Bronchial vasodilator pathways in the vagus nerve of dogs.

Bronchial vasodilation in dogs is mediated largely by vagal pathways. To examine the relative contribution of cholinergic and noncholinergic parasympathetic pathways and of sensory axon reflexes to vagal bronchial vasodilation, we electrically stimulated the peripheral vagus nerve in 10 chloralose-anesthetized dogs and measured bronchial artery flow. Moderate-intensity electrical stimulation (which did not activate C-fiber axons) caused a rapid voltage- and frequency-dependent vasodilation. After atropine, vasodilation was slower in onset and reduced at all voltages and frequencies: bronchial vascular conductance increased by 9.0 +/- 1.5 (SE) ml. min-1. 100 mmHg-1 during stimulation before atropine and 5. 5 +/- 1.4 ml. min-1. 100 mmHg-1 after (P < 0.02). High-intensity stimulation (sufficient to recruit C fibers) was not studied before atropine because of the resulting cardiac arrest. After atropine, high-intensity stimulation increased conductance by 12.0 +/- 2.5 ml. min-1. 100 mmHg-1. Subsequent blockade of ganglionic transmission, with arterial blood pressure maintained by a pressure reservoir, abolished the response to moderate-intensity stimulation and reduced the increase to high-intensity stimulation by 82 +/- 5% (P < 0.01). In 13 other dogs, we measured vasoactive intestinal peptide-like immunoreactivity in venous blood draining from the bronchial veins. High-intensity vagal stimulation increased vasoactive intestinal peptide concentration from 5.7 +/- 1.8 to 18.4 +/- 4.1 fmol/ml (P = 0.001). The results suggest that in dogs cholinergic and noncholinergic parasympathetic pathways play the major role in vagal bronchial vasodilation.

Neural regulation of bronchial blood flow.

The neural regulation of the bronchial vasculature differs from that of the general systemic circulation in that vasodilator reflexes play a major part in determining blood flow. These reflexes originate in the upper or lower airways, in carotid chemoreptors or in cardiac chemosensitive nerves; those arising in the lower airways are most potent and may increase bronchial blood flow several-fold and cause swelling of the airway mucosa. Lower airway reflexes have afferent and efferent pathways in the vagus nerves, the former including sensory C-fibers and rapidly adapting receptors, the latter involving both cholinergic and non-cholinergic transmitters. In addition, neuropeptides released from the C-fiber terminals provide a local mechanism for vasodilation independent of central reflex control. This so-called axon-reflex plays the major part in bronchial vasodilation in rodents but makes only a small contribution in larger animals. In larger animals centrally-mediated reflexes and vagal vasodilator pathways appear more important. Nevertheless, local neural vasodilation may be important in airway disease; the factors that favor its operation in animals other than rodents deserve to be explored.

Reflex bronchial vasodilation in dogs evoked by injection of a small volume of water into a bronchus.

Injection of water into a lobar bronchus stimulates airway C-fibers and rapidly adapting receptors and evokes airway defense reflexes. To determine whether this stimulus also evokes a reflex increase in bronchial blood flow (Qbr), we injected 1-2 ml of water into a lobar bronchus in anesthetized dogs. Injection decreased arterial pressure but increased Qbr from 9 +/- 1 to 21 +/- 3 ml/min. The increase had a latency of 6-8 s and reached a peak after approximately 20 s; Qbr returned to control after 60-90 s. Airway mucosal blood flow, measured by colored microspheres, increased in proportion to Qbr. In contrast, flow in an adjacent intercostal artery that did not supply the airway decreased slightly. Injection of isosmotic saline had little effect. In 13 of 16 dogs, the water-induced increase in Qbr was abolished by cutting or cooling the cervical vagus nerves and hence was entirely dependent on centrally mediated vagal pathways. When the vagus nerves were intact, about one-third of the vasodilator response remained after pharmacological blockade of muscarinic and adrenergic receptors. We conclude that in dogs the defense response to water in the lower airways includes a large increase in Qbr that is partly due to activation of nonadrenergic noncholinergic autonomic pathways.

Contribution of vagal afferents to respiratory reflexes evoked by acute inhalation of ozone in dogs.

Acute inhalation of ozone induces vagally mediated rapid shallow breathing and bronchoconstriction. In spontaneously breathing anesthetized dogs, we attempted to determine whether afferent vagal C-fibers in the lower airways contributed to these responses. Dogs inhaled 3 ppm ozone for 40-70 min into the lower trachea while cervical vagal temperature was maintained successively at 37, 7, and 0 degrees C. At 37 degrees C, addition of ozone to the inspired air decreased tidal volume and dynamic lung compliance and increased breathing frequency, total lung resistance, and tracheal smooth muscle tension. Ozone still evoked significant effects when conduction in myelinated vagal axons was blocked selectively by cooling the nerves to 7 degrees C. Ozone-induced effects were largely abolished when nonmyelinated vagal axons were blocked by cooling to 0 degree C, breathing during ozone inhalation at 0 degree C being generally similar to that during air breathing at 0 degree C, except that minute volume and inspiratory flow were higher. We conclude that afferent vagal C-fibers in the lower airways make a major contribution to the acute respiratory effects of ozone and that nonvagal afferents contribute to the effects that survive vagal blockade.

Acute inhalation of ozone stimulates bronchial C-fibers and rapidly adapting receptors in dogs.

To identify the afferents responsible for initiating the vagally mediated respiratory changes evoked by acute exposure to ozone, we recorded vagal impulses in anesthetized, open-chest, artificially ventilated dogs and examined the pulmonary afferent response to ozone (2-3 ppm in air) delivered to the lower trachea for 20-60 min. Bronchial C-fibers (BrCs) were the lung afferents most susceptible to ozone, the activity of 10 of 11 BrCs increasing from 0.2 +/- 0.2 to 4.6 +/- 1.3 impulses/s within 1-7 min of ozone exposure. Ten of 15 rapidly adapting receptors (RARs) were stimulated by ozone, their activity increasing from 1.5 +/- 0.4 to 4.7 +/- 0.7 impulses/s. Stimulation of RARs (but not of BrCs) appeared secondary to the ozone-induced reduction of lung compliance because it was abolished by hyperinflation of the lungs. Ozone had little effect on pulmonary C-fibers or slowly adapting pulmonary stretch receptors. Our results suggest that both BrCs and RARs contribute to the tachypnea and bronchoconstriction evoked by acute exposure to ozone when vagal conduction is intact and that BrCs alone are responsible for the vagally mediated tachypnea that survives vagal cooling to 7 degrees C.

Cooling the pulmonary blood in dogs alters activity of pulmonary vagal afferents.

In open-chest anesthetized dogs with left and right lungs ventilated separately, we recorded changes in firing of right lung vagal receptors when 1.25 ml/kg cold (5 degrees C, 20 degrees C) blood were injected into the nonperfused right pulmonary artery. With the right lung inflated at constant pressure, effects of cold blood on individual pulmonary stretch receptors (PSRs) were frequency dependent, with discharge increasing or remaining unchanged if control frequency was low and decreasing if it was high. Consequently average PSR discharge was unchanged by cold blood when airway pressure was maintained at 5 cmH2O, but it decreased at pressures of 10 and 15 cmH2O. Cold blood stimulated rapidly adapting receptors (RARs) at all three pressures. Injection of blood at 37 degrees C had no effect. We conclude that changes in PSR activity account for the tachypnea induced by pulmonary arterial injection of cold blood (G. G. Giesbrecht and M. Younes. J. Appl. Physiol. 69: 1435-1441, 1990). With the right lung phasically ventilated, cold blood decreased PSR discharge in inflation, caused high-threshold PSRs to fire in deflation, and stimulated RARs. Pulmonary C-fibers were unaffected by cold blood. We suggest that PSRs and RARs initiate respiratory changes during hypothermia or exercise-induced asthma.

Capsaicin-induced bronchial vasodilation in dogs: central and peripheral neural mechanisms.

In 21 anesthetized dogs, we placed a flow probe around the right bronchial artery and examined changes in bronchial blood flow and bronchial vascular conductance when pulmonary C-fibers were stimulated by right atrial injection of capsaicin. When vagus nerves were intact, capsaicin evoked a pulmonary depressor chemoreflex and increased bronchial blood flow by 125% and bronchial vascular conductance by 175%; flow in an adjacent intercostal artery did not increase. Injection of color-coded microspheres revealed that blood flow to mucosa of lower trachea and to a peripheral bronchus doubled, whereas flow to posterior tracheal wall increased little. Cooling (to -1 degree C) or cutting cervical vagi (in 17 dogs) abolished the pulmonary chemoreflex and abolished all bronchial vascular effects in nine dogs but 33% of the vasodilation persisted in eight. In five of six dogs, this persisting vasodilation was potentiated by phosphoramidon (a neutral endopeptidase inhibitor that retards breakdown of neuropeptides released by C-fibers). Atropine reduced the capsaicin-induced bronchial vasodilation by approximately 30%. We conclude that the bronchial vasodilation was largely due to a centrally mediated vagal reflex and that a neuropeptide-dependent axon-reflex component was also present in about one-half the dogs.

Vagal afferent and reflex responses to changes in surface osmolarity in lower airways of dogs.

In anesthetized dogs we examined the sensitivity of afferent vagal endings in the lungs to changes in airway fluid osmolarity. Injection of 0.25-0.5 ml/kg water or hyperosmotic sodium chloride solutions (1,200-2,400 mmol/l) into a lobar bronchus caused bradycardia, arterial hypotension, apnea followed by rapid shallow breathing, and contraction of tracheal smooth muscle. All effects were abolished by vagotomy. We examined the sensory mechanisms initiating these effects by recording afferent vagal impulses arising from the lung lobe into which the liquids were injected. Water stimulated pulmonary and bronchial C-fibers and rapidly adapting receptors; isosmotic saline and glucose solutions were ineffective. Hyperosmotic saline (1,200-9,600 mmol/l, 0.25-1 ml/kg) stimulated these afferents in a concentration-dependent manner. Stimulation began 1-10 s after the injection and sometimes continued for several minutes. Responses of slowly adapting stretch receptors varied. Our results suggest that non-isosmotic fluid in the lower airways initiates defense reflexes by stimulating pulmonary and bronchial C-fibers and rapidly adapting receptors. Conceivably, stimulation of these afferents as a result of evaporative water loss from airway surface liquid could contribute to exercise-induced asthma.

Pulmonary C-fiber stimulation by capsaicin evokes reflex cholinergic bronchial vasodilation in sheep.

We investigated changes in bronchial blood flow (Qbr) associated with capsaicin-induced stimulation of pulmonary C-fibers in seven anesthetized and two unanesthetized sheep. A Doppler flow probe chronically implanted around the common bronchial artery provided a signal (delta F, kHz) linearly related to bronchial arterial blood velocity (Vbr, cm/s), which was proportional to Qbr. An index of bronchial vascular conductance (Cbr, in arbitrary units) was calculated as the ratio of Vbr to systemic arterial pressure (Pa). Right atrial injection of capsaicin evoked a prompt pulmonary chemoreflex (apnea, bradycardia, and hypotension), with immediate increases in Vbr (average +34%) and Cbr (+63%) that reached a maximum approximately 7 s after the injection. A second increase in Vbr, but not in Cbr, occurred approximately 12 s later, coinciding with an increase in Pa. Vagal cooling (0 degrees C) prevented the pulmonary chemoreflex; it also abolished the immediate increases in Vbr and Cbr in four of six sheep and substantially reduced them in two sheep; it did not affect the late increases in Vbr and Pa. Results after atropine indicated that the immediate increases in Vbr and Cbr were mainly cholinergic. In two sheep a small residual vasodilation survived combined cholinergic and adrenergic blockade and may have been due to peripheral release of neurokinins.

Cigarette smoke in lungs evokes reflex increase in tracheal submucosal gland secretion in dogs.

Stimulation of pulmonary C-fibers (PCs) by capsaicin and of rapidly adapting receptors (RARs) by reduced lung compliance reflexly increases airway submucosal gland secretion in dogs. Because both PCs and RARs are stimulated by cigarette smoke (nicotine being the primary stimulus), we performed experiments in anesthetized open-chest artificially ventilated dogs (with aortic nerves cut) to determine whether cigarette smoke reflexly stimulates airway secretion. We measured submucosal gland secretion by counting the hillocks in a 1.2-cm2 field of tracheal epithelium coated with tantalum dust. Secretion was stimulated by delivery of 40-320 ml smoke from high-nicotine cigarettes to the lower trachea, secretion rate increasing from 7.4 +/- 1.3 to 48.1 +/- 5.1 hillocks.cm-2.min-1. Results of cutting the pulmonary vagal branches or carotid sinus nerves or both indicated that the secretory response was initiated by stimulation of lower respiratory vagal afferents and augmented several seconds later by stimulation of carotid chemoreceptors. Results of cooling the cervical vagus nerves to 7 and 0 degrees C indicated that most of the vagally mediated increase in secretion was due to stimulation of afferent lung C-fibers.

Response of slowly adapting pulmonary stretch receptors to reduced lung compliance.

We examined the steady-state response of slowly adapting pulmonary stretch receptors (SAPSRs) to reduced lung compliance in open-chest cats with lungs ventilated at eupneic rate and tidal volume (VT) and with a positive end-expiratory pressure (PEEP) of 3-4 cmH2O. Transient removal of PEEP decreased compliance by approximately 30% and increased transpulmonary pressure (Ptp) by 1-2.5 cmH2O. Reduction of compliance significantly decreased SAPSR discharge in deflation and caused a small increase in discharge at the peak of inflation; it had little effect on discharge averaged over the ventilatory cycle. Increasing VT to produce a comparable increase in Ptp significantly increased peak discharge. Thus unlike rapidly adapting receptors, whose discharge is increased more effectively by reduced compliance than by increased VT, SAPSRs are stimulated by increased VT but not by reduced compliance. We speculate that the most consistent effect of reduced compliance on SAPSRs (the decrease in deflation discharge) was due to the decreased time constant for deflation in the stiffer lung. This alteration in firing may contribute to the tachypnea evoked as the lungs become stiffer.

Intravenous injection of hypertonic NaCl solution stimulates pulmonary C-fibers in dogs.

Intravenous injection of hypertonic NaCl solution evokes reflex bradycardia and hypotension, effects thought to result from stimulation of afferent vagal endings in the lungs. To identify the afferents responsible for these effects, we recorded vagal impulses arising from endings in the lungs and lower airways of anesthetized dogs and examined the response to injection of hypertonic solutions into the pulmonary circulation. Injection of 4,800 mmol/l NaCl solution (1 ml/kg) stimulated 39 of 49 pulmonary C-fibers, their impulse frequency increasing 35-fold. Stimulation was concentration dependent, the minimum effective concentration being between 1,200 and 4,800 mmol/l. Rapidly adapting receptors were also stimulated in a concentration-dependent manner, 35 of 41 receptors being stimulated by 4,800 mmol/l NaCl solution, firing increasing fivefold. Bronchial C-fibers were not stimulated by injection into the pulmonary circulation but were by injection into the bronchial artery. Hypertonic urea solutions had qualitatively similar but smaller effects on pulmonary C-fibers and rapidly adapting receptors. The results suggest that the reflex effects of intravenous injection of hypertonic solutions result principally from stimulation of pulmonary C-fibers.

Reflex coronary vasodilation evoked by chemical stimulation of cardiac afferent vagal C fibres in dogs.

1. Veratridine injected into the coronary circulation stimulates afferent vagal endings in the heart to evoke bradycardia and systemic hypotension (Bezold-Jarisch reflex, coronary chemoreflex) and coronary vasodilation. We have examined certain features of the reflex coronary vasodilator response in anaesthetized dogs. 2. When the circumflex coronary artery was perfused at constant pressure (100 mmHg), injection of veratridine (0.3 micrograms kg-1) into the anterior descending artery decreased blood pressure and heart rate, and increased circumflex blood flow by 54%; when heart rate was kept constant, circumflex flow increased by 57%. The increase in circumflex flow was reduced 63% by atropine, and finally abolished by phentolamine. 3. During severe coronary underperfusion (perfusion pressure 45 mmHg), veratridine still increased coronary flow by 35%, an increase amounting to 24-64% of the coronary vascular reserve. Flow increased in all layers of the myocardium, but the relative distribution of flow between subendocardial and subepicardial layers was unaltered. 4. Veratridine stimulates both mechanosensitive and chemosensitive cardiac endings. Stimulating chemosensitive afferents selectively by injecting capsaicin (1.5 micrograms kg-1) into the anterior descending artery decreased blood pressure and heart rate, and increased circumflex flow by 50% (and by 36% when heart rate was kept constant). 5. In ten of fifteen dogs, veratridine and capsaicin still evoked coronary vasodilatation when vagal A fibres were blocked selectively by cooling to 7.5 degrees C, the increase in coronary flow averaging 45% of that at 37 degrees C. All responses were abolished by cooling to 0 degrees C. 6. We conclude that coronary vasodilatation can be evoked by selective stimulation of cardiac chemosensitive vagal C fibres, although the coronary vasodilation of the veratridine-induced Bezold-Jarisch reflex may be due to stimulation of both mechanosensitive and chemosensitive C fibres. We speculate that during periods of coronary underperfusion ischaemic stimulation of chemosensitive vagal C fibres evokes a reflex dilatation of the coronary vascular bed that supplements the dilatation dependent upon autoregulatory mechanisms.

Rapidly adapting receptors monitor lung compliance in spontaneously breathing dogs.

We examined the ability of rapidly adapting receptors (RARs) to monitor changes in dynamic lung compliance (Cdyn) in anesthetized spontaneously breathing dogs by recording RAR impulses from the vagus nerves. We decreased Cdyn in steps through the physiological range by briefly restricting lung expansion with an inflatable cuff around the chest and recording the response after deflating the cuff; we restored Cdyn to control by hyperinflating the lungs. Of 45 RARs, 34 were stimulated by a 40 +/- 2% reduction in Cdyn, their inspiratory discharge increasing on average more than threefold. Two-thirds of responsive RARs were stimulated by less than or equal to 20% reductions in Cdyn; in most, firing increased proportionately with lung stiffness (1/Cdyn) as Cdyn was decreased further. Stimulation by reduced Cdyn was not simply a function of the concomitant increase in transpulmonary pressure, because similar increases in pressure produced by increasing tidal volume produced smaller increases in firing. RAR stimulation was unaffected by atropine and, hence, was not dependent on neurally mediated changes in bronchomotor tone. Our results indicate that during spontaneous breathing RARs provide a signal inversely proportional to Cdyn.

Usefulness and limitations of regional cardiac sympathectomy by phenol.

We evaluated the completeness and extent of regional sympathetic denervation of the left ventricle after epicardial painting with phenol in anesthetized dogs. In a region encircled by phenol, the effect of electrical stimulation of efferent sympathetic fibers on myocardial contractility and coronary vascular resistance was completely abolished within 30 min. Denervation extended to untreated regions innervated by sympathetic fibers crossing the phenol line. For at least 4 h after phenol application, intravenous infusion of isoproterenol or coronary arterial infusion of tyramine increased myocardial contractility in the denervated region; norepinephrine content and neurotransmitter uptake were normal, indicating that nerve terminals, postjunctional receptors, and myocardium remained functional. However, after 3-14 days, tissue catecholamine content and transmitter uptake in the encircled area were markedly reduced. The results suggest that careful evaluation is necessary in selecting a fully innervated control region in studies employing regional sympathetic denervation with phenol.

Pulmonary rapidly adapting receptors reflexly increase airway secretion in dogs.

We attempted to determine whether stimulation of pulmonary rapidly adapting receptors (RARs) increase tracheal submucosal gland secretion in anesthetized open-chest dogs. Electroneurographic studies of pulmonary afferents established that RARs but not lung C-fibers were stimulated by intermittent lung collapse during deflation, collapse being produced by removing positive end-expiratory pressure (PEEP, 4 cmH2O) or by applying negative end-expiratory pressure (NEEP, -4 cmH2O). We measured tracheal secretion by the "hillocks" method. Removing PEEP or applying NEEP for 1 min increased secretion from a base line of 6.0 +/- 1.1 to 11.8 +/- 1.7 and 22.0 +/- 2.8 hillocks.cm-2.min-1, respectively (P less than 0.005). After PEEP was restored, dynamic lung compliance (Cdyn) was 37% below control, and secretion remained elevated (P less than 0.05). A decrease in Cdyn stimulates RARs but not other pulmonary afferents. Hyperinflation, which restored Cdyn and RAR activity to control, returned secretion rate to base line. Secretory responses to lung collapse were abolished by vagal cooling (6 degrees C), by pulmonary vagal section, or by atropine. We conclude that RAR stimulation reflexly increases airway secretion. We cannot exclude the possibility that reduced input from slowly adapting stretch receptors contributed to the secretory response.

Stimulation of vagal pulmonary C-fibers by a single breath of cigarette smoke in dogs.

Inhalation of cigarette smoke into the lower airway via a tracheostomy evokes immediate apnea, bradycardia, and systemic hypotension in dogs. These responses can still be evoked when conduction in myelinated vagal fibers is blocked preferentially by cooling but are abolished by vagotomy, suggesting that they are mediated by afferent vagal C-fibers. To examine this possibility, we recorded impulses in pulmonary C-fibers in anesthetized, open-chest dogs and delivered 120 ml cigarette smoke to the lungs in a single ventilatory cycle. Pulmonary C-fibers were stimulated within 1 or 2 s of the delivery of smoke generated by high-nicotine cigarettes, activity increasing from 0.3 +/- 0.1 to a peak of 12.6 +/- 1.3 (SE) impulses/s, (n = 60); the evoked discharge usually lasted 3-5 s. Smoke generated by low-nicotine cigarettes evoked a milder stimulation in 33% of pulmonary C-fibers but did not significantly affect the overall firing frequency (peak activity = 2.2 +/- 1.1 impulses/s, n = 36). Hexamethonium (0.7-1.2 mg/kg iv) prevented C-fiber stimulation by high-nicotine cigarette smoke (n = 12) but not stimulation by right atrial injection of capsaicin. We conclude that pulmonary C-fibers are stimulated by a single breath of cigarette smoke and that nicotine is the constituent responsible.