The thyrohyoid membrane was opened widely to observe the laryngeal vestibule and vocal cords, and to allow free efflux of air from the larynx. Laryngeal resistance was measured in two ways, with the larynx out of circuit or in circuit. (1) A constant stream of humidified warm air was passed through the upper cannula and the larynx, and out through the thyrohyoid opening at constant rates (5-25 l.min-'), measured with a rotameter. Inflow pressure was recorded and its ratio to the set laryngeal airflow was used as an index of laryngeal resistance. (2) The upper tracheal cannula was connected in series with the lower cannula by silicon rubber tubes containing a Fleisch pneumotachograph head (No. 1), so that the animal breathed through its larynx. The translaryngeal pressure and respiratory airflow were recorded and displayed on the two axes of a storage oscilloscope (Tektronix 513 N). The mean slopes of full inspiratory and expiratory limbs of the 'loops' were used for calculation of resistances. With both methods the laryngeal resistance was always higher in expiration than in inspiration (Figs. 1, 2 and 4). Upper tracheal pressure (Ptr) was measured with a strain-gauge manometer (Statham 15299); airflow (i) was measured from a Fleisch pneumotachograph head attached to the lower tracheal cannula; electrical integration provided tidal volume (VT). Systemic arterial blood pressure (B.P.) was recorded by a catheter inserted into the right femoral artery by means of a fluid-filled strain-gauge manometer (Bell & Howell 4-422). A Swan-Ganz catheter was inserted into a brachial vein and advanced to the pulmonary artery. Pulmonary arterial pressure (P.A.P.) was recorded by means of a saline-filled strain-gauge manometer (Bell & Howell 4422). Records of B.P., P.A.P., Ptr, V and VT were made on a Grass polygraph (Model 7B). Pressures are expressed as means of systolic and diastolic values. The Breuer-Hering reflex was evaluated by inflating the lungs with constant pressures up to 8 cmh2o, the maximum pressure depending on the sensitivity of each pig. Inflations were started immediately at the beginning of an expiratory pause. We measured the absolute duration of the apnoea from the end of the inspiration before inflation to the start of the first inspiration during inflation. We also calculated the ratio of the length of the apnoea divided by the previous expiratory time (mean value of three breaths). Microembolism was done by injection into the pulmonary artery of hardened red blood cells (3-5 ml of 3% v/v suspension) (Clement et al. 1983). Sufficient red cells were always injected to cause a clear pulmonary hypertension. PBG (25-75 #ug.kg-') and capsaicin (3-4 /ug.kg-1) were injected through a catheter in the right femoral vein; drugs were administered in control conditions and after pulmonary microembolism. In experiments with the larynx out of circuit, PBG and capsaicin were injected when the flow through the larynx was 15-2 l.min-1. Since flow through the larynx may influence breathing pattern (Bartlett et al. 1973; Gautier et al. 1973; Remmers & Bartlett, 1977), the effect of putting the larynx in and out of circuit was studied. The effects on the larynx of pulmonary microembolism and its associated pulmonary hypertension and of reflexes induced by drugs were studied before and after vagosympathectomy performed bilaterally in the mid-cervical region. Analysis of data Changes in laryngeal resistance were analysed as comparisons of three control breaths before and three breaths after an intervention if the response was approximately steady state, or peak maximum change if it was transient (as after injection of drugs). Pattern of breathing was analysed in terms of VT, inspiratory and expiratory times for control breaths, and when the larynx was taken in or out of circuit for the first breath to avoid changes in breathing due to alterations in blood gas concentrations. Statistical analyses were performed on paired values by Student's t test and the sign test. Values are given as means ±S.E.M.S.

3 REFLEXES AND THE PIG LARYNX 97 Table 1. Changes due to L v. injections of PBG and capsaicin, and the effects of pulmonary microembolism (p.m.e.) PBG Capsaicin Control During p.m.e. Control During p.m.e. Variable (n = 5/8) (n = 4/4) (n = 8/12) (n = 7/7) Apnoea (s) ** ** ** B.P. (mmhg) ** * ** ** H.R. (min-') ** * ** -58+1l1** P.A.P. (mmhg) * * Values are means+s.e.m. *P <-5, **P < 1. n = animals/tests. H.R., heart rate. RESULTS Responses to PBG and capsaicin PBG caused apnoea at end-expiratory level lasting an average of s longer than control expiratory duration, and was followed by rapid shallow breathing; B.P. fell by % and heart rate fell by % (P < 1, in all cases, eight tests in five pigs). P.A.P. did not change significantly. Table 1 summarizes the results. Fig. 1 shows that when the animals were breathing through the larynx there was a considerable but very variable increase in inspiratory and expiratory laryngeal resistances after the apnoeas. When the larynx was out of circuit, the larynx closed almost completely during the apnoeas (not illustrated), and its resistance was greater than control (but not significantly) when breathing started again (Fig. 2). In five tests on five of the same pigs, bilateral cervical vagotomy abolished the changes due to PBG and capsaicin in breathing, heart rate and B.P., and there were still no significant changes in P.A.P. The changes in laryngeal resistance were usually present when the larynx was in circuit (Fig. 1), but were absent when measured with the larynx out of circuit (Fig. 2) (see Discussion). In twelve tests on eight pigs, capsaicin had similar respiratory and cardiovascular effects to PBG, causing apnoeas s longer than the control expiratory durations, reducing B.P. by 9l % and heart rate by % (P < O1 in all cases) (Table 1). However, with capsaicin P.A.P. also fell significantly by % from a control value of mmhg. As shown in Figs. 1 and 2 there were significant increases in laryngeal resistance with both methods of determination and in both phases of breathing. After vagotomy, in six tests in six pigs, the effects on breathing, B.P., heart rate and P.A.P. were absent or weaker, and the effects on resistance of the larynx out ofcircuit were virtually abolished (Fig. 2); those on resistance of the larynx in circuit were still present (Fig. 1). Breuer-Hering reflex This was tested by constant-pressure inflations in nine pigs, before and after lung microembolism. The results were analysed both in terms of the absolute duration of expirations caused by inflation, and as the ratios of the expiratory duration to the previous control value. The latter method might make allowance for differences in breathing pattern. Fig. 3 shows the mean results in terms of inhibitory ratios for those pigs studied also after

4 98 G. AGUGGINI, M. G. CLEMENT AND J. G. WIDDICOMBE PBG Capsaicin 3 O** ~~~~~~~~ ~~~~~~~~ O/ to / /I I / X Control pin~e. v~ag. Control pin~e. v~ag. Fig. 1. Changes in laryngeal resistance (ordinate; mean flow-pressure relationship) in pigs breathing through the larynx, in response to injections of phenylbiguanide (PBG, left) and capsaicin (right). p.m.e. = after pulmonary microembolism; v.a.g. = after pulmonary microembolism and bilateral cervical vagotomy. Circles, expiratory resistance; crosses, inspiratory resistance. The continuous lines join points before injection and immediately after breathing starts after apnoea due to the drug, or the maximal response due to the drug. Vertical lines indicate S.E.M.s. All values are paired for each condition, and figures indicate numbers of tests in the same animals. * P < 1; *P < -5. I PBG Capsaicin 15 1 I ao / I 6l / VI Control pin~e. v~ag. Control pin~e. v-a-g. Fig. 2. As for Fig. 1 but showing changes in laryngeal resistance when the larynx was 'out of circuit' with the same constant airflow through it. With the vagi intact, there was usually almost complete closure of the larynx during the apnoeas induced by PBG and capsaicin, but these values are not included in the Figure which refers only to measurements when breathing restarted.

5 REFLEXES AND THE PIG LARYNX 99 lung microembolism. After bilateral vagotomy, inflation had no effect on expiratory duration, i.e. the inhibitory ratio was unity. Effect ofputting larynx in and out of circuit When the tracheostomy tube was opened so that the animals no longer breathed through the larynx, there were only small changes in the pattern of breathing; these were measured only for the first breath to prevent changes in blood gas tensions due to altered dead space having any action. Expiratory time decreased 7O%, inspiratory time decreased 5 % and VT increased 1%. None of these changes was statistically significant (P > 5). Flow-pressure relations through the larynx With the animals breathing through the lower tracheostomy, so that the larynx was isolated in situ, flow-pressure curves were determined during spontaneous breathing. Fig. 4 gives mean values for nine pigs, showing the relatively lower laryngeal resistance during inspiration compared with expiration, and the alinearity of the flow-pressure curves. Effects ofpulmonary microembolism Microembolism by hardened red blood cells increased pulmonary arterial pressure from to mmhg or by % (P < 1) without significant changes in B.P. and heart rate (Table 2). Microembolism induced only small changes in breathing pattern due principally to a decrease in the inspiratory time of % (P < 1), while the responses of VT and expiratory time were small. The effects of the larynx, changed from in to out of circuit, on breathing pattern after microembolism were insignificantly small and similar qualitatively to those seen before microembolism. The flow-pressure relationships of the isolated larynx were not significantly changed (Fig. 4). Microembolism did not appreciably change arterial blood gas concentrations: before microembolism the arterial 2 pressure (Pa,2) was mmhg, the arterial CO2 pressure (Pa C2) was mmhg and the ph was ; after microembolism, Pa,2 was mmhg, PaCo2 was mmhg and the ph was (n = 6). Fig. 3 shows the apnoea inhibitory ratios for the Breuer-Hering reflex for the pigs in which paired values for the same inflation pressures were used. There was considerable variation in the results after microembolism; this was due partly to the fact that pigs that had long apnoeas at low pressures (e.g. 3 and 7 cmh2o) were not tested at higher pressures; this variation would also affect the shape of the line. Seven of the nine pigs tested showed an enhanced Breuer-Hering reflex at all inflation pressures used before and after microembolism, and in four of the pigs the effect was very large, contributing to the large S.E.M.s. Two pigs showed small changes or no change in the strengths of the reflex at different pressures. Taking the results as a whole, there was a significant increase in the inhibitory ratio of % (P < 1). A similar conclusion is reached if the absolute values of the inflation-induced apnoeas are used. As shown in Figs. 1, 2 and 4, microembolism caused only small and insignificant changes in laryngeal resistance, whether the larynx was in or out of circuit. To test whether microembolism changes the strength of the vagal reflexes induced by PBG and capsaicin, the drugs were injected after microembolism and the responses were compared with earlier controls (Table 1, Figs. 1 and 2). After microembolism both drugs caused longer apnoeas than in the controls. However, the changes in B.P., heart rate and P.A.P. were very similar in size and not significantly different before and after micro-

6 1 G. AGUGGINI, M. G. CLEMENT AND J. G. WIDDICOMBE 17-5 o I o6 Cu-. 1* I 7o7 I k I o Inflation pressure (cmh2) Fig. 3. Strength of the Breuer-Hering reflex before and after microembolism and associated pulmonary hypertension. The ordinate gives the reflex inhibitory ratio, the ratio of the duration of inspiratory inhibition caused by lung inflation to the previous control expiratory duration. The abscissa gives inflation pressures. Crosses, control values with S.E.M.S; circles, mean values with S.E.M.s after pulmonary microembolism with figures indicating numbers of tests for each pressure. Only results when paired observations were made before and after microembolism are included. At all levels of inflation pressure the inhibitory ratios were increased during pulmonary hypertension. embolism. With regard to the larynx, tested by either method, the responses to PBG and capsaicin were not significantly different after microembolism compared with previous controls, although there was considerable variation in results (Figs. 1 and 2). Vagotomy, performed always after microembolism, caused qualitatively expected effects: an increase in VT and inspiratory time, and a decrease in breathing frequency and B.P. P.A.P. decreased but not nearly to control values. Perhaps unexpectedly, expiratory time decreased. The results are given in Table 2. DISCUSSION The purpose of the study was twofold: to study the control of laryngeal calibre in the pig by lung reflexes activated by PBG and capsaicin, drugs acting on pulmonary vagal receptors, and to study the effects of pulmonary hypertension caused by microembolism on lung and

7 REFLEXES AND THE PIG LARYNX 11 2 I t Flow (I. min-') Fig. 4. Flow-pressure relationships for the larynx isolated in situ during spontaneous breathing through a lower tracheal cannula. Crosses, during inspiration; circles, during expiration. Vertical crosses and filled circles, before pulmonary microembolism; diagonal crosses and open circles, after pulmonary microembolism. Vertical lines = S.E.M.s. There was no significant difference between the inspiratory and expiratory flow-pressure relationships before and after pulmonary microembolism. Table 2. Changes in variables due to pulmonary microembolism (p.m.e.) and effects of vagotomy p.m.e. (n = 1/1) Vagotomy (n = 7/7) Variable Control Change (%) Control Change (%) VT (ml) * f(min-') VE (l.min'1) ** t1 (s) l ** * te (S) * B.P. (mmhg) ± * H.R. (min') P.A.P. (mmhg) ** * Values are means+ S.E.M. *P< -5, **P<.1. n = animals/tests. f, breathing frequency; VE, expiratory airflow; t1, inspiratory time; te, expiratory time; H.R., heart rate.

8 12 G. AGUGGINI, M. G. CLEMENT AND J. G. WIDDICOMBE laryngeal respiratory reflex regulation. In particular, the pig has seldom been studied in relation to lung reflexes and apparently not at all for laryngeal control. In anaesthetized pigs, during quiet breathing, the laryngeal resistance has similar properties to those seen in cats, rabbits and dogs (Bartlett et al. 1973; Stransky et al. 1973; Dixon et al. 1974; Jammes et al. 1985). There is an alinear flow-pressure curve, with resistance being lower in inspiration when the larynx dilates. Of the two methods we used to assess laryngeal resistance, that with spontaneous flow with the larynx 'in circuit' might be inaccurate because flow-pressure loops are alinear and the average inspiratory and expiratory resistances assessed from them will depend on flow rate and pattern of breathing. However, they should give a qualitative indication of changes in laryngeal resistance. The assessment of resistance with the larynx 'out of circuit', with pressure measured at constant flow, should give a more precise indication of changes in laryngeal resistance at the given flow. In general the results with the two methods gave qualitatively similar results (Figs. 1 and 2). The method with larynx out of circuit allowed alinear flow-pressure volume curves to be plotted (Fig. 4). Strictly speaking laryngeal resistance is the slope of a curve at any point, and the flow-pressure ratios (as if straight lines were drawn through the origin) give only indications of changes in resistance. However, with rapidly changing conditions as after the injections of drugs, it is not practical to plot full flow-pressure curves. Paired values were always determined at the same flows and, if the shape of the curve remains constant, the change in ratios should be closely related to the change in resistance. Bartlett et al. (1973), Gautier et al. (1973) and Remmers & Bartlett (1977) showed that in cats the presence of the larynx in circuit significantly controls the duration of expiratory time and expiratory flow and therefore respiratory pattern. Our results show that in the anaesthetized pig the larynx is not a significant determinant of respiratory pattern, possibly because of the anaesthesia. Capsaicin and PBG have been used to elicit C fibre reflexes from the lungs in several mammalian species (Paintal, 1973; Coleridge & Coleridge, 1984, 1986), but have not been much studied in pigs. PBG and phenyldiguanide (PDG) have different chemical structures and both elicit lung reflexes by stimulation of lung C fibre receptors (Armstrong, Kay & Russell, 1985); it is not always clear which substance was used in some earlier studies. Some species show clear differences in sensitivity to capsaicin and PBG, such as the dog which does not respond appreciably to PBG. The pig responds to both drugs by vagal reflexes, presumably mediated by C fibres in the vagus nerves, as shown by the effects of vagotomy, which abolished the majority of the effects of both drugs; the exception was that some laryngeal changes remained, especially when the pigs were breathing through the larynx (Fig. 1). This last result is difficult to explain. 'Resistance' with the larynx in circuit is a mean value from a flow-pressure loop, which will depend on pattern of airflow in tidal breathing; however, vagotomy virtually abolished the respiratory effects of PBG and capsaicin. In other species PBG has been shown to have non-vagal effects (Glogowska & Widdicombe, 1973), although it had not been shown before that these were especially powerful on the larynx. These non-vagal effects could include a laryngeal constriction via the superior laryngeal nerves and the cricothyroid muscle; the main motor supply to the larynx, via the recurrent laryngeal nerves, is interrupted by cervical vagotomy and in this condition the cricothyroid acts as a constrictor of the larynx. Both PBG and capsaicin, when the vagi were intact, caused almost complete laryngeal closure during reflex apnoeas, as previously only shown in some species and usually hidden by the presence of tracheostomies. One of the main aims of this study was to try to clarify any lung reflex effects associated

9 REFLEXES AND THE PIG LARYNX 13 with the pulmonary hypertension due to microembolism and in particular to see whether reflexes acting on the larynx were activated. Results have only been given when the microembolism caused a large and clear pulmonary hypertension. We cannot say whether recorded changes were secondarily due to the pulmonary hypertension or to the primary microembolism, since the two were always associated. As Table 2 shows, during the large increases in P.A.P. after microembolism, other cardiovascular and respiratory changes were small in spite of the fact that the results of lung inflation and injection of drugs showed that pulmonary reflexes were potent. The only consistently significant effect due to microembolism was a decrease in inspiratory time. When lung reflexes were studied during microembolism, the most striking effect was a clear but variable enhancement of the Breuer-Hering reflex in the great majority of animals. This enhancement would be consistent with the shortening of inspiratory time. The responses to PBG and capsaicin were unaffected by microembolism with regard to changes in B.P., heart rate and P.A.P. However, the apnoeas induced by these drugs were far longer, and this might be correlated with the enhanced apnoeic reflex due to the Breuer-Hering reflex. The strength of the Breuer-Hering reflex, induced by constant-pressure inflations, would depend on the initial lung volume before inflation (functional residual capacity, F.R.C.). However it is unlikely that changes in F.R.C. would be a major influence, partly because microembolism did not induce changes in breathing that might be associated with an increase in F.R.C. (Table 2), and also because the size of the enchancement of the Breuer-Hering reflex (Fig. 3) was such that any increase in F.R.C. would have to be extremely large to explain the changes. Microembolism in itself had little consistent effect on laryngeal resistance, whether the larynx was in or out of circuit (Figs. 1 and 2). The laryngeal responses to PBG and capsaicin were not significantly changed in most instances after microembolism. A further consideration relevant to the reflexes involved in microembolism is the effect of bilateral cervical vagotomy, always performed after microembolism (Table 2). All the changes seen were 'classical' except that expiratory time was reduced whereas normally one associates bilateral vagotomy with an increase in expiratory time (Coleridge & Coleridge, 1986). The changes in expiratory time on vagotomy are related to the balance of activity of pulmonary stretch receptors for Breuer-Hering reflex (which extend expiratory time) and that of fibres from rapidly adapting irritant receptors (which shorten expiratory time) (Davies, Dixon, Callanan, Huszczuk, Widdicombe & Wise, 1978; Davies & Roumy, 1982). Therefore the results of vagotomy after microembolism are consistent with a relative enhancement of the Breuer-Hering reflex at the expense of lung irritant and C fibre reflexes. In conclusion, although it is known that pulmonary microembolism can stimulate C fibre receptors (Coleridge & Coleridge, 1984, 1986), rapidly adapting irritant receptors (Mills, Sellick & Widdicombe, 1969), and slowly adapting pulmonary stretch receptors (Armstrong, Luck & Martin, 1976), our results with this form of microembolism in the pig suggest that with microembolism and the associated pulmonary hypertension very considerable increases in P.A.P. can occur in animals which have all three types of reflex active; and yet the changes in pattern of breathing and response to reflex activation are consistent with only a small role for these reflexes. That which seems to be most changed is the Breuer-Hering inflation reflex mediated by slowly adapting pulmonary stretch receptors, although the mechanism whereby microembolism and pulmonary hypertension affects these nerve endings has not been clarified. This study was supported by grant N from the Italian National Research Council. We are grateful to Mrs M. A. Albertini, M. Marzani and M. Constanzi for their efficient technical assistance.

10 14 G. AGUGGINI, M. G. CLEMENT AND J. G. WIDDICOMBE REFERENCES ARMSTRONG, D. J., KAY, I. S. & RUSSELL, N. J. W. (1985). The pulmonary chemoreflexes evoked by phenyl biguanide, diguanide and guanidine in anaesthetized rabbits. Journal of Physiology 371, 115P. ARMSTRONG,. J., LUCK, J. C. & MARTIN, V. M. (1976). The effect of emboli upon intrapulmonary receptors in the cat. Respiration Physiology 26, BARTLETT JR, D., REMMERS, J. E. & GAUTIER, H. (1973). Laryngeal regulation of respiratory airflow. Respiration Physiology 18, CLEMENT, M. G., AGUGGINI, G. & MISEROCCHI, G. (1981). Respiratory modulation by vagal afferents in pigs. Quarterly Journal of Experimental Physiology 66, CLEMENT, M. G., DAMASIO, M., PARRAVICINI, R., RUSSO, V. & AGUGGINI, G. (1983). Respiratory responses to pulmonary microembolisation induced by hardened red blood cells. Italian Journal of Chest Diseases 37, COLERIDGE, H. M. & COLERIDGE, J. C. G. (1986). Reflexes evoked from tracheobronchial tree and lungs. In Handbook ofphysiology, section 3: The Respiratory System, vol. II. Control ofbreathing, part 1 ed. CHERNIACK, N. S. & WIDDICOMBE, J. G., pp Bethesda, MD: American Physiological Society. COLERIDGE, J. C. G. & COLERIDGE, H. M. (1984). Afferent vagal C-fiber innervation of the lungs and airways and its functional significance. Reviews of Physiology, Biochemistry and Pharmacology 99, DAVIES, A., DIXON, M., CALLANAN, D., HUSZCZUK, A., WIDDICOMBE, J. G. & WISE, J. C. M. (1978). Lung reflexes in rabbits during.pulmonary stretch receptor block by sulphur dioxide. Respiration Physiology 34, DAVIES, A. & ROUMY, M. (1982). The effect of transient stimulation of lung irritant receptors on the pattern of breathing in rabbits. Journal of Physiology 324, DIXON, M., SZEREDA-PRZESTASZEWSKA, M., WIDDICOMBE, J. G. & WISE, J. C. M. (1974). Studies on laryngeal calibre during stimulation of peripheral and central chemoreceptors, pneumothorax and increased respiratory loads. Journal of Physiology 239, GAUTIER, H., REMMERS, J. E. & BARTLETT JR, D. (1973). Control of the duration of expiration. Respiration Physiology 18, GLOGOWSKA, M., STRANSKY, A. & WIDDICOMBE, J. G. (1974). Reflex control of discharge in motor fibres to the larynx. Journal of Physiology 239, GLOGOWSKA, M. & WIDDICOMBE, J. G. (1973). Respiratory and circulatory responses to phenyl diguanide in rabbits. Archives Internationales de Pharmacodynamie et de Therapie 23, JAMMES, Y., DAVIES, A. & WIDDICOMBE, J. G. (1985). Tracheobronchial and laryngeal responses to hypercapnia, histamine and capsaicin in dogs. Clinical Respiratory Physiology 21, MILLS, J., SELLICK, H. & WIDDICOMBE, J. G. (1969). Activity of lung irritant receptors in pulmonary microembolism, anaphylaxis and drug-induced bronchoconstriction. Journal of Physiology 23, PAINTAL, A. S. (1973). Vagal sensory receptors and their reflex effects. Physiological Reviews 53, REMMERS, J. E. & BARTLETT JR, D. (1977). Reflex control of expiratory airflow and duration. Journal of Applied Physiology 42, SANT'AMBROGIO, G., MATHEW,. P., FISHER, J. T. & SANT'AMBROGIO, F. B. (1983). Laryngeal receptors responding to transmural pressure, airflow and local muscle activity. Respiration Physiology 54, STRANSKY, A., SZEREDA-PRZESTASZEWSKA, M. & WIDDICOMBE, J. G. (1973). The effect of lung reflexes on laryngeal resistance and motoneurone discharge. Journal of Physiology 231, WHITTERIDGE, D. (195). Multiple embolisms of the lung and rapid shallow breathing. Physiological Reviews 3, WIDDICOMBE, J. G. (1986). Reflexes from the upper respiratory tract. In Handbook of Physiology, section 3: The Respiratory System, vol. II, Control of Breathing, part 1, ed. CHERNIACK, N. S. & WIDDICOMBE, J. G., pp Bethesda, MD: American Physiological Society.