Intrapulmonary receptors in the Tegu lizard: I. Sensitivity to CO2

Seasonal variation of hypoxic and hypercarbic ventilatory responses in the lizard Tropidurus torquatus

Carbon dioxide (CO₂) and oxygen (O₂) influence the breathing pattern of reptiles, especially when CO₂ is in excess or O₂ at low concentrations and the effects of these gases on the respiratory response varies according to the species. In addition to respiratory gases, seasonal changes can also modulate breathing pattern and ventilatory responses to hypoxia and hypercarbia. Therefore, the present study investigated the breathing pattern and ventilatory responses to hypercarbia (5% CO₂) and hypoxia (5% O₂) of the Neotropical lizard Tropidurus torquatus over a period of one year, covering all seasons (summer, autumn, winter and spring). Our data suggest that like other ectothermic sauropsids, Tropidurus torquatus possesses distinct ventilatory responses to hypoxia and hypercarbia, being more sensitive to changes in CO₂ than in O₂. Additionally, the ventilatory responses to hypoxia were more pronounced during summer and hypercanic and pos-hypercapnic ventilatory response was reduced during spring, suggesting that seasonality modulates the control of ventilation in this species.

Adaptive trends in respiratory control: a comparative perspective

In 1941, August Krogh published a monograph entitled The Comparative Physiology of Respiratory Mechanisms (Philadelphia, PA: University of Pennsylvania Press, 1941). Since that time comparative studies have continued to contribute significantly to our understanding of the fundamentals of respiratory physiology and the adaptive trends in these processes that support a broad range of metabolic performance under demanding environmental conditions. This review specifically focuses on recent advances in our understanding of adaptive trends in respiratory control. Respiratory rhythm generators most likely arose from, and must remain integrated with, rhythm generators for chewing, suckling, and swallowing. Within the central nervous system there are multiple “segmental” rhythm generators, and through evolution there is a caudal shift in the predominant respiratory rhythm-generating site. All sites, however, may still be capable of producing or modulating respiratory rhythm under appropriate conditions. Expression of the respiratory rhythm is conditional on (tonic) input. Once the rhythm is expressed, it is often episodic as the basic medullary rhythm is turned on/off subject to a hierarchy of controls. Breathing patterns reflect differences in pulmonary mechanics resulting from differences in body wall and lung architecture and are modulated in different species by various combinations of upper and lower airway mechanoreceptors and arterial chemoreceptors to protect airways, reduce dead space ventilation, enhance gas exchange efficiency, and reduce the cost of breathing.

Evolutionary trends in airway CO2/H+ chemoreception

In many species of air-breathing vertebrates CO2-sensitive airway receptors play an important role in ventilatory control. In ectotherms, olfactory receptors often inhibit breathing and prolong breath holding when environmental CO2 levels are high. CO2/H+ sensitive pulmonary receptors (intra pulmonary chemoreceptors (IPC) and pulmonary stretch receptors (PSR)) regulate breathing patterns in all vertebrates in a manner that reduces dead space ventilation and enhances the efficiency of CO2 excretion under conditions of environmental hypercarbia, and/or reduces CO2 loss from hyperventilation. The greater CO2 sensitivity of IPC may allow them to also serve as a venous CO2 receptor (at least transiently when levels of metabolically produced CO2 begin to rise), prevent alkalosis during hyperpnea/polypnea, and may have contributed to the evolution of the extremely thin air/blood barrier and increased diffusion capacity associated with the rigid avian lung. The presence of all three receptor groups with different degrees of CO2 sensitivity in most reptiles, however, gives rise to what appear to be anomalous responses to environmental CO2.

Influence of hypercapnia and hypocapnia on bladder contractions and their respiratory consequences

Rhythmic contractions of the detrusor muscle, induced by gradual filling of the urinary bladder in decerebrate or anesthetized cats, are accompanied by decreased inspiratory activity in motor nerves to respiratory muscles, particularly those of the upper airway. We have examined the influence of hypercapnia and hypocapnia on these contractions and the accompanying activities of the phrenic and hypoglossal nerves in decerebrate, vagotomized, paralyzed and ventilated cats, some of which had denervated carotid chemoreceptors. Hypercapnia slowed, and then reversibly abolished bladder contractions in most animals, regardless of the state of the carotid chemoreceptors. Bladder contractions were well maintained in progressive hypocapnia, even at end-tidal CO(2) levels below the 'apneic' thresholds of the hypoglossal and phrenic activities. The reductions of the nerve activities in response to bladder contractions were not significantly altered by hypercapnia or hypocapnia. The abolition of bladder contractions by hypercapnia is unlikely to reflect a direct effect of CO(2) or H+ ion on the contractile mechanism of the detrusor muscle, but may be based on inhibition of stretch receptors in the bladder wall and/or an effect of CO(2) or H+ in or near the micturition centers in the brain stem.

Phylogeny of CO2/H+ chemoreception in vertebrates

The traditional view has been that respiratory chemoreceptors responsive to changes in P(CO(2))/pH first evolved in air breathing vertebrates at both peripheral and central sites. Recent evidence, however, suggests that fish also possess chemoreceptors responsive to changes in P(CO(2)) per se. In many species these receptors reside in the gills and respond primarily to changes in aquatic rather than arterial P(CO(2)). There is also scattered evidence to suggest that central CO(2)/H(+)-sensitive chemoreceptors may be present in representatives of all fish groups but only the data for air breathing fish are strong and convincing. The phylogenetic trends that are emerging indicate that the use of CO(2) chemoreception for cardiorespiratory processes arose much earlier than previously believed, (arguably) that CO(2) chemoreception may first have arisen in the periphery sensitive to the external environment and that central CO(2)/H(+) chemoreception subsequently arose multiple times in association with several of the independent origins of air breathing, and that the mechanisms of CO(2)/H(+) chemotransduction may be as varied as the different receptor groups involved.

Airway ammonia negates the normal ventilatory response to airway CO(2) in garter snakes

Earlier studies from this lab showed that unidirectionally ventilated snakes, maintained on high airway [CO(2)], breathe slowly with a large tidal volume. If airway [CO(2)] is abruptly reduced during inspiration, inspiratory duration (TI) and tidal volume increase. On the other hand, in an animal normally receiving fresh air (no CO(2) in the inspired air) on each inspiration, if fresh air is withheld for one breath, TI and tidal volume decrease. To test the effect of producing an alkaline environment in the presence of CO(2) during these two maneuvers, six unidirectionally ventilated snakes weighing 32+/-16 g were maintained on 4% CO(2) in air flowing at 300 ml/min even during inspiration. NH(3) (4%) was introduced into the lung during one inspiration. During that breath, TI increased significantly, as if CO(2) had been removed from the lung. In another study, six different snakes weighing 21+/-6 g received fresh air at each inspiration. At the end of inspiration [CO(2)] was again raised to 4%. However, for one breath, [CO(2)] was maintained high during inspiration and airway NH(3) (3--4%) was simultaneously introduced into the airstream. During this breath, TI was expected to decrease, owing to the elevated [CO(2)]; however, there was no significant difference in the TI of the test breath compared with the previous breath, indicating that ammonia interfered with the expected CO(2) response. This study suggests that pH (probably intracellular) rather than P(CO(2)) produces this ventilatory reflex.

Evidence of a functional role in lung inflation for the buccal pump in the agamid lizard, Uromastyx aegyptius microlepis

This study has demonstrated that the agamid desert lizard Uromastyx aegyptius microlepis ventilates its lungs both with a triphasic, thoracic aspiratory pump and by gulping air, using a buccal pump. These two mechanisms never occur simultaneously because bouts of buccal pumping are always initiated after the passive expiration that terminates a thoracic breath. Lung inflation arising from thoracic and buccal ventilation was confirmed by direct recording of volume changes using a whole-body plethysmograph. This observation was further confirmed by mechanical separation of the inflationary pressures associated with these two breathing mechanisms, enabling the effects of lung inflation on buccal breathing to be observed. This revealed that the buccal pump is influenced by a powerful Hering-Breuer-type reflex, further confirming its role in lung inflation. Bilateral thoracic vagotomy tended to increase the variance of the amplitude and duration of the breaths associated with the aspiration pump and abolished the effects of lung inflation on the buccal pump. Uromastyx has vagal afferents from pulmonary receptors that respond to changes in lung volume and appear not to be sensitive to CO(2). This study describes two lung-inflation mechanisms (an amphibian-like buccal pump and a mammalian-like aspiration pump) in a single extant amniote, both of which are subject to vagal feedback control.

Responses of laryngeal receptors to intralaryngeal CO2 in the cat

1. We recorded afferent activities of single fibres in the superior laryngeal nerves of decerebrate or anaesthetized, paralysed cats while 3, 5 and 10% CO2 was added to a constant flow of warm, humidified air through the isolated upper airway. 2. Fifty-three receptors with discharge frequencies modulated by intralaryngeal CO2 were studied. Of these, forty-eight showed CO2-induced attenuation of their firing rates. Pulses of 3, 5 or 10% CO2, alternating with air at intervals ranging from 1.5 to 60 s, also diminished the discharge frequencies. This diminution was greater with higher CO2 concentrations and longer pulse durations. 3. Five of the fifty-three receptors were stimulated by intralaryngeal CO2. The discharge frequencies of these units increased slowly and by only a few impulses per second during CO2 exposure. 4. Thirty-four of the CO2-sensitive receptors were tested with other stimuli, including water, saline, positive and negative intralaryngeal pressures and cold air. The responses to these stimuli varied among receptors, but many of the units that reduced their frequencies with intralaryngeal CO2 were consistently stimulated by positive and/or negative intralaryngeal pressures. 5. Thirty-six of the receptors were anatomically located by probing the upper airway. Twenty-six were in the larynx, and ten were in the rostral trachea, within 5 mm of the cricoid cartilage. 6. The results, which are directly applicable to the investigation of reflex responses reported in the preceding paper, indicate that the predominant initial response to intralaryngeal CO2 under the conditions of these studies is attenuation of laryngeal receptor activity.

The influence of venous CO2 on ventilation in garter snakes

Garter snakes were used to study the effects of venous CO2 loading using the skin as an exchanger. The gaseous environment surrounding the snake's body was isolated by placing the body in a plethysmograph with the head out. While the animal breathed room air, the carbon dioxide concentration within the plethysmograph was varied between 0 and 80%. Room air was drawn through a funnel placed over the snake's head, thus collecting the exhaled gases, and this gas was analyzed by O2 and CO2 analyzers. The descending aorta was cannulated to measure blood gases. Expired CO2 flow rose linearly with increasing cutaneous CO2. Ventilation increased 3.5-fold at 80% cutaneous CO2 compared with no cutaneous CO2 load. Neither the mean CO2 concentration in exhaled air nor arterial PCO2 changed when the snake was exposed to high levels of CO2 at the skin. Thus ventilation increased in proportion to the CO2 load, and was not driven by arterial hypercapnia. Bilateral vagotomy eliminated arterial CO2 homeostasis during cutaneous CO2 loading, and ventilation increased with increasing arterial PCO2. Therefore, these snakes respond to extra-arterial elevations in CO2 or to a changing CO2 signal. Furthermore, receptors responsible for the increase in ventilation when venous CO2 is elevated have neurons in the vagus nerves.

Temperature effects on pulmonary receptor responses to airway pressure and CO2 in Alligator mississippiensis

The effects of body temperature (Tb) on pulmonary stretch receptor (PSR) and CO2-sensitive intrapulmonary chemoreceptor (IPC) response characteristics may have important effects on ventilatory control in reptiles. In this study, three questions were addressed: (1) what are the effects of Tb on PSR and IPC responses to airway pressure (Paw) and lung CO2 (PCO2); (2) what are the effects of acute (less than 12 h) vs chronic (greater than 1 week) changes in Tb on both receptor groups; and (3) can predicted changes in the fractional dissociation of imidazole (alpha im), calculated via independent changes in Tb and PCO2, explain the CO2-sensitivity of either IPC or PSR? Single fiber PSR and IPC responses to Paw, PCO2 and Tb were determined in 11 anesthetized Alligator mississippiensis (pentobarbital; 30 mg/kg), acclimated at 20 degrees C (N = 5) or at 30 degrees C (N = 6). PSR activity increased as Paw increased at both Tb, but PSR activity and sensitivity to Paw were lower at 20 degrees C. The average Q10 was 2.1. Increasing inhaled CO2 from 1 to 7% decreased PSR activity by 27 +/- 6% at 20 degrees C and 18 +/- 5% at 30 degrees C. IPC activity decreased as PCO2 increased at both Tb, but IPC activity and sensitivity were reduced at 20 degrees C. The average Q10 was 3.2. Increasing Paw from 2 to 10 cm H2O had inconsistent effects on IPC activity. There were no differences between the effects of acute or chronic changes in Tb on either PSR or IPC responses. Predicted changes in alpha im could not explain the CO2-sensitivity of either IPC or PSR. We conclude that PSR and IPC adapt rapidly to Tb changes. The larger Q10 of IPC suggests that the relative role of IPC vs PSR in ventilatory control may be greater at elevated body temperatures.

Intrapulmonary CO2 inhibits inspiration in garter snakes

This study was undertaken to assess the role of intrapulmonary chemoreceptors in the control of breathing in reptiles. Garter snakes were tracheostomized to remove the influence of upper airway receptors. After recovery from anesthesia, the animals were unidirectionally ventilated or allowed to ventilate normally, but without glottal control of the breath-hold. Breathing movements were recorded by means of a pneumotachograph. When a bidirectionally breathing snake was presented with CO2 at the tracheal tube, inspiratory duration and volume decreased. When CO2 was removed from the inspired air between breaths, inspiratory duration and volume increased. Removing CO2 from the airstream of a unidirectionally ventilated snake during a breath-hold produced apnea, but removing CO2 at the beginning of inspiration caused the duration and volume of the inspiration to increase. This reflex appears to represent a positive feedback mechanism for enhancing inspiration once a breath has been initiated.

Intrapulmonary receptors in the garter snake (Thamnophis sirtalis)

Receptors in the respiratory system of anesthetized, unidirectionally ventilated garter snakes were analyzed from single-unit vagal recordings. Three types of receptors were found: rapidly adapting mechanoreceptors (RARs), slowly adapting pulmonary stretch receptors (SARs) and intrapulmonary chemoreceptors (IPCs). RARs gave a burst of activity when the lung was inflated or deflated. Discharge frequency of SARs increased during a sustained elevation of intrapulmonary pressure (PIP); a step increase in PIP usually caused an overshoot in receptor activity that slowly adapted to a new tonic level during the sustained inflation. The activity of intrapulmonary chemoreceptors (IPCs) decreased when airway CO2 was raised. A sudden removal of CO2 caused a striking overshoot in activity which slowly adapted. Receptor discharge of SARs and IPCs was studied at 20, 25 and 30 degrees C. Discharge frequency increased with increasing temperature. The Q10 of discharge frequency averaged 3.0 for IPCs and 1.5 for SARs. Temperature clearly had a profound effect on pulmonary receptor activity; however, the relative change of activity of IPCs or SARs to a given stimulus was similar regardless of body temperature, and since the body temperature of snakes varies considerably, this may have important implications regarding the control of breathing in these animals.

Effects of intrapulmonary CO2 and airway pressure on pulmonary vagal afferent activity in the alligator

The effects of airway CO2 and pressure on pulmonary vagal afferent fibers were studied in seven anesthetized alligators Alligator mississippiensis, at room temperature (24 degrees C). Of 49 receptors which fired in phase with ventilation, 13 behaved like mammalian rapidly adapting pulmonary stretch receptors, 19 like mammalian slowly adapting pulmonary stretch receptors (PSR), and 17 like avian intrapulmonary CO2-sensitive chemoreceptors (IPC). PSR and IPC were positively localized to the lung by punctate stimulation or response to airway CO2 changes during pulmonary artery occlusion. PSR discharge frequency (fPSR) was measured at airway pressures (Paw) from 0 to 15 cm H2O at FICO2 = 0.01 in 14 receptors. fPSR increased in all receptors throughout the range of Paw studied. In 13 PSR, increasing FICO2 from 0.01 to 0.07 decreased fPSR 23 +/- 13% (+/- SEM) at Paw = 2 cm H2O and 14 +/- 7% at 15 cm H2O. IPC discharge frequency (fIPC) decreased as FICO2 increased and most discharged less than 1 sec-1 at FICO2 = 0.03. In 7 IPC at FICO2 = 0.01, increasing Paw from 2 to 15 cm H2O increased fIPC 17 +/- 5% after pulmonary artery occlusion demonstrating some mechanosensitivity in alligator IPC. Although both IPC and PSR showed mechanosensitivity and CO2-sensitivity, the two receptor types were distinct. PSR were 13 times more sensitive to Paw changes than IPC and IPC were 14 times more sensitive to FICO2 changes than PSR. We did not find any receptors with intermediate CO2- or mechanosensitivities that could represent a transitional form of receptor. These results predict that IPC and PSR may have different roles in reflex ventilatory control.

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.

Effect of venous (gut) CO2 loading on intrapulmonary gas fractions and ventilation in the tegu lizard

Studies were conducted to determine regional pulmonary gas concentrations in the tegu lizard lung. Additionally, changes in pulmonary gas concentrations and ventilatory patterns caused by elevating venous levels of CO2 by gut infusion were measured. It was found that significant stratification of lung gases was present in the tegu and that dynamic fluctuations of CO2 concentration varied throughout the length of the lung. Mean FCO2 was greater and FO2 less in the posterior regions of the lung. In the posterior regions gas concentrations remained nearly constant, whereas in the anterior regions large swings were observed with each breath. In the most anterior sections of the lung near the bronchi, CO2 and O2 concentrations approached atmospheric levels during inspiration and posterior lung levels during expiration. During gut loading of CO2, the rate of rise of CO2 during the breathing pause increased. The mean level of CO2 also increased. Breathing rate and tidal volume increased to produce a doubling of VE. These results indicate that the method of introduction of CO2 into the tegu respiratory system determines the ventilatory response. If the CO2 is introduced into the venous blood a dramatic increase in ventilation is observed. If the CO2 is introduced into the inspired air a significant decrease in ventilation is produced. The changes in pulmonary CO2 environment caused by inspiratory CO2 loading are different from those caused by venous CO2 loading. We hypothesize that the differences in pulmonary CO2 environment caused by either inspiratory CO2 loading or fluctuations in venous CO2 concentration act differently on the IPC. The differing response of the IPC to the two methods of CO2 loading is the cause of the opposite ventilatory response seen during either venous or inspiratory loading.

Respiratory mechanics and control of the ventilatory cycle in the garter snake

We measured respiratory airflow and intrapulmonary pressure in spontaneously breathing garter snakes (Thamnophis sirtalis) before and after bilateral vagotomy. Each breathing cycle consisted of expiration, inspiration, and a breath-holding period with the glottis closed, as previously reported. Respiratory system compliance was very high compared with that of mammals (0.042 ml/cm H2O per g), but was considerably reduced when the animal was coiled. Resistance to airflow averaged 6.35 cm H2O per ml/sec. Lung volume during the breath-hold was usually above the relaxed volume of the system. Analysis of dynamic pressure-volume and flow-volume loops showed that both expiration and inspiration were active, and that little use was made of passive recoil pressure in expiration. Evidence that vagally mediated, volume-related feedback influenced the breathing pattern was found in some snakes, but this influence was weaker than in most mammals and other reptiles. The results suggest that breathing in snakes is mainly a centrally programmed process, not markedly dependent on respiratory system mechanics or neuromechanical feedback. This arrangement is advantageous in view of the high, posturally variable compliance and the frequent regional distortions of the respiratory system that result from the unique structure of these animals.

Acid-base balance during lactic acid infusion in the lizard Varanus salvator

Although reptiles rely heavily on anaerobic metabolism and lactic acid production during activity, little is known concerning their ventilatory response to the attendant metabolic acidosis. We measured arterial PCO2, H+ and lactate (L)ion concentrations, and the rates of CO2 (MCO2) and O2 (MO2) exchange in Varanus salvator (n = 9) during intravenous infusions of lactic acid (HL) or sodium lactate (NaL; 250 mM) at rest. Two protocols were used: (1) 15 min infusions of 0.42 ml/min at both 25 and 35 degrees C with measurements every 5 min; (2) 4.5 min infusions of 1.73 ml/min at 35 degrees C with measurements at 4.5 min. At 35 degrees C, control pH decreased from its value at 25 degrees C with a slope of -0.007/degrees C and PaCO2 increased. The results of HL infusion were: (1) [L]a increased, (2) MCO2 increased, (3) R (MCO2/MO2) increased, (4) pHa decreased, and (5) PaCO2 remained unchanged from control at both temperatures and at both infusion rates. The only significant changes in PaCO2 observed were following the termination of fast HL infusions, when PaCO2 decreased. In NaL infusions, only small changes were observed except in [L]a. The results indicate that: (1) delta pH/delta T in V. salvator is less than in other poikilothermic vertebrates, but consistent with other varanid lizards, (2) respiratory compensation is slight in response to acute metabolic acidosis in this species, and (3) ventilation follows changes in MCO2 rather closely, accounting for precise regulation of PaCO2 despite 4-fold increases in MCO2 elicited by bicarbonate buffering and increased metabolic rate.

Ventilatory response to inspired CO2 in the lizard, Tupinambis nigropunctatus

Elevating inspired levels of CO2 (1-4%) in Tupinambis nigropunctatus leads to an increase in tidal volume, mean expiratory flow, mean inspiratory flow, duration of the non-ventilatory period, inspiratory duration, expiratory duration and end inspiratory lung volume. Minute ventilation is variable and end expiratory volume decreases. An increase or decrease in CO2 concentration surrounding the head affects the duration of the non-ventilatory period before the altered CO2 concentration is inspired into the lungs. The change in duration of the non-ventilatory period before altered CO2 concentration is inspired into the lungs is probably mediated by CO2 sensitive receptors located in the mouth, nose or on the head surface.

CO2-sensitivity of stretch receptors in the marsupial lung

Discharge activity in single fibres of the vagus of the brush tailed possum was examined for evidence of pulmonary CO2 receptors by artificially ventilating the lungs with gas mixtures which preserved, abolished or reversed the normal tidal oscillation in FCO2. No specific CO2 receptors were observed. A quantitative study of the CO2 sensitivity of thirteen pulmonary stretch receptors was carried out after stabilizing FACO2 at high (6.4-7.8%), low (1.4-2.5%) as well as intermediate values. In addition receptor responses to a series of sustained augmenting inflations were examined at different intrapulmonary CO2 concentrations. All thirteen receptors showed CO2 sensitivity, their frequency of discharge being reduced by hypercapnia and increased by hypocapnia. Five were low threshold receptors which discharged throughout the ventilatory cycle while the remaining eight were only phasically active. High threshold receptors were more sensitive to FACO2 changes than were low threshold units. The results from the series of augmenting inflations suggest that it is the receptor's threshold, but not its sensitivity, to tracheal pressure that is modulated by the co2 signal.

Ventilatory response to inspired CO2 in the sea turtle: effects of body size and temperature

The ventilatory response of green turtles (Chelonia mydas) to inspired CO2 was tested. Both immature (0.9--1.35 kg) and adult (59--130 kg) animals responded with increased ventilation (VE) that was primarily (immature) or exclusively (adult) due to increased respiratory frequency. The VE of the immature turtles while breathing air increased with temperatures between 15 and 35 degrees C with a Q10 of 1.95, but the peak VE of these turtles while breathing 4.5% CO2 in air was not significantly different at 15, 25 or 35 degrees C. VE increased irregularly with time throughout an hour of CO2 breathing in both groups, although several of the adult animals exhibited a first-breath response to CO2 and all animals increased VE within 5 min after the onset of CO2 breathing. The VE of an immature turtle at 35 degrees C increased with graded increases in CO2 up to 6%, but at 15 and 25 degrees C VE increased only up to 4.5% and decreased with 6% CO2. The results are discussed in terms of possible receptor mechanisms involved.

Microanatomy of the lung parenchyma of a tegu lizard Tupinambis nigropunctatus

The combined techniques of light microscopy, scanning (SEM) and transmission (TEM) electron microscopy were used for the first time to study the structure of unicameral lungs of a Tegu lizard (Tupinambis nigropunctatus). The lungs are prolate spheroid bags with blood supplied by superficial branches of a dorsal pulmonary artery and returned by diffuse, more deeply located veins. The primary bronchus enters the medial aspect near the apex of the lung. The lung wall is composed of trabeculae: (1) arranged in a faviform pattern, (2) forming individual faveoli (gas exchange chambers) which appear deepest in the cranial one-half of the lung, (3) all of which have a smooth muscle core overlain by either a ciliated or nonciliated epithelium. A ciliated epithelium lines the luminal surfaces of the large primary trabeculae and parts of smaller secondary trabeculae; it is composed of cone-shaped cells with ciliated-microvillous surfaces, and of columnar serous secreting cells. Nonciliated epithelium covers the luminal surface of portions of some secondary trabeculae, abluminal surfaces of primary and secondary trabeculae and all surfaces of the small tertiary trabeculae forming the faveoli. The nonciliated epithelium overlies an extensive superficial capillary network. The blood-gas barrier (0.7-1.0 μm thick) is composed of a thin cytoplasmic flange of Type I pneumonocytes, a thick homogeneous basal lamina and an attenuated endothelial cytoplasm. Numerous surfactant-producing Type II pneumonocytes are closely associated with the Type I pneumonocytes. The nonrespiratory ciliated epithelium may function in humidification of air and clearing of the lungs.

Pulmonary receptor chemosensitivity and the ventilatory response to inhaled CO2 in the turtle

Ventilatory responses of unanesthetized turtles to changes in the intrapulmonary CO2 content of a vascularly isolated and an intact lung were measured during spontaneous breathing. The hyperpnea associated with inhalation of CO2 by the vascularly isolated lung was 19% of that associated with inhalation of CO2 by the intact lung. Transection of the vagus nerve supplying the isolated lung abolished this response. We conclude that both inhibition of pulmonary stretch receptor discharge with increasing levels of FICO2 and a functional increase in central inspiratory volume threshold during hypercapnia contribute to tidal volume increases following CO2 inhalation in normal animals. The major component of the ventilatory response of intact turtles to increasing levels of FICO2, however, was an increase in respiratory frequency. When CO2 was inspired only by the vascularly isolated lung the increase in respiratory frequency was only 21% of that recorded when the same levels of CO2 were inspired by the intact lung. Thus the ventilatory response of turtles to increasing levels of FICO2 is primarily dependent upon concomitant hypercapnia.

I. Pulmonary-CO2 ventilatory reflex in dogs: effective range of CO2 and results of vagal cooling

Bartoli et al. (1974) found in dogs with constant PaCO2 that an increase in PCO2 in the vascularly isolated lungs increased ventilatory drive by a vagal reflex. We have examined the range of lung PCO2 over which the reflex operates. In anaesthetized dogs we ventilated the lungs separately with O2, maintaining gas exchange with the right lung. When we occluded the left pulmonary artery, left lung PCO2 fell to 2--4 mm Hg, and phrenic nerve firing decreased significantly. Phrenic activity increased again when left lung PCO2 was raised in steps to 19, 32 and finally to 50 mm Hg. PaCO2 was unchanged. Phrenic responses were abolished by cutting the left vagus nerve or by cooling it to 7--8 degrees C. The largest increase in phrenic activity occurred when left lung PCO2 was increased from 2 to 19 mm Hg, and the smallest when PCO2 was raised from 32 to 50 mm Hg. Hence the significance of the pulmonary-CO2 ventilatory reflex may lie in depression of ventilatory drive when pulmonary CO2 falls below normal, rather than in stimulation of breathing when pulmonary CO2 increases above normal.

A study of the effect of SO2 on pacing and intrapulmonary chemoreceptor discharge in the domestic fowl

Kunz and Miller )1974) described a phenomenon called pacing in which FCO2 oscillations forced in the lungs of unanesthetized unidirectionally ventilated chickens cause one ventilatory movement per ECO2 oscillation. In the present study SO2 (0.08--0.32 vol%) added to the ventilating gas stream caused a loss of pacing in 11 of 12 birds. In 7 of the 11 birds in which SO2 was effective, pacing returned 3.2--7.0 min after removal of SO2. Intravenous infusion of a similar quantity of SO2 (dissolved in saline) did not block pacing. In a second series of experiments chickens were again unidirectionally ventilated and single nerve fiber recording was used to investigate the effect of SO2 on individual intrapulmonary chemoreceptors (IPC). Of necessity, this work was done on birds which were anesthetized and paralyzed. Discharge was abolished in 10 of 12 IPC by doses of SO2 similar to those which blocked pacing. In 6 of the 10 fibers in which SO2 was effective, discharge resumed after a mean period of quiescence lasting 7 min. It is concluded that pacing is mediated by intrapulmonary rather than systemic chemoreceptors.

Intrapulmonary receptors in the Tegu lizard: II. Functional characteristics and localization;

Intrapulmonary receptors identified in the Tegu lizard by single-unit vagal recording (Fedde et al., 1977) were subjected to a number of stimuli and localized within the lung. Some carbon dioxide receptors could follow periodic changes in intrapulmonary CO2 concentrations as rapidly as 1.3 Hz; No oxygen sensitivity was observed with this receptor type, and halothane markedly depressed the discharge frequency. In response to intravenously injected acetazolamide they increased their discharge frequency and became almost totally insensitive to CO2, suggesting molecular per se is not the direct controller of receptor discharge; These receptors show many of the functional characteristics described for those in the avian lung. Afferent activity from both CO2 and mechanoreceptors could be elicited by electrically stimulating the lung surface. The CO2 receptors appeared to be organized in a receptive field covering more than 1 cm2 of lung surface, multiple receptors being innervated by a single afferent fiber. Activity in afferent fibers from mechanoreceptors could be evoked from only one distinct spot on the lung surface. Conduction velocities of afferent fibers from CO2 receptors ranged from 1 to 3 m-sec-1; from mechanoreceptors, from 1.9 to 5.2 m-sec-1.