Temperature effects on CO2-sensitive intrapulmonary chemoreceptors in the lizard, Tupinambis nigropunctatus

Body temperature (Tb) effects on CO2 responses of 17 intrapulmonary chemoreceptors (IPC) were investigated in 9 anesthetized (pentobarbital; 30 mg/kg) and unidirectionally ventilated tegu lizards (Tupinambis nigropunctatus). At 30 degrees C, all IPC (n = 15) had a stable discharge pattern. At 20 degrees C, IPC discharge (n = 14) was stable at high PCO2 but irregular at low PCO2 and often (10/14) consisted of bursts of activity separated by one or more seconds of quiescence. Responses of IPC to static and dynamic changes in PCO2 were quantified at both Tb and the discharge rate vs PCO2 response curves were compared. Static discharge frequency (fSTAT) decreased as PCO2 increased at both Tb. At 20 degrees C: (1) fSTAT was diminished at all PCO2 levels relative to 30 degrees C; and (2) the slope of the fSTAT vs PCO2 relationship was markedly attenuated. The Q10 was 3.7 +/- 0.5 and was independent of PCO2. The peak discharge associated with a step decrease in PCO2 (dynamic response; fDYN) also decreased as PCO2 increased. At 20 degrees C: (1) fDYN was diminished at all PCO2 levels relative to 30 degrees C; but (2) the slope of the fDYN vs PCO2 relationship was similar at both Tb. The Q10 was 2.6 +/- 0.3 and was significantly less than the Q10 of fSTAT (P less than 0.05). Acute changes in Tb exert large effects on the CO2 response and discharge pattern of IPC; these effects on IPC may be important in ventilatory control at different Tb in lizards.

Release of multiple tachykinins from capsaicin-sensitive sensory nerves in the lung by bradykinin, histamine, dimethylphenyl piperazinium, and vagal nerve stimulation

Recent evidence suggests that activation of airway C-fibers, besides causing afferent transmission, also causes release of transmitters from peripheral endings, probably via local axon reflexes, resulting in effects on vascular and bronchial smooth muscle, i.e., vasodilatation, increase in vascular permeability, and bronchoconstriction. In the present study, the release of tachykinins was investigated in the perfused guinea pig lung by various ways of neuronal activation. Substance-P-like immunoreactivity (SP-LI) and neurokinin-A-like immunoreactivity (NKA-LI) was determined by radioimmunoassay in the perfusates. A significantly increased outflow of both SP-LI and NKA-LI was observed during perfusion of the lung with high potassium concentration (60 mM), the C-fiber activator capsaicin (1 microM), bradykinin (1 microM), histamine (100 microM), or the nicotinic agonist dimethylphenyl piperazinium (DMPP) (32 microM). Release of both SP-LI and NKA-LI could also be achieved by electrical stimulation of vagal nerves. The percental increase varied from 80 to 1,000% depending on the kind of stimulus. The release of tachykinins by K+ or capsaicin was greatly reduced in calcium-free medium. Release by histamine was completely inhibited by 1 microM mepyramine, and release by DMPP was abolished by 20 microM hexamethonium. High performance liquid chromatography indicated that NKA-LI consisted of several cross-reacting substances, presumably other peptides of the tachykinin family. Among the isolated mammalian tachykinins, NKA was the most potent one to contract tracheal smooth muscle of guinea pigs in vitro, followed by neurokinin B and by SP. Both NKA and SP relaxed the guinea pig pulmonary artery with similar potency.(ABSTRACT TRUNCATED AT 250 WORDS)

[Effect of chemical sympathectomy on the function of the pulmonary surfactant system in white rats]

Rats received guanethidine (50 mg/kg) daily during the period from 2 weeks to 3 months to exclude selectively sympathetic irritation in order to study its influence on the phospholipid status of the lung surfactant component. An increase in total phospholipids of lung extracts as well as that of separate fractions of phospholipids especially of phosphatidylcholine was revealed along with a direct interrelationship between the alveoli area and the phosphatidylcholine rate. The alterations in surfactant phospholipids are regarded as a compensatory reaction maintaining alveolar stabilization in the case of sympathectomy.

The effect of human heart-lung transplantation upon breathing at rest and during sleep

We have assessed the contribution of intrathoracic pulmonary nerves to the control of breathing in humans. During relaxed wakefulness and during sleep the level, pattern and variability of breathing have been quantified in 8 healthy patients 1 month to 2 years after combined heart--lung transplantation. These data have been compared with similar data from both of 2 matched control groups; either healthy normal controls, or healthy patients after heart transplantation alone. We found no significant differences in the mean levels of respiratory variables between the 3 groups either during relaxed wakefulness or sleep. There were no significant differences between groups (other than would be expected by chance alone) either in the variability of breathing, or in the shapes of the frequency distributions of respiratory variables during these states. There were no respiratory disorders associated with sleep nor any disturbances in blood gases in any group. We conclude that in man breathing is remarkably normal, during relaxed wakefulness and during sleep, after chronic pulmonary denervation. When the ventilatory demands are minimal the human ventilation system functions normally in the absence of a control loop involving pulmonary proprioceptors and the medullary respiratory centres.

Microspectrofluorimetric analysis of the formaldehyde-induced fluorophores of 5-hydroxytryptamine and dopamine in intrapulmonary neuroepithelial bodies after administration of L-5-hydroxytryptophan and L-dopa

To demonstrate the intracellular store of 5-hydroxytryptamine and dopamine in pulmonary neuroepithelial bodies of the neonatal rabbit after treatment with the corresponding amino-acid precursors L-5-hydroxytryptophan or L-3,4-dihydroxyphenylalanine, formaldehyde-induced fluorescence in combination with microspectrofluorimetric analysis has been used. Emission spectra and excitation spectra in an extended wavelength range from 240 to 460 nm, the displacement of excitation peaks after exposure to hydrochloric acid vapour, and calculation of peak ratio values 410/260, 380/320, 320/260 for phenylethylamine fluorophores and 385/315 for indolylethylamine fluorophores were performed. Thus, the presence of 5-hydroxytryptamine without occurrence of 5-hydroxytryptophan was demonstrated in pulmonary neuroepithelial bodies after administration of the corresponding biological precursor, while dopamine combined with 5-hydroxytryptamine were clearly revealed after administration of L-3,4-dihydroxyphenylalanine. The rate of photodecomposition always corroborated these findings.

Reciprocal action of pulmonary vagal afferents on tracheal smooth muscle tension in dogs

Tracheal smooth muscle usually relaxes when the lungs are transiently inflated, an effect attributed to inhibitory input from pulmonary stretch receptors (PSRs). Relaxation is often followed by contraction, however, and occasionally contraction is the sole response. We attempted to identify the afferents responsible for this reflex contraction. In anesthetized, artificially ventilated dogs with open chest we recorded transverse tension in an upper tracheal segment innervated only by the superior laryngeal nerves and periodically hyperinflated the lungs as the cervical vagus nerves were cooled. Hyperinflation usually evoked tracheal relaxation when vagal temperature was 37 degrees C, but contraction became more frequent as temperature decreased and was the sole response below 8 degrees C. We hypothesise that above 6 degrees C contraction was triggered by rapidly adapting receptors and lung C fibers, whereas below 6 degrees C only C fibers were involved. Contraction, which appeared to represent the bronchomotor counterpart of Head's paradoxical reflex, was abolished below 2 degrees C. Cooling alone without periodic hyperinflation increased baseline tracheal tension to a maximum at 7-8 degrees C; further cooling often decreased tension, sometimes to control levels. Cutting the pulmonary vagal branches abolished these effects. Our results indicate that PSRs and C fibers act reciprocally, one causing bronchodilation, the other bronchoconstriction, and that background activity in C fibers may contribute to bronchomotor tone, an effect unmasked by selectively blocking A fibers.

Power spectra of inspiratory nerve activity with lung inflations in cats

To investigate the effect of lung inflations on the high-frequency synchrony (70-122 Hz) observed in the inspiratory activity of respiratory motor nerves of decerebrate cats, I applied a step increase in lung inflation pressure at fixed delays into the inspiratory phase and computed power spectra of phrenic neurograms before and during inflation. In 25 decerebrate paralyzed cats the frequency of the high spectral peak was 92.3 +/- 11.1 Hz before and 105.3 +/- 12.1 Hz during the step in inflation pressure, shifting upward by 13.0 +/- 6.0 Hz. For 8 of the 25 cats, the recurrent laryngeal and phrenic neurograms were recorded simultaneously. The high spectral peak was present during inspiration in the recurrent laryngeal power spectra and coherent with the high peak in the phrenic power spectra. In response to lung inflation, the high peak disappeared from the power spectra of the recurrent laryngeal nerve as the inspiratory activity was inhibited; a shift upward in frequency was not detectable. Comparing inspiratory times (TI, based on the phrenic neurograms) for breaths with no lung inflations to those for breaths with lung inflations, I found that lung inflations early in inspiration caused a decrease in TI, lung inflations at intermediates times had no effect on TI, and lung inflations late in inspiration caused an increase in TI. Despite lung inflation decreasing, not affecting, or increasing inspiratory duration and amplitude of the phrenic neurogram, lung inflation always caused a shift upward in the high-frequency peak of the phrenic power density. The fact that lung inflation, a powerful respiratory stimulus, affected the frequency of the high peak in a consistent manner suggests that the high-frequency synchrony is an important and robust feature of the central respiratory pattern generator.

[Neural influences on pulmonary vasomotility in man]

In man, the extent, the physiological role and even the existence of neural influences on the pulmonary vasomotility are not defined. A major obstacle to these evaluations is passive adaptation of the lung vessels to changes in flow, making it hard to separate the active from the passive component of vasomotion during adrenergic activation. In this study, which was carried out in 8 patients during diagnostic procedures, the inflation of a balloon in the inferior vena cava was utilized to reduce blood flows through the lungs and to buffer its changes consequent to stimulation of the adrenergic system. Neural activation was obtained with the mental arithmetic and the cold pressor tests. When venous return was unimpeded, cardiac output rose by 2,060 ml during the former and remained stable during the latter test, and arteriolar resistance in the pulmonary circuit was significantly reduced and slightly increased, respectively. Under vena cava obstruction, cardiac output decreased in the baseline by 600 ml; it rose by 925 ml during the arithmetic test and again remained stable during the cold test; pulmonary arteriolar resistance was unchanged since before obstruction in the baseline and was significantly augmented by both test. These observations support the view that lung blood vessels in man are sensitive to adrenergic influences and the vasoconstrictor component of these becomes effective in circumstances in which the flow through the lungs reduced. The physiological importance of these influences remains unclear.

Attenuation of pulmonary afferent input by vagal cooling in dogs

In open chest, artificially ventilated, anesthetized dogs, we examined the effect of vagal cooling on the pulmonary afferent input evoked by hyperinflating the lungs to 3 VT, recording the activity of slowly adapting pulmonary stretch receptors (PSRs), rapidly adapting receptors (RARs) and pulmonary C fibers rostral to the cooling platform. At 15 degrees C and below, input in all three types of fiber was significantly reduced, attenuation being least marked in C fibers. Between 12 degrees C and 7 degrees C, attenuation of RAR input was significantly less than that of PSRs. At 7 degrees C, virtually none of the hyperinflation-evoked increase in PSR activity and only 10% of that in RARs passed the cooling platform--indeed RAR input was less than during normal ventilation at 37 degrees C; by contrast, 40% of the hyperinflation-evoked increase in C fiber activity was still transmitted. Cooling had similar effects on C fiber input evoked by capsaicin. If reflexes are attenuated in proportion to the attenuation of afferent input, our results suggest that a hyperinflation-evoked reflex that survives vagal cooling below 6 degrees C is almost certainly triggered by C fibers.

[Characteristics of the secretory activity of the respiratory cells of mouse lungs after partial chemical sympathectomy]

Partial "sympathectomy" in the neonatal BALB mice was achieved by the treatment with guanethidine. The number of neurons in the stellate ganglion decreased to 20% of the control values and remained constant throughout the subsequent period of 1 month. Partial "chemical sympathectomy" did not affect the postnatal growth and development of the lungs. Partial "chemical sympathectomy" significantly increased the number of secreting cells in bronchiolar and alveolar regions. Secretory activity of the alveolocyte population, type two, synthetizing and secreting surfactant also increased. It has been concluded that the partial "chemical sympathectomy" stimulated the alveolar surfactant secretion.

Calcitonin gene-related peptide and the lung: neuronal coexistence with substance P, release by capsaicin and vasodilatory effect

The occurrence and distribution of calcitonin gene-related peptide (CGRP) in the lower airways was studied by means of immunohistochemistry and radioimmunoassay (RIA) in combination with high performance liquid chromatography (HPLC). CGRP-like immunoreactivity (-LI) was observed in nerves from the epiglottis down to peripheral bronchi in rat, cat and guinea pig and also in human bronchi. Double staining revealed colocalization of CGRP-LI and substance P (SP)-LI in cell bodies of nodose and jugular ganglia as well as in axons and nerve terminals of the airways. Systemic capsaicin pretreatment induced a marked loss of the CGRP- and SP-immunoreactive (-IR) nerves in the lower airways. CGRP-IR was also present in epithelial endocrine cells and neuroepithelial bodies. The content of CGRP-LI as measured with RIA in guinea pig bronchi was significantly lower after capsaicin pretreatment. Analysis of human bronchial extracts revealed that CGRP-LI coeluted with synthetic human CGRP on HPLC. In the isolated perfused guinea pig lung capsaicin exposure caused overflow of CGRP-LI suggesting release from peripheral branches of sensory nerves. Both in vivo experiments in the guinea pig measuring insufflation pressure as well as in vitro studies on isolated guinea pig and human bronchi showed that whereas tachykinins contracted bronchial smooth muscle no contractile or relaxing effect was elicited by human or rat CGRP. However, CGRP caused relaxation of serotonin precontracted guinea pig and human pulmonary arteries. In conclusion, the presence and release of CGRP-LI from capsaicin sensitive nerves in the lower airways adds another possible mediator, in addition to tachykinins, of vascular reactions upon sensory nerve irritation.

Effect of sino-aortic denervation in comparison to cardiopulmonary deafferentiation on long-term blood pressure in conscious dogs

The isolated and combined influence of cardiopulmonary and sinoaortic denervation on long-term blood pressure (MAP), heart rate (HR), plasma renin activity (PRA) and plasma volume (PV) was studied in 11 conscious, chronically instrumented foxhounds receiving a normal sodium diet. MAP, HR, PV and PRA remained unchanged in the 5 dogs after bilateral thoracic vagal stripping, which eliminates the cardiopulmonary afferents. After sino-aortic denervation in another 5 dogs there was equally little change when compared to the control group. Only total baroreceptor and cardiopulmonary denervation (7 dogs) revealed significantly higher levels of MAP (119.6 +/- 4.6 vs. 100.4 +/- 1.5, P less than 0.01), HR (118.2 +/- 3.7; vs. 84.1 +/- 3.5; P less than 0.0001), and PRA (3.6 +/- 0.9 vs. 0.9 +/- 0.2; P less than 0.05). In conclusion, the function of either arterial baroreceptors or cardiopulmonary receptors is sufficient for normal circulatory control. When both groups of receptor afferents are interrupted, MAP, HR, and PRA rise to significantly higher levels. Thus, both systems interact in a sense of a non-additive attenuation on "cardiovascular centres". This may clarify previous disputes concerning neurogenic hypertension, and supplies information for the role of the renin-angiotensin system in blood pressure control.

Tonic sensory pathways of the respiratory system

Both respiratory centres and the preganglionic vagal motoneurones, which control respiratory (striated) and airway (smooth) muscles respectively, receive information on the lungs, the circulation and the skeletal and respiratory muscles. Each of these nervous pathways has two components: one is phasic, i.e. in phase with biological rhythms, and comes from mechanoreceptors connected to large myelinated fibres; the second has a tonic low frequency firing rate and corresponds to the spontaneous activity of polymodal receptors connected to thin sensory fibres, which act mostly as sensors of changes in extracellular fluid composition (O2 and/or CO2 partial pressure, pH, release of algesic agents etc...). Some of them also detect large mechanical disturbances or local temperature changes. The influence of tonic background sensory activity is well known in animals concerning the role played by arterial chemoreceptors in the control of ventilation and of thin vagal afferents from the lungs (bronchopulmonary C-fibres and irritant receptors) in reflex facilitation of the bronchoconstrictor vagal tone. Moreover, the stimulation of thin sensory fibres in particular circumstances is responsible for hyperventilation (arterial chemoreceptors and muscle afferents), increased airway tone (arterial chemoreceptors and mostly thin vagal afferent fibres) or bronchodilation (muscle afferents). These peripheral inputs project centrally on different structures and also on brain stem neurones, which integrate simultaneously chemosensory, vagal and muscle information. This results in complex interactions between the different sensory pathways.

Age-related changes in vasoactive intestinal polypeptide levels and distribution in the rat lung

Vasoactive intestinal polypeptide (VIP) levels and distribution were studied in the lung of young-adult (3-month-old) and aged (28-month-old) male Wistar rats by radioimmunoassay and immunofluorescence. VIP concentrations were reduced approximately by 60% as the animal ages. The density of VIP-immunoreactive nerve fibres was remarkably reduced within bronchial smooth muscle and bronchial glands. Moreover, the number of VIP-immunoreactive nerve cell bodies located in intraparenchymal ganglia was decreased in old rats. The density of VIP-containing perivascular plexuses was slightly reduced in senescence. The present data are indicative that VIP neuronal system is impaired in the lung of old rats. In view of the significant age-dependent loss of VIP-immunoreactive nerve fibres that supply the bronchial tree and bronchial glands it cannot be excluded that the relaxant action exerted by peptide on airway smooth muscle and the control of bronchial secretion exerted by VIP are impaired in old age.

[Morphology of the apudocytes and neuroepithelial bodies in the lungs of rats]

Endocrine cells (apudocytes) and neuroepithelial bodies (NEBs) were revealed in the lungs of rats of different ages by Grimelius' argyrophilic method. Solitary apudocytes were found among the bronchial epitheliocytes, they had the oval, columnar or triangular shape. NEBs comprise groups of argyrophilic cells, in some cases the penetration of neural fibers into the bodies and their branching in the terminals are observed. Apudocytes and NEBs in 1-, 7-, and 15-day-old rats are more numerous in the epithelium of small than big bronchi. In 21- and 30-day-old and in adult animals apudocytes and NEBs are very scarce. The elements under study are likely to take part in the early postnatal development of the rat lungs.

Volume acceleration as an index of neuromuscular output

At the transition from expiration to inspiration, when flow and volume changes are small, changes in the respiratory system driving pressure could determine the degree of volume acceleration (AI), which, in turn, could reflect the degree of respiratory center output. To test this hypothesis, we calculated AI occurring in each respiratory cycle at the transition from expiration to inspiration during CO2 rebreathing in 4 healthy supine subjects. To minimize the flow and volume change over the measurement interval, we measured AI just prior to inspiration within the limits of an expiratory flow of 0.2 L . sec -1 to zero flow using digital differentiation. We also measured mouth pressure 100 msec after the onset of inspiration (P0.1) during intermittent transient inspiratory airway occlusions. During CO2 rebreathing AI increased significantly with both increasing PCO2 and P0.1. We also compared pairs of rebreathing studies, performed without and with an alinear (16 cm H2O . L -1 . sec -1) inspiratory resistor (IR), repeated twice in the 4 subjects. IR markedly decreased delta VE/delta PCO2 and the slope of the increase in mean inspiratory flow rate with PCO2 (delta VT/TI/delta PCO2) but did not significantly alter either delta AI/delta PCO2 or the increase in P0.1 with PCO2 (delta P0.1/delta PCO2). However, the effects of IR on AI and P0.1 differed between the early and late phases of each rebreathing run; early in the rebreathing runs (PCO2 = 55 Torr) IR increased both AI and P0.1 by a similar amount, but near the end of rebreathing (PCO2 = 60 Torr) IR increased P0.1 but not AI. Our results are consistent with the possibility that AI reflects neuromuscular output under the conditions of the study. Hence this approach justifies further evaluation to determine its general applicability.

Influence of dopamine and norepinephrine on the central ventilatory response to hypoxia in conscious cats

The effects of intravenous administration of agonists and antagonists of dopamine (DA) and norepinephrine (NE) on the central ventilatory response to hypoxia were studied in unanesthetized cats. The experiments were performed in intact animals exposed to CO-hypoxia and in carotid-body denervated animals breathing a hypoxic mixture. The DA agonist, apomorphine (APO) significantly decreased minute ventilation in response to central hypoxia, whereas the opposite effect occurred with the DA antagonist, haloperidol (HAL). Indeed, the characteristic tachypnea elicited by CO or hypoxic hypoxia was inhibited by APO as the respiratory frequency markedly decreased while tidal volume concomitantly increased. Conversely, HAL administration enhanced the tachypnea during milder hypoxia or reversed the inhibitory action of APO. In contrast, the NE agonist, clonidine (CLO) and antagonists propranolol (PRO) and phenoxybenzamine (PHE) did not cause significant changes in minute ventilation and breathing pattern although CLO tended to attenuate the hypoxic tachypnea. This study confirms, therefore, that catecholamines are involved in the central ventilatory response to hypoxia and suggests that the brain dopaminergic system plays a major role in the CO or hypoxic tachypnea.

Autoradiographic visualization of muscarinic receptors in pulmonary nerves and ganglia

We investigated autoradiographically the distribution of muscarinic receptors in bovine airways using (-)-[3H]quinuclidinyl benzilate as radioligand. The autoradiographs demonstrated the presence of muscarinic receptors in smooth muscle as well as neuronal muscarinic receptors in pulmonary nerves and ganglia. It is reasonable to believe that the neuronal muscarinic receptors participate in the regulation of neurotransmitter release at the peripheral nerve terminals innervating the bronchial smooth muscle.

Connectivity of slowly adapting pulmonary stretch receptors with dorsal medullary respiratory neurons

1. Intracellular recordings were made from 50 dorsal respiratory group (DRG) neurons in the region of the ventrolateral nucleus of the solitary tract in anesthetized, paralyzed cats ventilated with a cycle-triggered pump whose inflation stroke was triggered by the onset of phrenic nerve inspiratory (I) discharge. Activity was recorded simultaneously in the ipsilateral nodose ganglion from sensory cell bodies of slowly adapting pulmonary stretch receptors (PSRs). 2. Respiratory cycle-related membrane potential changes of DRG neurons were recorded. Twenty-six neurons that did not exhibit spikes were classified as I alpha, I beta or pump (P)-cells by comparing their membrane potential trajectories during I in the presence of lung inflation with that observed during I, but with lung inflation withheld. The remaining 24 neurons were classified similarly, but the classification was based upon a comparison of their I-phase spike activity responses with and without lung inflation. I phase-related histograms of either membrane potential or spike activity were constructed to facilitate DRG neuronal classification. Additionally, steady lung inflation of varying magnitudes was applied during the expiratory phase. This prolonged expiration and produced different responses in the neurons. Generally, I beta and P-cells were depolarized, whereas I alpha cells were hyperpolarized. 3. Low-intensity electrical stimulation of the ipsilateral vagus nerve evoked excitatory postsynaptic potentials (EPSPs) in all three DRG neuronal types. P-cells and I beta cells exhibited EPSPs in response to the lowest intensity; generally this intensity was below threshold for the simultaneously recorded PSR. Overall, EPSPs in I alpha cells had the highest thresholds, but some EPSPs could be evoked at thresholds similar to those of the I beta cells. The distributions of the average onset latency of the evoked EPSP overlapped considerably. Thus vagal electrical stimulation cannot be used for unequivocal classification of DRG neurons into I alpha, I beta, and P-cell subpopulations. 4. Using intracellular spike-triggered averaging, single PSRs were shown to generate monosynaptic EPSPs in I beta neurons and P-cells but not I alpha cells. Divergence of single PSR afferents also was observed. Relationships between EPSP shape factors, amplitudes, and PSR afferent conduction velocity are similar to those previously observed for monosynaptic EPSPs in hindlimb motoneurons generated by spinal afferents.

Control of discharge patterns of medullary respiratory neurons by pulmonary vagal afferent inputs

To provide a better understanding of the central mechanisms by which pulmonary afferents reflexly control breathing, the responses of single respiratory neurons to vagal afferent patterns were analyzed. Respiratory-related unit (RRU) recordings were obtained from inspiratory (I), expiratory (E), and phase-spanning neurons in the ventral medulla of halothane-anesthetized, paralyzed, ventilated, vagotomized, mongrel dogs. Electrical stimulation of the largest vagal fibers was used to reflexly alter I and E durations (TI and TE) and to present various temporal input patterns to RRU. The net response was quantified by taking the difference between cycle-triggered histograms (CTH) of activity obtained during an input and the spontaneous control (no input) CTH. For step frequency patterns confined to either the I or E phase, 127 responses in 41 neurons were analyzed. The average step response time was greater than 500 ms. In general the time courses of the control and test-input discharge patterns were linearly related to one another. For I neurons the slopes (beta) of these relationships were linear functions of the vagal step frequency (Fv). Linear relationships were also obtained for 1/TI vs. Fv and 1/beta vs. TI. These results suggest that the vagal control of the discharge patterns of these neurons and phase timing is mediated via a process similar to gain modulation.

Differences in respiratory patterns after acute and chronic pulmonary denervation

The role of pulmonary vagal information in the control of respiratory patterns was assessed in awake and anaesthetised rats in which pulmonary denervation was effected by bilateral cervical vagotomy or by right cervical vagotomy combined with left pneumonectomy or left intrathoracic vagotomy. Acute denervation led to increases of tidal volume (VT), inspiratory duration (TI) and expiratory duration (TE) in both awake and halothane anaesthetised animals; in awake rats the increase of TE rapidly subsided. Chronic pulmonary denervation produced markedly smaller increases of VT and TI and no change of TE from control values. In hypercapnia, awake animals with combined pneumonectomy and vagotomy consistently increased respiratory frequency by reductions in TI and TE; awake animals with combined intrathoracic and cervical vagotomy showed no increase in f because decreases in TI offset increases in TE; in anaesthetised rats with acute bilateral cervical vagotomy there was a consistent fall in respiratory frequency due to an expiratory pause. The results demonstrate that (1) the role of vagal activity in the production of respiratory patterns is unlikely to be accounted for solely in terms of influences arising from pulmonary stretch receptors; (2) vagal influences of TE are transitory; (3) under halothane anaesthesia hypercapnia induces an expiratory pause; and (4) the combination of pneumonectomy with contralateral vagotomy makes possible studies in awake rats although pulmonary denervation is less complete than with bilateral intrathoracic vagotomy.

Muscarinic M1 receptors mediate the increase in pulmonary resistance during vagus nerve stimulation in dogs

The physiologic roles of the 2 muscarinic receptors (M1 and M2) in the vagal control of pulmonary resistance were studied by comparing the effects of pirenzepine (PZ, M1-blocker), gallamine (GAL, M2-blocker), and atropine (AT, M1- and M2-blocker) on the increase in pulmonary resistance (RL) and on the reduction in heart rate (HR) during bilateral cervical vagus nerve stimulation in 18 anesthetized (chloralose and urethane) and paralyzed (vecuronium) dogs. PZ, AT, and GAL all inhibited the reduction in HR during vagus nerve stimulation, although the inhibition required relatively high doses of PZ and GAL. AT and PZ inhibited the increase in RL during vagus nerve stimulation. The ratio of the dose needed to inhibit by 50% the HR response to the dose needed to inhibit by 50% the RL response was approximately 45:1 for PZ, 12:1 for AT, and less than 0.4:1 for GAL. Thus, compared with AT, PZ is a more selective blocker of vagally induced increases in RL, indicating that M1 receptors are present in the airway smooth muscle of intact anesthetized dogs. In the same dose range as that which caused the inhibition of the HR response, GAL had no consistent effect on the increase in RL during vagus nerve stimulation, indicating that M2 receptors do not mediate the increase in RL in intact anesthetized dogs.