Acid-Base Regulation and Temperature in Selected Invertebrates as a Function of Temperature

The pH of the hemolymph of selected invertebrates decreasesas their body temperature increases. The magnitude of this change(pH/°C) is very similar to the change of the pH of waterwith temperature (pN/°C) and suggests that these invertebrates,like poikilothermous vertebrates, regulate the pH of their extracellularfluid so that its degree of alkalinity relative to the pH ofwater remains constant. The degree of alkalinity (pH_blood-pN)varies between species, but seems to be fixed for any givenspecies. In Limulus pH-pN was essentially the same for in vivosamples, measured after the whole animal had been acclimatedto different temperatures, as it was for in vitro samples inwhich the hemolymph was cooled or warmed anaerobically, suggestingthat the CO₂ content of the extracellular fluid is constantas the temperature changes. The P_CO2 of the hemolymph is invariablylower in animals breathing water than in those breathing air.In the invertebrates, as in the vertebrates, manipulation ofP_CO2 and HCO₃- is probably the major mechanism in the regulationof the relative alkalinity of the extracellular fluid.     

Episodic Breathing in Frogs: Converging Hypotheses on Neural Control of Respiration in Air Breathing Vertebrates

SYNOPSIS. The episodic, or intermittent, breathing of frogsand many ectothermic vertebrates results in important fluctuationsof arterial blood gases. This pattern of breathing differs fromthe rhythmic and continuous alternation of inspiration observedin most homeotherms, which maintain O₂ and CO₂ levels withinnarrow ranges. These differences in pattern of breathing indicatethat the respiratory control systems of ectotherms and homeothermsdiffer substantially. The results of recent studies using invitro brainstemspinal cord preparations of adult frogs and premetamorphictadpoles (Rana catesbeiana and Rana pipiens) demonstrate, however,that the mechanisms for rhythm generation and pattern formationdescribed previously for mammals are also key features of therespiratory control system of frogs. These findings thereforesupport the hypothesis that the respiratory control system ishighly conserved amongst air breathing vertebrates, whetherthey breathe continuously or episodically.     

Respiratory,cardiovascular, and hemolymph acid‐base changes in the amphibious crab,Cardisoma guanhumi,during immersion and emersion

The cardiorespiratory and hemolymph acid base status of bimodal breathing crabs, Cardisoma guanhumi, was monitored during the transition from breathing air to breathing water. Upon immersion, oxygen uptake (MO₂) decreased by half. Ventilatory frequency (fsc) increased more than 5 fold, causing a decrease in hemolymph carbon dioxide partial pressure (PCO₂). This was nearly fully compensated for by a gradual decrease in hemolymph bicarbonate concentration ([HCO₃ ^‐]) over 96 hours post‐immersion. After one to two weeks of immersion, when crabs were removed from the water, oxygen uptake initially increased, but eventually returned to the initial immersed value. Heart rate was unchanged but fsc slowed dramatically. The decreased ventilation resulted in a buildup of hemolymph PCO₂, causing a respiratory acidosis that was slowly compensated for by increased hemolymph [HCO₃ ^‐]. C. guanhumi appears to be a truly amphibious crab with respiratory and acid‐base adaptations found in both fully aquatic and fully terrestrial species.     

Morphological, Behavioral, and Physiological Characterization of Bimodal Breathing Crustaceans

SYNOPSIS Bimodal breathing crustaceans, while representing astage in the transition from the aquatic to terrestrial habitat,also constitute a distinct group that can be characterized bymorphological, behavioral, and physiological traits. Morphologically,this group displays reduced gill surface area and enlarged branchialchambers. The lining of the branchial chambers, the branchiostegites,also has increased surface area and is highly vascularized.The branchiostegites can be smooth, cutaneous epithelia, orthey can have more complex evaginations or invaginations tofurther increase surface area. In addition, the thickness ofthe branchiostegal epithelium is greatly reduced, compared tothat in the gills, thus minimizing the diffusion distance betweenair and hemolymph. These animals maintain a store of water inthe branchial chamber that covers the gills and allows for simultaneousgas exchange with two media (air and water). There is also apartitioning of gas exchange, with oxygen uptake occurring preferentiallyfrom air across the branchiostegites, and carbon dioxide excretionoccurring across the gills into the branchial water. One criticalfactor that appears to separate bimodal breathing crustaceansfrom fully terrestrial, exclusively air-breathers is the abilityof the latter group to excrete CO₂directly into air across thegills and branchiostegites. It is suggested that the incorporationof the enzyme carbonic anhydrase into the membrane fractionof the branchiostegites may have been one of the key molecularevents which allowed for pulmonary CO₂ excretion into air.     

Effect of prior O2 breathing on ventilatory response to sustained isocapnic hypoxia in adult humans

Honda, Y., H. Tani, A. Masuda, T. Kobayashi, T. Nishino, H. Kimura, S. Masuyama, and T. Kuriyama. Effect of priorO₂ breathing on ventilatoryresponse to sustained isocapnic hypoxia in adult humans.J. Appl. Physiol. 81(4):1627-1632, 1996.Sixteen healthy volunteers breathed 100%O₂ or room air for 10 min in random order, then their ventilatory response to sustained normocapnic hypoxia (80% arterial O₂saturation, as measured with a pulse oximeter) was studied for 20 min.In addition, to detect agents possibly responsible for the respiratorychanges, blood plasma of 10 of the 16 subjects was chemically analyzed.1) Preliminary O₂ breathing uniformly andsubstantially augmented hypoxic ventilatory responses.2) However, the profile ofventilatory response in terms of relative magnitude, i.e., biphasichypoxic ventilatory depression, remained nearly unchanged.3) Augmented ventilatory incrementby prior O₂ breathing wassignificantly correlated with increment in the plasma glutamine level.We conclude that preliminary O₂administration enhances hypoxic ventilatory response without affectingthe biphasic response pattern and speculate that the excitatory aminoacid neurotransmitter glutamate, possibly derived from augmentedglutamine, may, at least in part, play a role in this ventilatoryenhancement.

     

Some Thermal Requirements of Fiddler Crabs of the Temperate and Tropical Zones and Their Influence on Geographic Distribution

Thermal stress is one environmental parameter that has greatlyinfluenced the migration of crustaceans from the sea to land.Since a greater number of species of terrestrial crabs are foundin the tropics than in the temperate zone, comparative studiesof the influence of temperature on latitudinally separated populationswere undertaken. Two tropical species, U. rapax and U. thayeri,may occur as far north as St. Augustine, Florida, or, followinga severe winter, may be rare north of Cape Kennedy. The lethaleffect of the low temperatures recorded during one severe winter(1957–58) is supported by laboratory studies in whichLD₅₀, deaths occurred in 4.5 days at 10°C for U. rapax acclimatedto 18°C. The experiment demonstrates that U. rapax cannotacclimate to and survive low temperatures. This contrasts markedlywith the situation in semi-terrestrial crabs of the temperatezone, which are able to acclimate to cold. The distribution of Uca around Cape Cod Bay correlates wellwith the coastal hydrographic thermal gradient and supportsPassano's suggestion that temperatures below 20° may belimiting as they inhibit proecdysis in U. pugnax. Such an inhibitionis found experimentally in U. pugilator and in the tropicalspecies, U. rapax. It is hypothesized that a shift in the thermodynamicsof the processes underlying molting has not occurred in Ucaof the temperate zone. The paucity of semi-terrestrial Brachyurain the temperate zone may be due to the failure of many speciesto evolve capacity-adaptations to carry out all requisite lifeprocesses at temperatures below 20°, or the resistance-adaptationsnecessary to survive the low temperatures of winter.     

Ecological and Evolutionary Aspects of Integumentary Respiration: Body Size, Diffusion, and the Invertebrata

SYNOPSIS. Most animal phyla lack specialized respiratory surfacesand all phyla contain groups that, for some part of their lifehistory, depend entirely upon integumental diffusion of respiratorygases. Animals that are diffusion-limited, yet function aerobicallyare generally small with large surface areas and there has beenconvergence for this among all phyla including the coelomateinvertebrates. Acoelomates lack specialized respiratory structuresbut have highly modified integuments, functional specializations,and features ranging from symbioses to air gulping that compensatefor diffusion limitation. The diversity of structures functioningfor integumentary respiration is much greater among invertebratesthan vertebrates. Among the higher invertebrates with respiratorysurfaces, accessory integumentary O₂ uptake is usually 20 to50% of total respiration. The high diffusion constant of O₂in air minimizes boundary effects on gas transfer and permitslarger body size, although this is limited by dry conditions.Terrestrial annelids and flatworms, both confined to moist habitats,are larger than aquatic forms which often have accessory gills.Size differences between terrestrial forms in these two phylareflect the presence of a circulation in the annelids. Ontogenetictransitions from skin breathing to other respiratory structuresoccur among marine invertebrates and vertebrates. Vertebratesapparently exercise greater integratory control over integumentalrespiration through adjustment of ventilation and perfusion;however, it is not known if these processes occur in some invertebrates.     

Accumulation of Uric Acid in the Land Crab, Cardisoma guanhumi

Variable amounts of white particulate matter occur in the hemocoelof the land crab, Cardisoma guanhumi. This material, mainlyuric acid, accounts for 0.2–15.9% of the total dry weightof a series of crabs. The material appears to increase duringintermolt. Concentrations of uric acid in the blood varied from0.2 to 13.2 mg/100 ml. Urine and feces contained little or nouric acid. Estimates of NH₃-N (3.9 mg/100 ml) in the blood of C. guanhumiare higher, and those of non-protein N (10.2 mg/100 ml) arelower, than corresponding values reported for other decapods.The crab's rate of nitrogen release (0.4 mg/10 g/day) into wateris similar to that reported for other terrestrial crustaceans.About one-third of this is released as NH₃. The urine of C.guanhumi is frequently nearly free of nitrogen, and its stomachfluid contains about five to seven times as much as does itsblood. Implications of these findings in the crab's metabolismand excretion are discussed.     

A microsensor for direct measurement of O2 partial pressure within plant tissues3

Widespread use of O₂ microsensors to measure O₂ partial pressure(pO₂) in plant tissues has been limited in part because of difficultyof construction and other technical obstacles. By modifyingpublished techniques, an O₂ microsensor was constructed thatcombined the advantages of Clark-type microsensors with lesscomplicated construction techniques. The specifications andsome performance characteristics of the microsensor are: tipdiameter 1–5 µm; sensitivity 7.5–25 pA kPa^–1;negligible stir-induced current; response time 540 ms. The microsensorcan be used in air or solution, and each sensor can be usedfor several experiments. The sensitivity of the microsensorwas unchanged during measurements over the physiological rangeof pO₂ in intact, growing maize (Zea mays L.) primary roots,and was thus unaffected by cellular fluids and turgor pressure.Use of the microsensor to compare pO₂ profiles in vermiculite-and solution-grown roots is described. The O₂ microsensor couldfind application in studies in which information on tissue pO₂is needed, but for which conventional O₂ probes are too large. Key words: Oxygen microsensor, Zea mays L., roots, oxygen partial pressure     

The Foregut of Gecarcinus lateralis as an Organ of Salt and Water Balance

The permeability of the foregut of the land crab, Gecarcinuslateralis, to tritiated water (THO), Na²², and Cl³⁶ was studiedin vitro during the intermolt period and after ecdysis. In crabswith eyestalks, the foregut is permeable to water and ions inthe direction hemolymph-to-lumen and lumen-to-hemolymph, bothduring the intermolt period and after ecdysis. However, theforegut of animals without eyestalks is impermeable after ecdysis. The movement of THO always follows the movement of Na²² acrossthe wall of the foregut, while the movement of Cl³⁶ may or maynot be correlated with the movement of Na²² and THO. Comparisonof the ratio of water to ions in the hemolymph with the ratiocalculated from radioisotope flux indicates that Na⁺ and waterare probably moving isosmotically, although not necessarilyaccompanied by Cl^– When an extract of the thoracic ganglionic mass of G. lateralisis added to the "hemolymph side" of the foregut in vitro, thereis immediately a large increase in permeability to water andsalts. This occurs in the foregut of crabs with eyestalks duringintermolt and also in eyestalkless crabs after ecdysis. Thus, not only is the foregut of Gecarcinus lateralis permeableto water and salts in both directions, but also the extent ofits permeability is under neuroendocrine control. As a consequence,the animal may be able to move water and salts into the foregutor out of it into the hemolymph as needed. This may be an importantadaptation for a terrestrial crab that must conserve water,especially at the critical time of ecdysis.     

Hypoxic Reptiles: Blood Gases, Body Temperature and Control of Breathing

Our contribution to this symposium is a review of recent modelsand experimental cdata on oxygen homeostasis in vertebrateswith normal intracardiac shunts; i.e., amphibians and reptiles.We focus on the interactions among hemoglobin function, bodytemperature regulation, and cardiovascular shunts under normalconditions (i.e., breathing fresh air at or near sea level)and during external hypoxia (e.g., altitude, burrows) and internalhypoxia (e.g., anemia, hemorrhage). Mathematical models andexperimental data suggest that animals with venous admixturefrom cardiovascular shunts will show biphasic arterial and mixedvenous Po₂ responses to warming; i.e., first increasing andthen, as the dissociation curve shifts too far to the right,decreasing. This has implications for many physiological functionsincluding oxygen consumption by tissues, control of breathing,as well as preferred body temperature and its regulation. Wepresent some of the recent experiments that have explored theseimplications.     

Simultaneous CO2- and 16O2/18O2-gas exchange and fluorescence measurements indicate differences in light energy dissipation between the wild type and the phytochrome-deficient aurea mutant of tomato during water stress

The CO₂-, H₂O- and ¹⁶O₂/¹⁸O₂ isotopic-gas exchange and the fluorescencequenching by attached leaves of the wild-type and of the phytochrome-deficienttomato aurea mutant was compared in relation to water stressand the photon fluence rate. The chlorophyll content of aurealeaves was reduced and the ultra-structure of the chloroplastswas altered. Nevertheless, the maximum rate of net CO₂ uptakein air by the yellow-green leaves of the aurea mutant was similarto that by the dark-green wild-type leaves. However, less O₂was produced by the leaves of the aurea mutant than by leavesof the wild-type. This result indicates a reduced rate of photosyntheticelectron flux in aurea mutant leaves. No difference in bothphotochemical and non-photochemical fluorescence quenching wasfound between wild-type and aurea mutant leaves. Water stresswas correlated with a reversible decrease in the rates of bothnet CO₂ uptake and transpiration by wild-type and aurea mutantleaves. The rate of gross ¹⁶O₂ evolution by both wild-type andaurea mutant leaves was fairly unaffected by water stress. Thisresult shows that in both wild-type and aurea leaves, the photochemicalprocesses are highly resistant to water stress. The rate ofgross ¹⁸O₂ uptake by wild-type leaves increased during waterstress when the photon fluence rate was high. Under the sameconditions, the rate of gross ¹⁸O₂ uptake by ^aurea mutant leavesremained unchanged. The physiological significane of this differencewith respect to the (presumed) importance of oxygen reductionin photoprotection is discussed. Key words: Water stress, gas exchange, fluorescence quenching, Lycopersicon esculentum, mutant (tomato, aurea), energy dissipation     

Scent of death: Evolution from sea to land of an extreme collective attraction to conspecific death

All living organisms must eventually die, though in some cases their death can bring life‐giving opportunities. Few studies, however, have experimentally tested how animals capitalize on conspecific death and why this specialization would evolve. Here, we conducted experiments on the phylogenetically most closely‐related marine and terrestrial hermit crabs to investigate the evolution of responses to death during the sea‐to‐land transition. In the sea, death of both conspecifics and heterospecifics generates unremodeled shells needed by marine hermit crabs. In contrast, on land, terrestrial hermit crabs are specialized to live in architecturally remodeled shells, and the sole opportunity to acquire these essential resources is conspecific death. We experimentally tested these different species’ responsiveness to the scent of conspecific versus heterospecific death, predicting that conspecific death would have special attractive value for the terrestrial species. We found the terrestrial species was overwhelmingly attracted to conspecific death, rapidly approaching and forming social groupings around conspecific death sites that dwarfed those around heterospecific death sites. This differential responsiveness to conspecific versus heterospecific death was absent in marine species. Our results thus reveal that on land a reliance on resources associated exclusively with conspecifics has favored the evolution of an extreme collective attraction to conspecific death.     

Interactions Between Limb Regeneration and Molting in Decapod Crustaceans

Molting and regeneration of lost appendages are tightly-coupled,hormonally-regulated processes in decapod crustaceans. Precociousmolts are induced by eyestalk ablation, which reduces circulatingmolt-inhibiting hormone (MIH) and results in an immediate risein hemolymph ecdysteroids. Precocious molts are also inducedby autotomy of 5–8 walking legs; adult land crabs (Gecarcinuslateralis) molt 6–8 wk after multiple leg autotomy (MLA).Autotomy of one or more of the 1° limb buds (LBs) that formafter MLA before a critical period interrupts proecdysis until2° LBs re-regenerate and grow to the approximate size ofthose lost. Based on these observations, Skinner proposed thatlimb buds produce two factors that control proecdysial events.Limb Autotomy Factor–Anecdysis (LAF_an), produced by 1°LBs when at least five legs are autotomized, stimulates anecdysialanimals to enter proecdysis. Limb Autotomy Factor–Proecdysis(LAF_pro), produced by 2° LBs in premolt animals when atleast one 1° LB is autotomized, inhibits proecdysial processes.Initial characterizations suggest that LAF_pro is a MIH-likepolypeptide that inhibits the synthesis and secretion of ecdysteroidby the Y-organs.     

Assessing the genetic relatedness of higher ozone sensitivity of modern wheat to its wild and cultivated progenitors/relatives

Modern wheat (Triticum aestivum L.) is one of the most ozone(O₃)-sensitive crops. However, little is known about its geneticbackground of O₃ sensitivity, which is fundamental for breedingO₃-resistant cultivars. Wild and cultivated species of winterwheat including donors of the A, B and D genomes of T. aestivumwere exposed to 100 ppb O₃ or charcoal-filtered air in opentop chambers for 21 d. Responses to O₃ were assessed by visibleO₃ injury, gas exchange, chlorophyll fluorescence, relativegrowth rate, and biomass accumulation. Ozone significantly decreasedlight-saturated net photosynthetic rate (–37%) and instantaneoustranspiration efficiency (–42%), but increased stomatalconductance (+11%) and intercellular CO₂ concentration (+11%).Elevated O₃ depressed ground fluorescence (–8%), maximumfluorescence (–26%), variable fluorescence (–31%),and maximum photochemical efficiency (–7%). Ozone alsodecreased relative growth rate and the allometric coefficient,which finally reduced total biomass accumulation (–54%),but to a greater extent in roots (–77%) than in the shoot(–44%). Winter wheat exhibited significant interspeciesvariation in the impacts of elevated O₃ on photosynthesis andgrowth. Primitive cultivated wheat demonstrated the highestrelative O₃ tolerance followed by modern wheat and wild wheatshowed the lowest. Among the genome donors of modern wheat,Aegilops tauschii (DD) behaved as the most O₃-sensitive followedby T. monococcum (AA) and Triticum turgidum ssp. durum (AABB)appeared to be the most O₃-tolerant. It was concluded that thehigher O₃ sensitivity of modern wheat was attributed to theincreased O₃ sensitivity of Aegilops tauschii (DD), but notto Triticum turgidum ssp. durum (AABB) during speciation. Key words: Biomass, Chl a fluorescence, genome, ozone sensitivity, relative growth rate, stomatal conductance, winter wheat Received 20 September 2007; Revised 30 November 2007 Accepted 16 January 2008     

EFFECTS OF TEMPERATURE ON CHRONIC HYPOXIA TOLERANCE IN THE NON-INDIGENOUS BROWN MUSSEL, PERNA PERNA (BIVALVIA: MYTILIDAE) FROM THE TEXAS GULF OF MEXICO

Effects of temperature (15°, 20° and 25°C), O₂ partialpressure (P_O2=0, 1, 2, 4, and 6 kPa), and individual size(12–79 mm shell length; SL) on survivorship of specimensof the non-indigenous, marine, brown mussel, Perna perna, fromTexas were investigated to assess its potential distributionin North America. Its hypoxia tolerance was temperature-dependent,survivorship being significantly extended at lower temperaturesunder all tested lethal P_O2. Incipient tolerated P_O2 was 4 and6 kPa at 15 and 20°C, respectively, with >50% mortalityoccurring at 25°C at all tested levels of hypoxia. P_O2 hadless of an effect on survival of hypoxia than temperature. At25°C, survivorship was not different over a P_O2 range of0–2 kPa and increased only at 4 and 6 kPa. Survivorshipwas size-dependent. Median survival times increased with increasingSL in anoxia and P_O2=1 kPa, but at 2, 4 and 6 kPa,smaller individuals survived longer than larger individuals.With tolerance levels similar to other estuarine bivalve species,P. perna should withstand hypoxia encountered in estuarine environments.Thus, its restriction to intertidal rocky shores may be dueto other parameters, particularly its relatively low temperaturetolerance. (Received 26 January 2004; accepted 31 March 2005)     

Cardiopulmonary Gas Exchange in the Turtle: A Model Analysis

The performance of the cardiorespiratory gas exchange systemof the turtle, Pseudemys scripla, at rest and at three bodytemperatures (15, 25, 35°C), was assessed in reference totheory and experimental data. The primary processes governinggas exchange(ventilation, perfusion, diffusion) were found tobe uniformly balanced in regard to CO₂ exchange. This was nottrue for O₂ transport where, at higher body temperature, thesystemis characterized by underventilation and overperfusionin conjunction with declining O₂ saturation levels. Adjustmentsin convection of air or blood, which favor improvement in arterialO₂ saturation levels, are incompatible with stability of theCO₂stores and acid-base state. The latter were preserved acrossthe temperature range. Comparisons between bird, mammal, andturtle illustrate close similarity in the relative roles ofthe primary processes governing CO₂ exchange. This persistsin the face of wide differences inmetabolic intensity, cardiopulmonaryarchitecture and complexity, and body temperatures characterizingthe species which were compared. The close control of carbondioxide transport may reflect a fundamental adjustment to airbreathing by lungs as primitive vertebrates emerged from waterto the terrestrial domain. From a regulatory point of view thestudy emphasizes the importance of both structural and functionalcontrol of CO₂ homeostasis. It does not rule out regulatoryfeatures which may insure more effective O₂ transport; however,the latter are not evident within the context of the steadystate assumptions underlying this analysis.     

Photosynthetic efficiency of Panicum hians and Panicum milioides in relation to C3 and C4 plants

Panicum hians and Panicum milioides were found to have characteristicsintermediate to those of C₃ and C₄ species with respect to CO₂compensation point, percentage inhibition of photosynthesisby O₂ at various O₂/CO₂ solubility ratios, and water use efficiency.C₄ species have a higher carboxylation efficiency than eitherthe intermediate or C₃ species. During photosynthesis, evenunder 2.5% O₂, C₄ species have a higher affinity for intercellularCO₂ (Km 1.6 µM) apparently due to the initial carboxylationthrough PEP carboxylase. Under low O₂ the intermediate and C₃species had a similar affinity for intercellular CO₂ duringphotosynthesis (Km 5–7 µM) consistent with carboxylationof atmospheric CO₂ through RuDP carboxylase. There were considerablevariation in photosynthesis/unit leaf area at saturating CO₂levels in the species examined which in part is due to differencesin RuDP carboxylase /unit leaf area. The highest rates of photosynthesis/unitleaf area under CO₂-saturating conditions were with the C₃ specieswhich had a correspondingly high level of RuDP carboxylase/unitleaf area. Possibilities for the greater efficiency of P. hiansand P. milioides in comparison to C₃ species in utilizing lowlevels of CO₂ in the presence of atmospheric O₂ are discussed. ¹ This research was supported by the College of Agriculturaland Life Sciences, University of Wisconsin, Madison; and theUniversity of Wisconsin Research Committee with funds from theWisconsin Alumni Research Foundation. (Received June 25, 1977; )     

Important role of carotid afferents in control of breathing

The purpose of the present study was todetermine the effect on breathing in the awake state of carotid bodydenervation (CBD) over 1-2 wk after denervation. Studies werecompleted on adult goats repeatedly before and1) for 15 days after bilateral CBD (n = 8),2) for 7 days after unilateral CBD(n = 5), and3) for 15 days after sham CBD(n = 3). Absence of ventilatorystimulation when NaCN was injected directly into a common carotidartery confirmed CBD. There was a significant(P < 0.01) hypoventilation during the breathing of room air after unilateral and bilateral CBD. Themaximum Pa_CO2 increase (8 Torr forunilateral and 11 Torr for bilateral) occurred ~4 days afterCBD. This maximum was transient because by 7 (unilateral)to 15 (bilateral) days after CBD, Pa_CO2 was only 3-4 Torr above control.CO₂ sensitivity was attenuated from control by 60% on day 4 afterbilateral CBD and by 35% on day 4 after unilateral CBD. This attenuation was transient, because CO₂ sensitivity returned tocontrol temporally similar to the return ofPa_CO2 during the breathing of room air.During mild and moderate treadmill exercise 1-8 days afterbilateral CBD, Pa_CO2 was unchanged fromits elevated level at rest, but, 10-15 days after CBD,Pa_CO2 decreased slightly from restduring exercise. These data indicate that1) carotid afferents are animportant determinant of rest and exercise breathing and ventilatoryCO₂ sensitivity, and2) apparent plasticity within theventilatory control system eventually provides compensation for chronicloss of these afferents.