Changes in ventilation and breathing pattern produced by changing body temperature and inspired CO2 concentration in turtles

Cross-Species Evaluation of Bioaccumulation Thresholds for Air-Breathing Animals

In air-breathing organisms, an organic chemical's susceptibility to elimination via urinary excretion and respiratory exhalation can be judged on the basis of the octanol-water partition ratio (K_OW) and the octanol-air partition ratio (K_OA), respectively. Current regulations specify that chemicals with K_OW values of <10² and K_OA values of <10⁵ may be screened as non-bioaccumulative in air breathers. Here we used a model-based approach to evaluate whether these thresholds are consistent with a biomagnification factor of 1 for 141 different mammals, birds, and reptiles. Animals with lower rates of respiration (e.g., manatees and sloths) and those ingesting high-lipid diets (e.g., polar bears and carnivorous birds) were predicted to be able to biomagnify persistent chemicals with K_OA values of <10⁵. This was also observed for several temperate reptiles due to their lower respiration rates and internal temperatures. Protective K_OA thresholds were determined to be <10^4.85 for mammals, <10^4.60 for birds, <10^4.60 for reptiles at >25 °C, and <10^3.95 for reptiles at ≤25 °C. For all animals, urination alone was not efficient to prevent the biomagnification of any organic chemical. For chemicals with K_OW values of <10¹, we found that biomagnification of persistent chemicals was constrained by the water-air partition ratio (K_WA) rather than K_OA. Differences in physiology may need to be considered in bioaccumulation assessments of air-breathing species.

Structural determinants of CO2-sensitivity in the β connexin family suggested by evolutionary analysis

A subclade of connexins comprising Cx26, Cx30, and Cx32 are directly sensitive to CO2. CO2 binds to a carbamylation motif present in these connexins and causes their hemichannels to open. Cx26 may contribute to CO2-dependent regulation of breathing in mammals. Here, we show that the carbamylation motif occurs in a wide range of non-mammalian vertebrates and was likely present in the ancestor of all gnathostomes. While the carbamylation motif is essential for connexin CO2-sensitivity, it is not sufficient. In Cx26 of amphibia and lungfish, an extended C-terminal tail prevents CO2-evoked hemichannel opening despite the presence of the motif. Although Cx32 has a long C-terminal tail, Cx32 hemichannels open to CO2 because the tail is conformationally restricted by the presence of proline residues. The loss of the C-terminal tail of Cx26 in amniotes was an evolutionary innovation that created a connexin hemichannel with CO2-sensing properties suitable for the regulation of breathing.

Isolated adult turtle brainstems exhibit central hypoxic chemosensitivity

During hypoxia, red-eared slider turtles increase ventilation and decrease episodic breathing, but whether these responses are due to central mechanisms is not known. To test this question, isolated adult turtle brainstems were exposed to 240 min of hypoxic solution (bath PO₂ = 32.6 ± 1.2 mmHg) and spontaneous respiratory-related motor bursts (respiratory event) were recorded on hypoglossal nerve roots. During hypoxia, burst frequency increased during the first 15 min, and then decreased during the remaining 35-240 min of hypoxia. Burst amplitude was maintained for 120 min, but then decreased during the last 120 min. The number of bursts/respiratory event decreased within 30 min and remained decreased. Pretreatment with either prazosin (α₁-adrenergic antagonist) or MDL7222 (5-HT₃ antagonist) blocked the hypoxia-induced short-term increase and the longer duration decrease in burst frequency. MDL7222, but not prazosin, blocked the hypoxia-induced decrease in bursts/respiratory event. Thus, during bath hypoxia, isolated turtle brainstems continued to produce respiratory motor output, but the frequency and pattern were altered in a manner that required endogenous α₁-adrenergic and serotonin 5-HT₃ receptor activation. This is the first example of isolated reptile brainstems exhibiting central hypoxic chemosensitivity similar to other vertebrate species.

Effects of environmental hypoxia and hypercarbia on ventilation and gas exchange in Testudines

Background

Ventilatory parameters have been investigated in several species of Testudines, but few species have had their ventilatory pattern fully characterized by presenting all variables necessary to understand changes in breathing pattern seen under varying environmental conditions.

Methods

We measured ventilation and gas exchange at 25 °C in the semi-aquatic turtle Trachemys scripta and the terrestrial tortoise Chelonoidis carbonarius under normoxia, hypoxia, and hypercarbia and furthermore compiled respiratory data of testudine species from the literature to analyze the relative changes in each variable.

Results

During normoxia both species studied showed an episodic breathing pattern with two to three breaths per episode, but the non-ventilatory periods (T_NVP) were three to four times longer in T. scripta than in C. carbonarius. Hypoxia and hypercarbia significantly increased ventilation in both species and decreased T_NVP and oxygen consumption in T. scripta but not in C. carbonarius.

Discussion

Contrary to expectations, the breathing pattern in C. carbonarius did show considerable non-ventilatory periods with more than one breath per breathing episode, and the breathing pattern in T. scripta was found to diverge significantly from predictions based on mechanical analyses of the respiratory system. A quantitative analysis of the literature showed that relative changes in the ventilatory patterns of chelonians in response to hypoxia and hyperbarbia were qualitatively similar among species, although there were variations in the magnitude of change.

A metabolic hypothesis for the evolution of temperature effects on the arterial PCO2 and pH of vertebrate ectotherms

Body temperature increases in ectothermic vertebrates characteristically lead to both increases in arterial PCO2 (PaCO2) and declines in resting arterial pH (pHa) of about 0.017 pH units/°C increase in temperature. This ‘alphastat’ pH pattern has previously been interpreted as being evolutionarily-driven by the maintenance of a constant protonation state on the imidazole moiety of histidine protein residues, hence stabilizing protein structure-function. Analysis of the existing data for interclass responses of ectothermic vertebrates show different degrees of PaCO2 increases and pH declines with temperature between the classes with reptiles&gt;amphibians&gt;fish. The PaCO2 at the temperature where maximal aerobic metabolism (VO2max) is achieved is significantly and positively correlated with temperature for all vertebrate classes. For ectotherms, the PaCO2 where VO2max is greatest is also correlated with VO2max indicating there is an increased driving force for CO2 efflux that is lowest in fish, intermediate in amphibians and highest in reptiles. The pattern of increased PaCO2 and the resultant reduction of pHa to increased body temperature would serve to increase CO2 efflux, O2 delivery, blood buffering capacity and maintain ventilatory scope. This represents a new hypothesis for the selective advantage of arterial pH regulation from a systems physiology perspective in addition to the advantages of maintenance of protein structure-function.

Acid-base balance in the developing marsupial: from ectotherm to endotherm

Marsupial joeys are born ectothermic and develop endothermy within their mother's thermally stable pouch. We hypothesized that Tammar wallaby joeys would switch from α-stat to pH-stat regulation during the transition from ectothermy to endothermy. To address this, we compared ventilation (V̇e), metabolic rate (V̇o2), and variables relevant to blood gas and acid-base regulation and oxygen transport including the ventilatory requirements (V̇e/V̇o2 and V̇e/V̇co2), partial pressures of oxygen (PaO2), carbon dioxide (PaCO2), pHa, and oxygen content (CaO2) during progressive hypothermia in ecto- and endothermic Tammar wallabies. We also measured the same variables in the well-studied endotherm, the Sprague-Dawley rat. Hypothermia was induced in unrestrained, unanesthetized joeys and rats by progressively dropping the ambient temperature (Ta). Rats were additionally exposed to helox (80% helium, 20% oxygen) to facilitate heat loss. Respiratory, metabolic, and blood-gas variables were measured over a large body temperature (Tb) range (∼15–16°C in both species). Ectothermic joeys displayed limited thermogenic ability during cooling: after an initial plateau, V̇o2 decreased with the progressive drop in Tb. The Tb of endothermic joeys and rats fell despite V̇o2 nearly doubling with the initiation of cold stress. In all three groups the changes in V̇o2 were met by changes in V̇e, resulting in constant V̇e/V̇o2 and V̇e/V̇co2, blood gases, and pHa. Thus, although thermogenic capability was nearly absent in ectothermic joeys, blood acid-base regulation was similar to endothermic joeys and rats. This suggests that unlike some reptiles, unanesthetized mammals protect arterial blood pH with changing Tb, irrespective of their thermogenic ability and/or stage of development.

Daily and seasonal rhythms in the respiratory sensitivity of red-eared sliders (Trachemys scripta elegans)

The purpose of the present study was to determine whether the daily and seasonal changes in ventilation and breathing pattern previously documented in red-eared sliders resulted solely from daily and seasonal oscillations in metabolism or also from changes in chemoreflex sensitivity. Turtles were exposed to natural environmental conditions over a one year period. In each season, oxygen consumption, ventilation and breathing pattern were measured continuously for 24 h while turtles were breathing air and for 24 h while they were breathing a hypoxic-hypercapnic gas mixture (H-H). We found that oxygen consumption was reduced equally during the day and night under H-H in all seasons except spring. Ventilation was stimulated by H-H but the magnitude of the response was always less at night. On average, it was also less in the winter and greater in the reproductive season. The data indicate that the day-night differences in ventilation and breathing pattern seen previously resulted from daily changes in chemoreflex sensitivity whereas the seasonal changes were strictly due to changes in metabolism. Regardless of mechanism, the changes resulted in longer apneas at night and in the winter at any given level of total ventilation, facilitating longer submergence at times of the day and year when turtles are most vulnerable.

Spinal cord injury-induced changes in breathing are not due to supraspinal plasticity in turtles (Pseudemys scripta)

After occurrence of spinal cord injury, it is not known whether the respiratory rhythm generator undergoes plasticity to compensate for respiratory insufficiency. To test this hypothesis, respiratory variables were measured in adult semiaquatic turtles using a pneumotachograph attached to a breathing chamber on a water-filled tank. Turtles breathed room air (2 h) before being challenged with two consecutive 2-h bouts of hypercapnia (2 and 6% CO2or 4 and 8% CO2). Turtles were spinalized at dorsal segments D8–D10so that only pectoral girdle movement was used for breathing. Measurements were repeated at 4 and 8 wk postinjury. For turtles breathing room air, breathing frequency, tidal volume, and ventilation were not altered by spinalization; single-breath (singlet) frequency increased sevenfold. Spinalized turtles breathing 6–8% CO2had lower ventilation due to decreased frequency and tidal volume, episodic breathing (breaths/episode) was reduced, and singlet breathing was increased sevenfold. Respiratory variables in sham-operated turtles were unaltered by surgery. Isolated brain stems from control, spinalized, and sham turtles produced similar respiratory motor output and responded the same to increased bath pH. Thus spinalized turtles compensated for pelvic girdle loss while breathing room air but were unable to compensate during hypercapnic challenges. Because isolated brain stems from control and spinalized turtles had similar respiratory motor output and chemosensitivity, breathing changes in spinalized turtles in vivo were probably not due to plasticity within the respiratory rhythm generator. Instead, caudal spinal cord damage probably disrupts spinobulbar pathways that are necessary for normal breathing.

Ventilatory responses to changes in temperature in mammals and other vertebrates

This article reviews the relationship between pulmonary ventilation (VE) and metabolic rate (oxygen consumption) during changes in ambient temperature. The main focus is on mammals, although for comparative purposes the VE responses of ectothermic vertebrates are also discussed. First, the effects of temperature on pulmonary mechanics, chemoreceptors, and airway receptors are summarized. Then we review the main VE responses to cold and warm stimuli and their interaction with exercise, hypoxia, or hypercapnia. In these cases, mammals attempt to maintain both oxygenation and body temperature, although conflicts can arise because of the respiratory heat loss associated with the increase in ventilation. Finally, we consider the VE responses of mammals when body temperature changes, as during torpor, fever, sleep, and hypothermia. In ectotherms, during changes in temperature, VE control becomes part of a general strategy to maintain constant relative alkalinity and ensure a constancy of pH-dependent protein functions (alphastat regulation). In mammals on the other hand, VE control is aimed to balance metabolic needs with homeothermy. Therefore, alphastat regulation in mammals seems to have a low priority, and it may be adopted only in exceptional cases.

Seasonal changes in the cardiorespiratory responses to hypercarbia and temperature in the bullfrog, Rana catesbeiana

We assessed the seasonal variations in the effects of hypercarbia (3 or 5% inspired CO2) on cardiorespiratory responses in the bullfrog Rana catesbeiana at different temperatures (10, 20 and 30 degrees C). We measured breathing frequency, blood gases, acid-base status, hematocrit, heart rate, blood pressure and oxygen consumption. At 20 and 30 degrees C, the rate of oxygen consumption had a tendency to be lowest during winter and highest during summer. Hypercarbia-induced changes in breathing frequency were proportional to body temperature during summer and spring, but not during winter (20 and 30 degrees C). Moreover, during winter, the effects of CO2 on breathing frequency at 30 degrees C were smaller than during summer and spring. These facts indicate a decreased ventilatory sensitivity during winter. PaO2 and pHa showed no significant change during the year, but PaCO2 was almost twice as high during winter than in summer and spring, indicating increased plasma bicarbonate levels. The hematocrit values showed no significant changes induced by temperature, hypercarbia or season, indicating that the oxygen carrying capacity of blood is kept constant throughout the year. Decreased body temperature was accompanied by a reduction in heart rate during all four seasons, and a reduction in blood pressure during summer and spring. Blood pressure was higher during winter than during any other seasons whereas no seasonal change was observed in heart rate. This may indicate that peripheral resistance and/or stroke volume may be elevated during this season. Taken together, these results suggest that the decreased ventilatory sensitivity to hypercarbia during winter occurs while cardiovascular parameters are kept constant.

Hypoxia, temperature, and pH/CO2 effects on respiratory discharge from a turtle brain stem preparation

An in vitro brain stem preparation from adult turtles (Chrysemys picta) was used to examine the effects of anoxia and increased temperature and pH/CO2 on respiration-related motor output. At pH approximately 7.45, hypoglossal (XII) nerve roots produced patterns of rhythmic bursts (peaks) of discharge (O.74 +/- 0.07 peaks/min 10.0 +/- 0.6 s duration) that were quantitatively similar to literature reports of respiratory activity in conscious, vagotomized turtles. Respiratory discharge was stable for 6 h at 22 degrees C; at 32 degrees C, peak amplitude and frequency progressively and reversibly decreased with time. Two hours of hypoxia had no effect on respiratory discharge. Acutely increasing bath temperature from 22 to 32 degrees C decreased episode and peak duration and increased peak frequency. Changes in pH/CO2 increased peak frequency from zero at pH 8.00-8.10 to maxima of 0.81 +/- 0.01 and 1.44 +/- 0.02 peaks/min at 22 degrees C (pH 7.32) and 32 degrees C (pH 7.46), respectively; pH/CO2 sensitivity was similar at both temperatures. We conclude that 1) insensitivity to hypoxia indicates that rhythmic discharge does not reflect gasping behavior, 2) increased temperature alters respiratory discharge, and 3) central pH/CO2 sensitivity is unaffected by temperature in this preparation (i.e., Q10 approximately 1.0).

Temperature and cerebral blood flow regulation in the freshwater turtle, Pseudemys scripta

Chemical regulation of cerebral blood flow (CBF) by CO2 has been demonstrated in an ectohermic vertebrate (Davies, Am. J. Physiol. 260: R382, 1991). Cerebrovascular sensitivity to CO2 (delta CBF/delta PaCO2), a measure of the vascular reactivity of the cerebral blood vessels to CO2, was found to be 0.7 ml.min-1.100 g-1.Torr-1 during normoxia and 3.4 during anoxia in the freshwater turtle, Pseudemys scripta. In the present study, the effect of body temperature on delta CBF/delta PaCO2 was studied. delta CBF/delta PaCO2 was not significantly affected by body temperature. It was concluded that if delta CBF/delta PaCO2 remains constant with changes in body temperatures and CBF is controlled by CO2, CBF should increase with temperature due to the temperature-induced increase in PaCO2.

Tissue-specific metabolism during normothermy and daily torpor in deer mice (Peromyscus maniculatus)

Previous work with deer mice (Peromyscus maniculatus) has demonstrated that a significant acidosis occurs during daily torpor. In addition, carbohydrate levels are significantly lower, whereas fatty acid and ketone levels are significantly higher during torpor. The present study examined the effects of these in vivo acid-base and metabolite adjustments on in vitro 14C-glucose metabolism in tissues taken from normothermic and torpid deer mice. Glucose oxidation in liver and vastus lateralis taken from normothermic animals was reduced by a change in incubation temperature from 37 degrees C to 25 degrees C (liver, 0.44 to 0.23 mumoles/g.h; vastus, 0.66 to 0.25 mumoles/g.h), whereas heart from normothermic mice exhibited an increase from 0.57 to 0.99 mumoles/g.h at the lower temperature. Altering acid-base conditions or metabolite levels had no effect on glucose metabolism in the heart or liver. However, both of these factors significantly influenced metabolism in vastus. Vastus taken from normothermic mice had an increased glucose oxidation rate under the more acidic torpor conditions (0.25 to 0.33 mumoles/g.h), whereas a reduction in oxidation occurred when incubated with torpor substrate concentrations (0.33 to 0.22 mumoles/g.h). In vastus taken from torpid mice, changing acid-base state and metabolites to torpor levels resulted in a significant reduction in glucose oxidation (0.26 to 0.12 mumoles/g.h). Under most incubation conditions, glucose oxidation was significantly lower in tissues taken from torpid mice than in these tissues from normothermic mice, suggesting that adjustments in addition to changes in acid-base and substrate parameters may be necessary to account for metabolic modifications during daily torpor.

Effects of vagotomy on ventilatory responses to CO2 in alligators

Reptiles increase ventilation during hypercapnia at a constant temperature. In this study, the contributions of vagal vs non-vagal receptors to CO2 ventilatory responses were investigated in 16 sedated Alligator mississippiensis (25 mg/kg pentobarbital; 3 days prior to data collection). Four animals served as controls to assess the effects of time and/or anesthetic drift on ventilation and blood gases; significant ventilatory drift was not detected during the observation period. The effects of bilateral vagotomy on CO2 ventilatory responses were determined during spontaneous breathing (n = 6) and unidirectional ventilation (UDV; n = 6) at two body temperatures (Tb = 30 and 20 degrees C). Resting PaCO2, minute ventilation (VI), tidal volume (VT) and breathing frequency (f) were elevated at 30 degrees C relative to 20 degrees C in spontaneously breathing alligators. Increasing inspired CO2 to 5% increased PaCO2, f, VT and VI at both levels of Tb. Ventilatory sensitivity to CO2 (S = delta VI/delta PaCO2) was higher at 30 degrees C with a temperature coefficient (Q10) of 2.3. Vagotomy increased PaCO2 and VT, decreased f and had no effect on VI at either Tb. After vagotomy, hypercapnia had no effects on ventilation. When CO2 feedback loops were opened by UDV at a high flow rate (greater than 2 L/min), Tb had no effects on ventilatory efforts at constant PCO2, but hypercapnia significantly increased f, VT and VI. S was variable with a Q10 of 2.1. After vagotomy, a significant CO2-ventilatory response remained during UDV, but S was unaffected by Tb (Q10 = 0.8). The results indicate that non-vagal chemoreceptors contribute to CO2 ventilatory responses in alligators, although their contribution following vagotomy is evident only during unidirectional ventilation. Although tentative, the data also suggest that CO2-sensitive vagal receptors may be necessary for the temperature dependency of S.

Metabolic, respiratory and cardiac activity in the shrew Crocidura russula

Oxygen consumption, respiratory rate, heart rate and body temperature of resting common white-toothed shrews (Crocidura russula, mean = 11.4 g), have been determined at ambient temperatures (Ta) between 0 and 37 degrees C. Mean basal oxygen consumption (Ta = 30 degrees C, Tb = 35.4 degrees C) was 2.3 ml.g-1.h-1 and was about 12% above the value expected on the basis of the allometric relationship applying for mammals. At 0 degree C oxygen consumption was 4.2 times that in the thermal neutral zone (TNZ) which is located at Ta of about 30 degrees C. The mean basal respiratory rate was 103 min-1 (Ta = 30 degrees C), fully 40% below the predicted value. The respiratory rate increased at 0 degree C to 3.8 times that in TNZ. The amount of oxygen consumed per breath was rather constant, increasing from a mean of 4.3 microliters (Ta = 30 degrees C) to 4.9 microliters (Ta = 0 degree C) which was only 15% above the basal value. Comparing the great changes of respiratory rate with the small alterations of oxygen consumed per breath, a dominant influence of respiratory rate in the regulation of respiration is shown. Basal heart rate was 444 min-1 (Ta = 30 degrees C), in agreement with the expected value. Heart rate increased only 1.75-fold at an ambient temperature of 0 degree C. Oxygen pulses depend very strongly on ambient temperature, increasing from 0.98 microliters (Ta = 30 degrees C) to 2.5 microliters at 0 degree C. Beat frequency and stroke volume regulation are salient features of heart function.