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

Inaugural Review Prize 2023: The exercise hyperpnoea dilemma: A 21st-century perspective

During mild or moderate exercise, alveolar ventilation increases in direct proportion to metabolic rate, regulating arterial CO2 pressure near resting levels. Mechanisms giving rise to the hyperpnoea of exercise are unsettled despite over a century of investigation. In the past three decades, neuroscience has advanced tremendously, raising optimism that the 'exercise hyperpnoea dilemma' can finally be solved. In this review, new perspectives are offered in the hope of stimulating original ideas based on modern neuroscience methods and current understanding. We first describe the ventilatory control system and the challenge exercise places upon blood-gas regulation. We highlight relevant system properties, including feedforward, feedback and adaptive (i.e., plasticity) control of breathing. We then elaborate a seldom explored hypothesis that the exercise ventilatory response continuously adapts (learns and relearns) throughout life and ponder if the memory 'engram' encoding the feedforward exercise ventilatory stimulus could reside within the cerebellum. Our hypotheses are based on accumulating evidence supporting the cerebellum's role in motor learning and the numerous direct and indirect projections from deep cerebellar nuclei to brainstem respiratory neurons. We propose that cerebellar learning may be obligatory for the accurate and adjustable exercise hyperpnoea capable of tracking changes in life conditions/experiences, and that learning arises from specific cerebellar microcircuits that can be interrogated using powerful techniques such as optogenetics and chemogenetics. Although this review is speculative, we consider it essential to reframe our perspective if we are to solve the till-now intractable exercise hyperpnoea dilemma.

Effects of prolonged lung inflation or deflation on pulmonary stretch receptor discharge in the alligator (Alligator mississippiensis)

The American alligator (Alligator mississippiensis) is a semi-aquatic diving reptile that has a periodic breathing pattern. Previous work identified pulmonary stretch receptors, that are rapidly and slowly adapting, as well as intrapulmonary chemoreceptors (IPC), sensitive to CO2, that modulate breathing patterns in alligators. The purpose of the present study was to quantify the effects of prolonged lung inflation and deflation (simulated dives) on pulmonary stretch receptors (PSR) and/or IPC discharge characteristics. The effects of airway pressure (0-20 cm H2O), hypercapnia (7% CO2), and hypoxia (5% O2) on dynamic and static responses of PSR were studied in juvenile alligators (mean mass=246 g) at 24°C. Alligators were initially anesthetized with isoflurane, cranially pithed, tracheotomized and artificially ventilated. Vagal afferent tonic and phasic activity was recorded with platinum hook electrodes. Receptor activity was a mixture of slowly adapting PSR (SAR) and rapidly adapting PSR (RAR) with varying thresholds and degrees of adaptation, without CO2 sensitivity. Receptor activity before, during and after 1 min periods of lung inflation and deflation was quantified to examine the effect of simulated breath-hold dives. Some PSR showed a change in dynamic response, exhibiting inhibition for several breaths after prolonged lung inflation. Following 1 min deflation, RAR, but not SAR, exhibited a significant potentiation of burst frequency relative to control. For SAR, the post-inflation receptor inhibition was blocked by CO2 and hypoxia; for RAR, the post-inflation inhibition was potentiated by CO2 and blocked by hypoxia. These results suggest that changes in PSR firing following prolonged inflation and deflation may promote post-dive ventilation in alligators. We hypothesize that PSR in alligators may be involved in recovery of breathing patterns and lung volume during pre- and post-diving behavior and apneic periods in diving reptiles.

Gestation increases the energetic cost of breathing in the lizard, Tiliqua rugosa

High gestational loads result in fetuses that occupy a large proportion of the body cavity and may compress maternal organs. Compression of the lungs results in alterations in breathing patterns during gestation which may affect the oxidative cost of breathing. In this study, the oxidative cost of breathing during gestation was determined in the viviparous skink, Tiliqua rugosa. Radiographic imaging showed progressive lung compression during gestation and a 30% reduction in the lung compression index (rib number at which the caudal margin of the lung was imaged). Pneumotachography and open flow respirometry were used to measure breathing patterns and metabolic rates. Gestation induced a two fold increase in minute ventilation via increases in breathing frequency but no change in inspired tidal volume. The rates of O2 consumption and CO2 production did not change significantly during gestation. Together, these results suggest that a relative hyperventilation occurs during gestation in Tiliqua rugosa. This relative hyperventilation suggests that diffusion and/or perfusion limitations may exist at the lung during gestation. The oxidative cost of breathing was estimated as a percentage of resting metabolic rate using hypercapnia to stimulate ventilation at different stages of pregnancy. The oxidative cost of breathing in non pregnant lizards was 19.96±3.85% and increased 3 fold to 62.80±10.11% during late gestation. This significant increase in the oxidative cost of breathing may have significant consequences for energy budgets during gestation.

Low cost of ventilation in the vagotomised alligator (Alligator mississippiensis)

Pulmonary ventilation requires energy, but the estimated costs of breathing in reptiles vary from 1 to 30% of resting metabolic rate. The low values have been estimated from changes in oxygen uptake during hypoxia or hypercapnia, but it remains possible that these treatments affected metabolism. We equipped alligators with masks for simultaneous measurements of ventilation and oxygen uptake during hypercapnia, hypoxia and bilateral vagotomy. Hypercapnia and hypoxia caused a marked increase in total ventilation, but oxygen uptake remained unchanged indicating a very low energetic cost of breathing. Upon vagotomy, breathing pattern changed to occasional and exceedingly deep breaths (76.1+/-11.1 ml kg(-1)) followed by buccal oscillations and shallower breaths (22.2+/-2.3 ml kg(-1)) interspersed between long non-ventilatory periods. This change in breathing pattern did not affect oxygen uptake. The duration of inspiration increased substantially upon vagotomy, so that inspiratory flow rate did not increase proportionally to tidal volume. These prolonged inspiratory times leads to less flow resistance and may explain that tidal volume could be increased substantially without a measurable energetic cost.

Development of avian intrapulmonary chemoreceptors

Although avian intrapulmonary chemoreceptors (IPC) have been studied extensively in adults, the maturation of IPC CO(2) sensitivity during development is completely unknown. To begin investigating IPC development we asked two fundamental questions: (1) Are IPC capable of sensing CO(2) during early development, and, if so, how early? And, (2) does IPC CO(2) sensitivity during early development exhibit postnatal maturation compared to IPC discharge characteristics in adult ducks? We addressed these questions by recording from single IPC Anas platyrhynchos ducklings beginning approximately 6 h prior to internal pipping through 4 days of postnatal development. We then compared mean IPC discharge characteristics during early development with mean IPC activity from adult ducks greater than 12 weeks old. In total, we recorded 28 individual IPC from 5 ducklings and 12 adult ducks. Results show that IPC were capable of responding to rapid step changes in CO(2) before hatching occurred, during the paranatal developmental period. We also found that mean IPC activity during early development had increased peak discharge frequencies, greater spike frequency adaptation, and less tonic CO(2) sensitivity when compared to adults (P< or =0.05). These results suggest that during early development phasic IPC CO(2) sensitivity is fully developed, yet tonic IPC CO(2) sensitivity exhibits postnatal maturation possibly associated with hatching. These results also suggest that the mechanisms that underlie phasic and tonic IPC action potential discharge, and therefore the degree of partial spike frequency adaptation, may be independent processes with different developmental trajectories.

Regulation of ventilation in the caiman (Caiman latirostris): effects of inspired CO2 on pulmonary and upper airway chemoreceptors

In order to study the relative roles of receptors in the upper airways, lungs and systemic circulation in modulating the ventilatory response of caiman (Caiman latirostris) to inhaled CO2, gas mixtures of varying concentrations of CO2 were administered to animals breathing through an intact respiratory system, via a tracheal cannula by-passing the upper airways (before and after vagotomy), or via a cannula delivering gas to the upper airways alone. While increasing levels of hypercarbia led to a progressive increase in tidal volume in animals with intact respiratory systems (Series I), breathing frequency did not change until the CO2 level reached 7%, at which time it decreased. Despite this, at the higher levels of hypercarbia, the net effect was a large and progressive increase in total ventilation. There were no associated changes in heart rate or arterial blood pressure. On return to air, there was an immediate change in breathing pattern; breathing frequency increased above air-breathing values, roughly to the same maximum level regardless of the level of CO2 the animal had been previously breathing, and tidal volume returned rapidly toward resting (baseline) values. Total ventilation slowly returned to air breathing values. Administration of CO2 via different routes indicated that inhaled CO2 acted at both upper airway and pulmonary CO2-sensitive receptors to modify breathing pattern without inhibiting breathing overall. Our data suggest that in caiman, high levels of inspired CO2 promote slow, deep breathing. This will decrease dead-space ventilation and may reduce stratification in the saccular portions of the lung.

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.

CO2 transduction mechanisms in avian intrapulmonary chemoreceptors: experiments and models

Intrapulmonary chemoreceptors (IPC) are neurons that sense tonic and phasic CO2 stimuli in the lungs of birds and diapsid reptiles. IPC are different from most other vertebrate respiratory CO2 receptors because: (1) they are stimulated by low PCO2 and inhibited by high PCO2, (2) they have extremely rapid response characteristics, (3) their CO2 sensitivity is nearly abolished by intracellular inhibitors of carbonic anhydrase, and (4) their CO2 sensitivity is strongly depressed by inhibiting Na+/H+ antiport exchange. Experimental evidence suggests that IPC respond to intracellular pH, not CO2 directly, and that intracellular pH and IPC discharge are determined by a kinetic balance between CO2 hydration/dehydration rates, transmembrane acid/base exchange rates, and intracellular buffering. We review experimental evidence for and against various mechanisms of IPC CO2 chemotransduction, present a conceptual and mathematical model of the proposed mechanisms, and compare this model to CO2 transduction in other respiratory chemoreceptors.

Cost of ventilation and effect of digestive state on the ventilatory response of the tegu lizard

We performed simultaneous measurements of ventilation, oxygen uptake and carbon dioxide production in the South American lizard, Tupinambis merianae, equipped with a mask and maintained at 25 degrees C. Ventilation of resting animals was stimulated by progressive exposure to hypercapnia (2, 4 and 6%) or hypoxia (15, 10, 8 and 6%) in inspired gas mixture. This was carried out in both fasting and digesting animals. The ventilatory response to hypercapnia and hypoxia were affected by digestive state, with a more vigorous ventilatory response in digesting animals compared to fasting animals. Hypoxia doubled total ventilation while hypercapnia led to a four-fold increase in total ventilation both accomplished through an increase in tidal volume. Oxygen uptake remained constant during all hypercapnic exposures while there was an increase during hypoxia. Cost of ventilation was estimated to be 17% during hypoxia but less than 1% during hypercapnia. Our data indicate that ventilation can be greatly elevated at a small energetic cost.

CO2 transduction in avian intrapulmonary chemoreceptors is critically dependent on transmembrane Na+/H+ exchange

Avian intrapulmonary chemoreceptors (IPC) are vagal respiratory afferents that are inhibited by high lung Pco(2) and excited by low lung Pco(2). Previous work suggests that increased CO(2) inhibits IPC by acidifying intracellular pH (pH(i)) and that pH(i) is determined by a kinetic balance between the rate of intracellular carbonic anhydrase-catalyzed CO(2) hydration/dehydration and transmembrane extrusion of acids and/or bases by various exchangers. Here, the role of amiloride-sensitive Na(+)/H(+) exchange (NHE) in the IPC CO(2) response was tested by recording single-unit action potentials from IPC in anesthetized ducks, Anas platyrhynchos. For each of the IPC tested, blockade of the NHE using dimethyl amiloride (DMA) elicited a marked (>50%) dose-dependent decrease in mean IPC discharge (P < 0.05), suggesting that NHE is important for pH(i) regulation and CO(2) transduction in IPC. In addition, activation of the NHE using 12-O-tetradecanoylphorbol 13-acetate stimulated six of the seven IPC tested, although the overall effect was not statistically significantly (P = 0.07). Taken together, these findings suggest that CO(2) transduction in IPC is dependent on transmembrane NHE although it is likely to be much slower than carbonic anhydrase-catalyzed hydration-dehydration of CO(2).

Effects of temperature and hypercapnia on ventilation and breathing pattern in the lizard Uromastyx aegyptius microlepis

In most reptiles, the ventilatory response to hypercapnia consists of large increases in tidal volume (V(T)), whereas the effects on breathing frequency (f(R)) are more variable. The increased V(T) seems to arise from direct inhibition of pulmonary stretch receptors. Most reptiles also exhibit a transitory increase in ventilation upon removal of CO(2) and this post-hypercapnic hyperpnea may consist of changes in both V(T) and f(R). While it is well established that increased body temperature augments the ventilatory response to hypercapnia, the effects of temperature on the post-hypercapnic hyperpnea is less described. In the present study, we characterise the ventilatory response of the agamid lizard Uromastyx aegyptius to hypercapnia and upon the return to air at 25 and 35 degrees C. At both temperatures, hypercapnia caused large increases in V(T) and small reductions in f(R), that were most pronounced at the higher temperature. The post-hypercapnic hyperpnea, which mainly consisted of increased f(R), was numerically larger at 35 compared to 25 degrees C. However, when expressed as a proportion of the levels of ventilation reached during steady-state hypercapnia, the post-hypercapnic hyperpnea was largest at 25 degrees C. Some individuals exhibited buccal pumping where each expiratory thoracic breath was followed by numerous small forced inhalations caused by contractions of the buccal cavity. This breathing pattern was most pronounced during severe hypercapnia and particularly evident during the post-hypercapnic hyperpnea.

Chemosensitivity of serotonergic neurons in the rostral ventral medulla

The medullary raphé contains two subtypes of chemosensitive neuron: one that is stimulated by acidosis and another that is inhibited. Both types of neuron are putative chemoreceptors, proposed to act in opposite ways to modulate respiratory output and other pH sensitive brain functions. In this review, we will discuss the cellular properties of these chemosensitive raphé neurons when studied in vitro using brain slices and primary dissociated cell culture. Quantification of chemosensitivity of raphé neurons indicates that they are highly sensitive to small changes in extracellular pH (pH(o)) between 7.2 and 7.6. Stimulation by acidosis occurs only in the specific phenotypic subset of neurons within the raphé that are serotonergic. These serotonergic neurons also have other properties consistent with a specialized role in chemoreception. Homologous serotonergic neurons are present within the ventrolateral medulla (VLM), and may have contributed to localization of respiratory chemoreception to that region. Chemosensitivity of raphé neurons increases in the postnatal period in rats, in parallel with development of respiratory chemoreception in vivo. An abnormality of serotonergic neurons of the ventral medulla has been identified in victims of sudden infant death syndrome (SIDS). The cellular properties of serotonergic raphé neurons suggest that they play a role in the CNS response to hypercapnia, and that they may contribute to interactions between the sleep/wake cycle and respiratory control.

Acidosis-Stimulated Neurons of the Medullary Raphe Are Serotonergic

Neurons of the medullary raphe project widely to respiratory and autonomic nuclei and contain co-localized serotonin, thyrotropin-releasing hormone (TRH), and substance P, three neurotransmitters known to stimulate ventilation. Some medullary raphe neurons are highly sensitive to pH and CO2 and have been proposed to be central chemoreceptors. Here it was determined whether these chemosensitive neurons are serotonergic. Cells were microdissected from the rat medullary raphe and maintained in primary cell culture for 13–70 days. Immunoreactivity for serotonin, substance P, and TRH was present in these cultures. All acidosis-stimulated neurons ( n = 22) were immunoreactive for tryptophan hydroxylase (TpOH-IR), the rate-limiting enzyme for serotonin biosynthesis, whereas all acidosis-inhibited neurons ( n= 16) were TpOH-immunonegative. The majority of TpOH-IR medullary raphe neurons (73%) were stimulated by acidosis. The electrophysiological properties of TpOH-IR neurons in culture were similar to those previously reported for serotonergic neurons in vivo and in brain slices. These properties included wide action potentials (4.55 ± 0.5 ms) with a low variability of the interspike interval, a postspike afterhyperpolarization (AHP) that reversed 25 mV more positive than the Nernst potential for K+, prominent A current, spike frequency adaptation and a prolonged AHP after a depolarizing pulse. Thus the intrinsic cellular properties of serotonergic neurons were preserved in cell culture, indicating that the results obtained using this in vitro approach are relevant to serotonergic neurons in vivo. These results demonstrate that acidosis-stimulated neurons of the medullary raphe contain serotonin. We propose that serotonergic neurons initiate a homeostatic response to changes in blood CO2 that includes increased ventilation and modulation of autonomic function.

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.

Central control of the cardiovascular and respiratory systems and their interactions in vertebrates

This review explores the fundamental neuranatomical and functional bases for integration of the respiratory and cardiovascular systems in vertebrates and traces their evolution through the vertebrate groups, from primarily water-breathing fish and larval amphibians to facultative air-breathers such as lungfish and some adult amphibians and finally obligate air-breathers among the reptiles, birds, and mammals. A comparative account of respiratory rhythm generation leads to consideration of the changing roles in cardiorespiratory integration for central and peripheral chemoreceptors and mechanoreceptors and their central projections. We review evidence of a developing role in the control of cardiorespiratory interactions for the partial relocation from the dorsal motor nucleus of the vagus into the nucleus ambiguus of vagal preganglionic neurons, and in particular those innervating the heart, and for the existence of a functional topography of specific groups of sympathetic preganglionic neurons in the spinal cord. Finally, we consider the mechanisms generating temporal modulation of heart rate, vasomotor tone, and control of the airways in mammals; cardiorespiratory synchrony in fish; and integration of the cardiorespiratory system during intermittent breathing in amphibians, reptiles, and diving birds. Concluding comments suggest areas for further productive research.

Breathing pattern and cost of ventilation in the American alligator

The energetic cost of pulmonary ventilation is termed the "cost of breathing" and is commonly determined from the change in oxygen uptake with altered ventilation. Previous analyses of lung mechanics predicts increased tidal volume would be more expensive than increased breathing frequency. Existing studies on the oxidative cost of breathing have, however, not addressed breathing pattern. We stimulated ventilation in juvenile alligators by either hypoxia or hypercapnia. Both hypoxia and hypercapnia increased ventilation (ten- and six-fold, respectively), but through entirely different changes in frequency and tidal volume combination. Hypoxia increased frequency from 1.4 to 6.0 breaths min -1 and tidal volume from 11.3 to 25.9 ml kg -1. During hypercapnia frequency remained constant, while tidal volume increased from 8.7 to 63.2 ml kg -1. Oxygen uptake remained constant at approximately 0.65 ml O2 kg -1 min -1 during all hypercapnic exposures, whereas oxygen uptake doubled during severe hypoxia. Extrapolating oxygen uptake to zero ventilation provides an estimate of non-ventilatory metabolic rate. Thus, ventilatory contributions to overall metabolic rate can be calculated. The cost of breathing estimated by hypoxic exposures (15% of total metabolic rate at rest) is markedly higher than that provided by hypercapnia (1-5% of total metabolic rate at rest). These data are in contrast to the predictions based on pulmonary mechanics.

Episodic breathing in alligators: role of sensory feedback

The episodic breathing pattern in many reptiles consists of two or more clustered breaths separated by variable non-ventilatory periods. This pattern is commonly postulated to result from oscillations in lung and/or blood PO2 or PCO2 via chemoreceptor feedback. We tested this hypothesis by monitoring breathing pattern in: (1) awake, undisturbed alligators and (2) sedated alligators (approx. 25 mg/kg pentobarbital, i.p.; 3 days prior to data collection). In sedated alligators, measurements were made: (1) before and after bilateral cervical vagotomy, a procedure that removes peripheral arterial chemoreceptors, CO2-sensitive intrapulmonary chemoreceptors and pulmonary stretch receptors (n = 6); and (2) during unidirectional ventilation (UDV) at high flow rates (greater than 2 L/min), thereby minimizing oscillations in lung and blood PO2 and PCO2 (n = 6). Measurements on sedated alligators were made at 30 and 20 degrees C in each of these conditions. In awake, undisturbed alligators, breathing was typically episodic with 2-7 breaths/cluster, although the pattern was easily altered (increased breaths/cluster) by even seemingly minor disturbances. In sedated alligators, episodic breathing was still evident after vagotomy, but only at increased inspired CO2; at 5% CO2 four of six alligators exhibited episodic breathing consisting of 2-3 breaths/cluster interspersed with occasional single breaths. An episodic breathing pattern was also evident during UDV; at low levels of CO2, 2-4 breaths/cluster interspersed with occasional single breaths were evident in four alligators, while two had 6-8 breaths/cluster. Increasing CO2 in the UDV gas stream generally increased the number of breaths/cluster. After vagotomy, all six alligators could manifest an episodic breathing pattern during UDV in at least one CO2 condition (greater than 2 breaths/cluster interspersed with occasional single breaths). The episodic breathing pattern was very labile, sometimes changing to single breaths without apparent cause. The results suggest: (1) episodic breathing requires neither feedback from vagal sensory receptors nor oscillations in respiratory gases; and (2) changes in arterial PCO2 modulate, but do not initiate episodic breathing. Episodic breathing in alligators may be due to complex interactions of higher brain structures with the central rhythm generator.

Temperature effects on ventilation and metabolism in the lizard, Ctenophorus nuchalis

The effect of temperature upon ventilation and metabolism was measured on a breath by breath basis in the lizard Ctenophorus nuchalis. This species displayed a typical reptilian breathing pattern; groups of breaths separated by periods of breath holding. The lengths of the non-ventilatory periods (NVP) decreased as temperature increased. As a result, overall breathing frequency (f) displayed a thermal dependence. Outside the preferred temperature range (27-37 degrees C) f was also affected by changes in the frequency of breathing that occurred during periods of ventilation (f'). On the other hand, tidal volume (VT) was maintained constant at all temperatures except 42 degrees C ('panting' threshold). Thus, due to the thermal dependency of f, ventilation (VE) displayed a significant increase between 18 and 37 degrees C. Metabolism (VO2 and VCO2) also increased within this temperature range. However, as a result of a lower Q10 for ventilation, ventilatory requirement (VE/VO2 and VE/VCO2) decreased. In addition, lung volume (LV) increased with temperature. Examination of the alveolar ventilation-PCO2 relationship revealed that as a result of the increase in LV the level of intrapulmonary CO2 at the end of an NVP was maintained irrespective of temperature. This implies a temperature-independent chemical threshold for the onset of breathing. It is postulated that with temperature the observed increase in LV could offset the increase in metabolism by slowing the rise of intrapulmonary CO2 levels. This would delay the onset of breathing which in turn would lower VE/VCO2 (and VE/VO2) by preventing an effectual hyperventilation. This supports the notion, at least in C. nuchalis, that relative alkalinity is maintained.

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.