Protoveratrines A and B increase sleep apnea index in Sprague-Dawley rats

The action of protovertarines A and B, which stimulate carotid sinus baroreceptors and vagal sensory endings in the heart as well as pulmonary bed, were assessed on spontaneous and postsigh central sleep apneas in freely moving Sprague-Dawley rats. During the 6-h recording period, animals were simultaneously monitored for sleep by using electroencephalogram and electromyogram recordings, for respiration by single-chamber plethysmography, and for blood pressure and heart period by using radiotelemetry. After administration of 0.2, 0.5, or 1 mg/kg sc of protoveratrines, cardiopulmonary changes lasting at least 6 h were observed in all three behavioral states [heart period increased up to 23% in wakefulness, 21% in non-rapid-eye-movement (non-REM) sleep, and 20% in REM sleep; P < 0.005 for each]. At the same time, there was a substantial increase in the number of spontaneous (375% increase; P = 0.04) and postsigh (268% increase, P = 0.0002) apneas. Minute ventilation decreased by up to 24% in wakefulness, 25% in non-REM, and 35% in REM sleep (P < 0.05 for each). We conclude that pharmacological stimulation of baroreflexes promotes apnea expression in the sleeping rat.

Central and peripheral effects of the non-neural substances on respiration before and after vagotomy

The central effects of capsaicin, veratrine, histamine and bradykinin were studied by injecting them directly into the oerebrospinal fluid and their peripheral effects were examined by injecting into femoral vein. Our experiments were performed in Na-pentobarbital-anaesthetized dogs. Tidal volume (VT), respiratory frequency (f/min), systemic arterial pressure (BP) were recorded. A significant increase in f, and an initial apnea or hypoventilation followed by a significant increase in VT were observed with central and peripheral capsaicin. Vagotomy removed the peripheral VT response, but not the central one. While central capsaicin administration increased BP, peripheral administration decreased. After vagotomy, a significant increase was observed in BP for both administrations. Respiratory responses to central and peripheral administrations of veratrine were similar to those of capsaicin. Significant increases were observed in f and VT of the intact group in response to central and peripheral administration of histamine. Response to peripheral administration disappeared after vagotomy. While central and peripheral bradykinin increased VT significantly, there was no significant change in f. Vagotomy only removed the increase in VT in response to peripheral administration. In conclusion, respiratory responses to central administration of capsaicin and veratrine are due to direct effects of these substances on respiratory neurons. In peripheral administration, disappearance of the responses after vagotomy indicate that the responses are brought about by stimulation of the lung receptors.

Activation of pulmonary rapidly adapting receptors does not induce bronchoconstriction in dogs

Activation of pulmonary rapidly adapting receptors (RARs) is believed to constrict airways by a vagally mediated reflex. We tested this hypothesis in dogs anesthetized with sufentanil citrate. We ventilated both lungs separately at a positive end-expiratory pressure of 4 cmH2O. We stimulated RARs in one lung under three different conditions: 1) deflation of the lung; 2) decrease in lung compliance; and 3) aerosolization of methacholine. We monitored the airway pressure in the nonstimulated lung as an index for airway muscle tone and could not detect increases in the pressure swing under these conditions. On the other hand, electrical stimulation of the distal end of cervical vagus nerve increased the pressure swing bilaterally (ipsilateral dominant), suggesting that reflex response could be detected in our preparation. Moreover, deflation (or inflation) of either lung increased (or suppressed) diaphragmatic activity. The results indicate intact vagal afferents and central response in our preparation. We conclude that activation of RARs located below the carina does not induce bronchoconstriction by a centrally mediated reflex. If any effect is present, it appears to be small.

CO2-sensitive olfactory and pulmonary receptor modulation of episodic breathing in bullfrogs

Breathing was monitored during normocarbia, hypercarbia (6% CO2 in air), and the period immediately after the return to normocarbic conditions in intact, olfactory-denervated, and vagotomized bullfrogs. In intact frogs, ventilation increased during hypercarbia, but the breathing pattern remained episodic. Immediately upon return to air, there was a further paradoxical increase in breathing frequency, and breathing became continuous in most frogs. Results obtained from animals after olfactory receptor denervation indicate that tonic stimulation of olfactory receptors by airway CO2 inhibited breathing during hypercarbia. Measurements of the kinetics of changes in airway and arterial blood CO2 levels support the suggestion that the sudden release of this inhibition on the return to normocarbic conditions was responsible for the posthypercarbic hyperpnea. Vagotomy increased ventilation during normocarbia. Hypercarbia now caused a change in breathing pattern but had no net effect on total ventilation, suggesting that pulmonary vagal feedback inhibited ventilation during normocarbia but stimulated ventilation during hypercarbia. Although olfactory and pulmonary receptor feed-back shape the breathing pattern, they were not responsible for initiating or terminating the episodes of breathing.

Flecainide blocks the stimulatory effect of veratridine on slowly adapting pulmonary stretch receptors in anaesthetized rabbits without changing lung mechanics

We examined the responses of slowly adapting pulmonary stretch receptors (PSRs), total lung resistance (RL) and dynamic lung compliance (Cdyn) to administered veratridine before and after pretreatment with atropine or flecainide in anaesthetized, artificially ventilated rabbits with bilateral vagotomy. Administration of veratridine (10 and 30 micrograms kg-1) caused vigorous stimulation of PSRs, resulting in a tonic discharge of receptors during both inflation and deflation, but did not significantly alter either RL or Cdyn. The veratridine-induced PSR stimulation became more prominent, as the dose of this alkaloid was increased. Pretreatment with atropine (1 or 2 mg kg-1) had no significant effect on the excitatory response of PSRs to veratridine. The veratridine-induced PSR stimulation was inhibited by treatment with flecainide (1, 2 and 3 mg kg-1), a sodium channel blocker, and this inhibition was dose-dependent. These results suggest that activation of PSRs following veratridine administration probably related to the increased influx of sodium ions into the receptive terminals but does not depend upon bronchoconstriction.

The response of rapidly adapting pulmonary stretch receptors elicited by sodium cyanide is augmented by vagotomy in the rabbit

The responses of rapidly adapting pulmonary stretch receptor (RAR) activity, tidal volume (VT), inspiratory airflow (VI), transpulmonary pressure (Ptrans) and dynamic lung compliance (Cdyn) to administration of NaCN (20 and 40 micrograms/kg) were compared before and after vagal section in anesthetized, spontaneously breathing rabbits. The responses of RARs were recorded from a partially dissected branch of the left vagus nerve (LVN). Before vagal section, hyperpnea following NaCN administration led to increases in RAR activity, VT, VI and Ptrans, but caused a decrease in Cdyn. Those responses obtained became more prominent by increasing the does of NaCN. Under these conditions the increased receptor activity fired throughout the whole respiratory cycle. The responses of RAR activity, VT and Ptrans to NaCN administration were augmented by a vagal section that was produced by denervating the rest of the LVN still intact and right vagus nerve, and the discharge of receptors showed a predominantly inspiratory pattern that corresponded to augmentation of both Ptrans and VT. Vagal section, however, had no significant effect on the changes of VI and Cdyn induced by NaCN. In addition, mean percent changes of increased VT and Ptrans produced by NaCN administration after vagal section were similar to those before vagal section. Furthermore, augmentation of VT and elevation in baseline VT, when NaCN after bilateral vagotomy was administered i.v., were preceded by simultaneously increased Ptrans and RAR activity. These results suggest that augmentation of NaCN-induced RAR stimulation in animals without intact vagus nerves is related to concomitant changes in the respiratory mechanics such as Ptrans and VT, probably involving the increased sensitivity of receptors to these two factors.

Role of the parabrachial nuclei in the airway dilation evoked by the Hering-Breuer reflex

We tested the hypothesis that blockade of glutamatergic receptors in the parabrachial nucleus (PBN) of chloralose-anesthetized cats attenuated the reflex airway dilation evoked by activation of pulmonary stretch receptors. Unilateral microinjection of kynurenic acid (50 nl, 100 mM) into the PBN reversibly attenuated the reflex relaxation of the trachealis muscle in 7 cats. These findings suggest that the PBN is part of the central pathway mediating the airway dilation component of the Hering-Breuer reflex.

Effects of thromboxane A2 agonist STA2 on rapidly adapting pulmonary stretch receptors in vagotomized rabbits

We investigated the responses of rapidly adapting pulmonary stretch receptors (RARs) and tracheal pressure (PT) to right atrial injections of the thromboxane A2 (TXA2) stable analogue STA2 (0.3, 1.0, and 3.0 microgram/kg) before and after administration of atropine sulfate (1 mg/kg), isoprenaline (200 micrograms/kg), indomethacin (1 mg/kg), or S-145 (0.5 mg/kg) in artificially ventilated, bilaterally vagotomized rabbits. The RARs increased their activity after STA2 administration, and the increase was dose-dependent. However, intraatrial injections of STA2 at all the doses examined had no significant effect on PT. The excitatory responses of RAR activity to STA2 (0.3-3.0 micrograms/kg) were not significantly altered by administration of atropine sulfate (anticholinergic agent), isoprenaline (bronchodilator), or indomethacin (cyclooxygenase inhibitor). However, S-145 treatment (TXA2 antagonist) blocked the STA2-induced RAR stimulation. To determine whether or not administration of STA2 causes release of acetylcholine (ACh), we also examined the effects of vagal efferent stimulation (10-15 V, 10 Hz, 1 ms), STA2 administration (3.0 micrograms/kg), and their combination on PT in rabbits associated with both artificial ventilation and bilateral vagotomy. The vagally mediated bronchoconstriction that led to an increase in PT was not enhanced by simultaneous administration of STA2 at 3.0 micrograms/kg in all of the tested animals. These results suggest that the stimulation of RARs by STA2 is not mediated by the release of ACh from the nerve endings but is probably due to a local inflammatory bronchoconstriction that does not significantly alter the value of PT.

Effects of veratridine and nifedipine on ammonia-induced rapidly adapting pulmonary stretch receptor stimulation in vagotomized rabbits

We studied the effects of aerosol administration of veratridine (a sodium channel opener) or nifedipine (a calcium channel blocker) on the responses of rapidly adapting pulmonary stretch receptors (RARs) and dynamic lung compliance (Cdyn) to aerosols of 2 and 4% ammonia solutions in anesthetized spontaneously breathing rabbits without intact vagi. The RARs increased their activity following ammonia aerosol, and the increase was concentration-dependent. However, ammonia aerosol did not significantly alter the value of Cdyn. The RARs following aerosol administration of veratridine (about 200 micrograms) showed their characteristic firing pattern with several phases; each phase was characterized by the long high-frequency continuous discharges. Under these conditions, the response was not associated with any significant change in Cdyn. Even though the change in receptor activity produced by veratridine was restored to control level, subsequent aerosol application of ammonia led to similar firing patterns, as veratridine was given by aerosol, but had no significant effect on Cdyn. Following aerosol administration of nifedipine (about 1 and 2 mg) the RAR activity and Cdyn were similar to those during control. Furthermore, the ammonia-induced RAR stimulation was not significantly affected by nifedipine aerosol. These results suggest that the stimulation of RARs by ammonia in vagotomized rabbits is independent of changes in Cdyn and speculate that their excitatory effect is at least in part related to the activation of Na+ influx to the receptive terminals but is not involved in the secondary entry of Ca2+ ions to the receptor membrane, through voltage-dependent calcium channels.

Effects of isoprenaline on the responses of slowly adapting pulmonary stretch receptors to reduced lung compliance and to administered histamine

To define the difference between the responses of slowly adapting pulmonary stretch receptors (SARs) to reduced dynamic lung compliance (Cdyn) and to administered histamine, experiments were performed in open-chest, artificially ventilated, bilaterally vagotomized rabbits with positive end-expiratory pressure (PEEP). Both stimuli caused an increase in tracheal pressure and produced augmentation of SAR activities during inflation and deflation. Isoprenaline treatment that blocked the responses of SARs and PT to histamine had no effect on those to reduced Cdyn. The results suggest that the response characteristics of SARs provoked by histamine administration do not involve the contribution of decreased Cdyn.

Responses of tracheobronchial receptors to halothane, enflurane, and isoflurane in anesthetized dogs

We investigated the effects of halothane, enflurane, and isoflurane on the activity of 43 tracheobronchial slowly adapting stretch receptors (SARs) and 16 rapidly adapting irritant receptors (RARs) in 5 anesthetized, vagotomized, paralyzed, and artificially ventilated dogs. The 43 SARs were classified into 2 subtypes: (i) 17 low-threshold SARs with an expiratory discharge at FRC that were active throughout the respiratory cycle and (ii) 26 high-threshold SARs active only in inspiration. Ventilating the lungs with 5% of each anesthetic caused a significant increase in the inspiratory discharge of low-threshold SARs, whereas the expiratory discharge was inhibited or altogether silenced. While the activity of the majority of high-threshold SARs increased during the administration of the three volatile anesthetics, it decreased in those with a particularly high recruitment threshold. There was, however, a consistent increase in the pressure threshold at which all SARs were recruited. Ventilating the lungs with 5% of each anesthetic caused a significant decrease in activity of RARs. Our results indicate that all three halogenated anesthetics inhibit RARs at concentrations ranging from 1% to 5%.

Role of N-methyl-D-aspartate receptors in the pontine pneumotaxic mechanism in the cat

Systemic injection of MK-801, an N-methyl-D-aspartate (NMDA) receptor-associated channel blocker, induces an apneusis in vagotomized cats similar to that produced by pontine respiratory group (PRG) lesions, suggesting the possible involvement of NMDA receptors in the pontine pneumotaxic mechanism. Previous results from our laboratory indicate that the efferent limb of the pontine pneumotaxic mechanism is unlikely to require NMDA receptor-mediated neurotransmission. Therefore, the present study examined the potential involvement of PRG NMDA receptors in the pontine pneumotaxic mechanism. Experiments were conducted in decerebrate, paralyzed, and ventilated adult cats. The effects on inspiratory time (TI) of MK-801 microinjection into PRG were tested in 12 cats. Pressure microinjection of MK-801 (15 mM, 80-3,000 nl) significantly prolonged TI in all animals when lung inflation was withheld. TI progressively increased in most animals for > or = 30 min. After this period, partial recovery of the effect occurred in eight cats as TI shortened toward predrug levels. In three animals, microinjection of MK-801 induced a complete apneusis in the absence of lung inflation from which there was no detectable recovery. Microinjections into regions approximately 2 mm distant from PRG produced little or no effect. These results provide evidence that NMDA receptors located in the region of PRG play an important functional role in the control of the breathing cycle.

Substance P antagonist does not block the stimulation of rapidly adapting pulmonary stretch receptors by ammonia

We investigated the effects of the substance P (SP) blocker [D-Pro2,D-Trp7,9]-SP on the response of rapidly adapting pulmonary stretch receptors (RARs) to SP administered into the right atrium, or ammonia vapor inhaled into the lungs in anesthetized, spontaneously breathing rabbits. Right atrial administration of SP (0.3, 1.0, and 3.0 micrograms/kg) caused an increase in the RAR activity, and this increase became more prominent as the dose of SP was increased. The RARs increased their activity following inhalation of vapor from 5 and 10% ammonia solutions, and the increase was concentration dependent. The excitatory responses of RAR activity to SP at different doses were greatly diminished or completely blocked by administration of the selective SP antagonist (300 and 500 micrograms/kg). However, the ammonia-induced RAR stimulation was not significantly altered by prior treatment with the SP blocker (300 and 500 micrograms/kg). These results suggest that the stimulation of RARs by ammonia does not occur as a result of the release of SP from sensory nerves in the airways and lungs.

Responsiveness of the guinea pig trachea to stretch: role of the epithelium and cyclooxygenase products

The role of the epithelium and cyclooxygenase products was investigated in the responses of isolated airways to sudden stretch. Strips of guinea pig trachea, in some of which the epithelium had been removed mechanically, were suspended in organ chambers; isometric tension was recorded. After rapid stretching to their optimal tension, the preparations (with and without epithelium) relaxed initially and then contracted to a level close to the imposed tension. Afterward, tissues with epithelium maintained this level of tension, but those without epithelium relaxed. After treatment with papaverine or isoproterenol (at concentrations causing maximal relaxation), stretch was followed only by a decrease in tension; a similar response to stretch was also obtained in tissues treated with indomethacin or acetylsalicylic acid (inhibitors of cyclooxygenase). Dazmegrel (an inhibitor of thromboxane synthase) and SQ-29548 (an antagonist of prostaglandin H2 or thromboxane A2 receptors) did not affect the response of tissues with epithelium but abolished the stretch-induced contraction in those without epithelium. Tranylcypromine, which inhibits prostacyclin synthase, and tetrodotoxin, which blocks local reflexes, did not significantly affect the responses of the tissues to stretch. These observations suggest that thromboxane may mediate the epithelium-independent contraction and that another product of cyclooxygenase contributes to the maintenance of tension on stretching observed in tissues with epithelium.

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.

H1- and H2-receptor influences of histamine and ammonia on rapidly adapting pulmonary stretch receptor activities

Studies were designed to establish the difference of the excitatory mechanisms between histamine and ammonia on rapidly adapting pulmonary stretch receptors (RARs). We therefore examined the responses of RARs to histamine administered as an aerosol and ammonia inhaled as a vapor before and after aerosol administration of mequitazine or cimetidine in spontaneously breathing rabbits. The excitatory responses of RARs to aerosols of histamine at different concentrations were completely blocked by administration of aerosol mequitazine but potentiated by aerosol cimetidine. However, the increases of RAR activity produced by inhalation of ammonia vapor at different concentrations were not significantly affected by aerosol administration of either a H1-receptor blocker or a H2-receptor antagonist. These results suggest that the stimulation of RARs by aerosol histamine occurs as a result of the interaction between H1 (excitatory)- and H2 (inhibitory)-receptor effects, whereas these two receptor effects do not contribute to the ammonia-induced RAR stimulation.

Effects of calcium channel and H1-receptor blockers on the responses of slowly adapting pulmonary stretch receptors to histamine in vagotomized rabbits

We studied the effects of calcium channel antagonists (verapamil and nifedipine) and H1-receptor blockers (mequitazine) on changes in the slowly adapting pulmonary stretch receptors (SARs) located below the carina in response to right atrial injections of histamine (60 and 80 micrograms/kg) in anesthetized artificially ventilated rabbits with bilateral vagotomy. After histamine was injected into the right atrium, the SARs became more active during expiration but decreased their activity during inspiration. These changes were more pronounced by increasing the dosage of histamine. However, administration of histamine had no significant effect on tracheal pressure (PT). Verapamil treatment (1 mg/kg) did not alter the SAR response to histamine, whereas the responses of SARs to histamine at different dosages were significantly diminished by treatment with nifedipine (1 mg/kg). Mequitazine (1 mg/kg), a potent H1-receptor blocker, blocked completely all the responses of SAR activity to histamine. These results suggest that the effect of histamine 60-80 micrograms/kg on SAR activity is mediated by the activation of H1-receptors of the peripheral airway smooth muscle and that this activation, at least in part, involves the opening of calcium channels of the airway smooth muscle.

Effects of the thromboxane A2 mimetic, U46,619, on pulmonary vagal afferents in the cat

Release of thromboxane A2 (TxA2) or infusion of the TxA2 mimetic U46,619 in the cat elicits pulmonary hypertension and rapid shallow breathing (Shams et al., Respir. Physiol. 71: 169-183, 1988). The vagus nerve mediates the observed respiratory, but not the circulatory, effects (Shams and Scheid, J. Appl. Physiol. 68: 2042-2046, 1990). To identify the type of lung vagal afferent fibers involved in this respiratory response to TxA2, we have recorded the functional single-unit activity and its response to infusion of U46,619 in fine strands of the vagus nerve in the artificially ventilated cat and rabbit. The fibers were classified as originating from slowly adapting (SAR) or rapidly adapting (RAR) stretch receptors by their response to sustained pulmonary inflation (intrapulmonary pressure of 20-25 cmH2O) or as C-fibers, by their response to a bolus injection of phenylbiguanide. C-fibers responded variably to lung inflation. U46,619 infusion caused only a small increase in SAR or RAR activity along with increases in end-inspiratory tracheal airway pressure (Paw), but evoked a marked increase in the firing rate of C-fibers, independent of their response to lung inflation. This increase in C-fiber activity was unrelated to the increase in Paw, which accompanied the infusion of U46,619. Since these responses remained the same after indomethacin they appear to be due to a direct action of U46,619, and not to be mediated by prostanoids that might be released by U46,619. These data suggest that C-fibers are indeed involved in the respiratory effects of TxA2. Since the effects exerted on C-fibers by U46,619 were unrelated to increased Paw, TxA2 is likely to stimulate the nerve endings directly, rather than via smooth muscle contraction. On the other hand, the small stimulating effect of U46,619 on SAR and RAR may be mediated by bronchoconstriction.

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.

MDL 72222 (a selective 5-HT3 receptor antagonist) prevents stimulation of intrapulmonary C fibres by pulmonary embolization in anaesthetized rabbits

We have investigated the effects of the intravenous administration of 5-hydroxytryptamine upon the discharge frequency of pulmonary stretch receptors, irritant receptors and C fibres. Only C fibres were stimulated by this autacoid. MDL 72222 a selective 5-HT3 receptor antagonist blocks the C fibre stimulation by 5-hydroxytryptamine. It also blocks the C fibre response to miliary pulmonary embolism. These data confirm the hypothesis that 5-hydroxytryptamine is the humoral link between pulmonary embolism and tachypnoea.

Indirect effects of histamine on pulmonary rapidly adapting receptors in cats

We did experiments to determine the relative importance of lung mechanical changes during histamine induced activation of pulmonary rapidly adapting receptors (RARs). In anesthetized, open-chest, artificially ventilated cats, we recorded RAR activity and injected histamine (25-50 micrograms/kg) into the right atrium. Histamine initially increased RAR activity from 1.1 +/- 0.2 to 3.6 +/- 0.6 imp/sec (n = 30) at 15.6 +/- 0.8 sec when dynamic lung compliance (CDYN) was decreased by 29.1 +/- 1.5%. The firing pattern of RARs changed from a relatively irregular pattern to a pronounced respiratory modulation. RAR activity reached its peak (5.6 +/- 0.8 imp/sec) at 36.3 +/- 3.3 sec. The firing pattern further changed to a cardiac modulation, and the activity closely correlated with cardiac output. Comparing the initial response of RARs to histamine with the response to mechanically decreasing CDYN, we found that the activities were similar when CDYN was decreased by the same amount. Our experiments suggest that in cats the initial increase of RAR activity in response to histamine is related to lung mechanical changes, but the later increase is related to cardiovascular functions.

Effects of histamine on slowly adapting pulmonary stretch receptor activities in vagotomized rabbits

Thirty-two SAR fibers located below the carina were used in 32 anesthetized, bilaterally vagotomized, and artificially ventilated rabbits. After intravenous administration of histamine (20, 40, and 80 micrograms/kg, n = 13) or ACh (10, 20, and 40 micrograms/kg, n = 13), SARs became active during expiration but decreased their inspiratory activity. The effects of both drugs were dose-dependent. The injection of histamine or ACh at all doses examined had no effect on tracheal pressure (PT). Atropine (3 mg/kg) and isoprenaline (100 micrograms/kg) blocked the responses of SAR activity to low doses of histamine (20 and 40 micrograms/kg) and to all doses of ACh. The response of SARs to 80 micrograms/kg of histamine was not altered by atropine (n = 10) nor by isoprenaline (n = 10). These results suggest that low-dose effects of histamine on SARs occur as the result of ACh release whereas with high doses the effect of histamine on the receptors is mainly independent of ACh released from the nerve endings. In another series of experiments (n = 6) where animals were treated with isoprenaline (100 micrograms/kg) and, subsequently, physostigmine (200 micrograms/kg), histamine (20 and 40 micrograms/kg) or ACh (10 and 20 micrograms/kg) increased the inspiratory discharge in SARs and the level of PT. From these results, it is assumed that the changes of SAR activity following histamine injection in vagotomized animals reflect a local bronchomotor effect which takes place in a peripheral bronchial tree that does not affect the level of PT. Histamine seems to release ACh and to elicit bronchoconstriction that can be manifested by physostigmine treatment.

Excitation of both slowly and rapidly adapting pulmonary stretch receptors attenuates tachypnea induced by ammonia

To elucidate the mechanism of attenuating the ammonia-induced tachypnea, the present study examined the discharge patterns and rates of slowly adapting pulmonary stretch receptors (SARs) and rapidly adapting pulmonary stretch receptors (RARs) in relation to the change in respiration produced by ammonia inhalation in anesthetized, spontaneously breathing rabbits. Extracellular action potentials of these two receptors were recorded at the peripheral cut-end of the left vagus nerve. A prolongation of expiration following ammonia inhalation occurred during the discharge of receptors increased continuously, particularly when the level of the discharge rate during expiration reached to approximately 20-fold, and under such circumstances the respiratory response was regularly associated with gasps. On the other hand, RARs increased their activity during only inspiration; this increased activity correlated with augmentation of inspiration. Furthermore, the prolongating effect of expiration due to ammonia inhalation was not observed after surgical denervation of the remaining right vagus nerve. These results suggest that vigorous stimulation of the SAR activity induced by ammonia inhalation can elicit a prolongation of expiration possibly resulting from augmentation of the Hering-Breuer inflation reflex and that augmentation of the increased RAR activity after ammonia inhalation counteracts the Hering-Breuer inflation reflex to shorten inspiration. Therefore, it is conceivable that strong stimulation of the SAR activity after ammonia inhalation counteracts the ammonia-induced tachypnea.

Location of pulmonary stretch receptors in the guinea-pig

The site of pulmonary slowly adapting stretch receptors (SRs) was investigated in anaesthetized, thoracotomized and artificially ventilated guinea-pigs. The location of SRs within the lungs and airways was determined by analyzing the changes of SR single fibres discharge patterns in response to (a) occlusion of the airways, (b) local probing, and (c) microinjection of the non-diffusible local anaesthetic cinchocaine into the presumed receptor site. The great majority (92%) of the 79 SRs examined was localized in small airways or in lung parenchyma ('peripheral SRs'), whereas only 8% were located in large airways, i.e., in the trachea, main bronchi and lobar bronchi ('central SRs'). The discharge responses to lung inflation and to ammonia inhalation slightly differed between these two SR groups. With the pronounced prevalence of peripheral SRs, the guinea-pig seems to take a unique position among the species examined hitherto.