Evolution of vertebrate respiratory central rhythm generators

Tracing the evolution of the central rhythm generators associated with ventilation in vertebrates is hindered by a lack of information surrounding key transitions. To begin with, central rhythm generation has been studied in detail in only a few species from four vertebrate groups, lamprey, anuran amphibians, turtles, and mammals (primarily rodents). Secondly, there is a lack of information regarding the transition from water breathing fish to air breathing amniotes (reptiles, birds, and mammals). Specifically, the respiratory rhythm generators of fish appear to be single oscillators capable of generating both phases of the respiratory cycle (expansion and compression) and projecting to motoneurons in cranial nerves innervating bucco-pharyngeal muscles. In the amniotes we find oscillators capable of independently generating separate phases of the respiratory cycle (expiration and inspiration) and projecting to pre-motoneurons in the ventrolateral medulla that in turn project to spinal motoneurons innervating thoracic and abdominal muscles (reptiles, birds, and mammals). Studies of the one group of amphibians that lie at this transition (the anurans), raise intriguing possibilities but, for a variety of reasons that we explore, also raise unanswered questions. In this review we summarize what is known about the rhythm generating circuits associated with breathing that arise from the different rhombomeric segments in each of the different vertebrate classes. Assuming oscillating circuits form in every pair of rhombomeres in every vertebrate during development, we trace what appears to be the evolutionary fate of each and highlight the questions that remain to be answered to properly understand the evolutionary transitions in vertebrate central respiratory rhythm generation.

Different strategies for convective O2 transport in high altitude birds: A graphical analysis

For illustrative purposes, in this article we use "Johansen Plots" as a graphical way of simultaneously visualizing the inter-connected variables that compose the convective steps of the gas transport cascade. These plots are used to reflect on some of the physiological characteristics seen in five species of birds, four of which sojourn to, or are native to, high altitudes (the barnacle goose, bar-headed goose, Andean goose, speckled teal and ruddy duck). These species were chosen to emphasize the diversity of responses to hypoxia that can exist within a single family. This diversity likely arose for many possible reasons, including local adaptation to hypoxia, differences in flight or diving abilities, or as a result of other phylogenetically-based differences across waterfowl in physiology, behaviour, and/or life style.

Heart mitochondria from naked mole-rats (Heterocephalus glaber) are more coupled, but similarly susceptible to anoxia-reoxygenation stress than in laboratory mice (Mus musculus)

Naked mole-rats (Heterocephalus glaber; NMRs) are among the most hypoxia-tolerant mammals described to date and exhibit plastic responses during hypoxia exposure. The goal of the present study was to determine if heart mitochondria from NMRs functionally differ from those of hypoxia-intolerant common laboratory mice (Mus musculus). We assessed heart mitochondrial respiratory flux, proton leak kinetics, responses to in vitro anoxia-recovery, and maximal complex enzyme activities. When investigated at their respective body temperatures (28 °C for NMR and 37 °C for mice), NMR heart mitochondria had lower respiratory fluxes relative to mice, particularly for state 2 and oligomycin-induced state 4 leak respiration rates. When leak respiration rates were standardized to the same membrane potential, NMR mitochondria had lower complex II-stimulated state 2 respiration rates than mice. Both mice and NMRs responded similarly to an in vitro anoxia-recovery challenge and decreased state 3 respiration rate post-anoxia. Finally, NMRs had overall lower maximal complex enzyme activities compared with mice, but the magnitude of the difference did not correspond with observed differences in respiratory fluxes. Overall, heart mitochondria from NMRs appear more coupled than those of mice, but in both species the heart appears equally susceptible to ischemic-reperfusion injury.

The roller coaster flight strategy of bar-headed geese conserves energy during Himalayan migrations

The physiological and biomechanical requirements of flight at high altitude have been the subject of much interest. Here, we uncover a steep relation between heart rate and wingbeat frequency (raised to the exponent 3.5) and estimated metabolic power and wingbeat frequency (exponent 7) of migratory bar-headed geese. Flight costs increase more rapidly than anticipated as air density declines, which overturns prevailing expectations that this species should maintain high-altitude flight when traversing the Himalayas. Instead, a "roller coaster" strategy, of tracking the underlying terrain and discarding large altitude gains only to recoup them later in the flight with occasional benefits from orographic lift, is shown to be energetically advantageous for flights over the Himalayas.

Validation of the i-STAT and HemoCue systems for the analysis of blood parameters in the bar-headed goose, Anser indicus

Every year, bar-headed geese (Anser indicus) perform some of the most remarkable trans-Himalayan migrations, and researchers are increasingly interested in understanding the physiology underlying their high-altitude flight performance. A major challenge is generating reliable measurements of blood parameters on wild birds in the field, where established analytical techniques are often not available. Therefore, we validated two commonly used portable clinical analysers (PCAs), the i-STAT and the HemoCue systems, for the analysis of blood parameters in bar-headed geese. The pH, partial pressures of O2 and CO2 (PO2 and PCO2), haemoglobin O2 saturation (sO2), haematocrit (Hct) and haemoglobin concentration [Hb] were simultaneously measured with the two PCA systems (i-STAT for all parameters; HemoCue for [Hb]) and with conventional laboratory techniques over a physiological range of PO2, PCO2 and Hct. Our results indicate that the i-STAT system can generate reliable values on bar-headed goose whole blood pH, PO2, PCO2 and Hct, but we recommend correcting the obtained values using the linear equations determined here for higher accuracy. The i-STAT is probably not able to produce meaningful measurements of sO2 and [Hb] over a range of physiologically relevant environmental conditions. However, we can recommend the use of the HemoCue to measure [Hb] in the bar-headed goose, if results are corrected. We emphasize that the equations that we provide to correct PCA results are applicable only to bar-headed goose whole blood under the conditions that we tested. We encourage researchers to validate i-STAT or HemoCue results thoroughly for their specific study conditions and species in order to yield accurate results.

The effect of sustained hypoxia on the cardio-respiratory response of bowfin Amia calva: implications for changes in the oxygen transport system

This study examined mechanisms underlying cardio-respiratory acclimation to moderate sustained hypoxia (6.0 kPa for 7 days at 22° C) in the bowfin Amia calva, a facultative air-breathing fish. This level of hypoxia is slightly below the critical oxygen tension (pcrit ) of A. calva denied access to air (mean ± s.e. = 9.3 ± 1.0 kPa). Before exposure to sustained hypoxia, A. calva with access to air increased air-breathing frequency on exposure to acute progressive hypoxia while A. calva without access to air increased gill-breathing frequency. Exposure to sustained hypoxia increased the gill ventilation response to acute progressive hypoxia in A. calva without access to air, regardless of whether they had access to air or not during the sustained hypoxia. Additionally, there was a decrease in Hb-O2 binding affinity in these fish. This suggests that, in A. calva, acclimation to hypoxia elicits changes that increase oxygen delivery to the gas exchange surface for oxygen uptake and reduce haemoglobin affinity to enhance oxygen delivery to the tissues.

The paradox of extreme high-altitude migration in bar-headed geese Anser indicus

Bar-headed geese are renowned for migratory flights at extremely high altitudes over the world's tallest mountains, the Himalayas, where partial pressure of oxygen is dramatically reduced while flight costs, in terms of rate of oxygen consumption, are greatly increased. Such a mismatch is paradoxical, and it is not clear why geese might fly higher than is absolutely necessary. In addition, direct empirical measurements of high-altitude flight are lacking. We test whether migrating bar-headed geese actually minimize flight altitude and make use of favourable winds to reduce flight costs. By tracking 91 geese, we show that these birds typically travel through the valleys of the Himalayas and not over the summits. We report maximum flight altitudes of 7290 m and 6540 m for southbound and northbound geese, respectively, but with 95 per cent of locations received from less than 5489 m. Geese travelled along a route that was 112 km longer than the great circle (shortest distance) route, with transit ground speeds suggesting that they rarely profited from tailwinds. Bar-headed geese from these eastern populations generally travel only as high as the terrain beneath them dictates and rarely in profitable winds. Nevertheless, their migration represents an enormous challenge in conditions where humans and other mammals are only able to operate at levels well below their sea-level maxima.

The phylogeny of central chemoreception

Respiratory chemoreceptors responsive to changes in CO(2)/H(+) appear to be present in all vertebrates from fish to birds and mammals. They appear to have arisen first in the periphery sensitive to the external environment. Thus, in most fish CO(2)/H(+) chemoreceptors reside primarily in the gills and respond to changes in aquatic rather than arterial P(CO)₂ . In the air-breathing tetrapods (amphibians, mammals, reptiles and birds), the branchial arches regress developmentally and the derivatives of the branchial arteries are now exclusively internal. The receptors associated with these arteries now sense only arterial (not environmental) P(CO)₂/pH . Central CO(2)/H(+) chemoreception also appears to have arisen with the advent of air breathing, presumably as a second line of defense. These receptors may have arisen multiple times in association with several (but not all) of the independent origins of air breathing in fishes. There is strong evidence for multiple central sites of CO(2)/H(+) sensing, at least in amphibians and mammals, suggesting that it may not only have originated multiple times in different species but also multiple times within a single species.

Species differences in respiratory rhythm generation in rodents

We examined the role of riluzole (RIL)- and flufenamic acid (FFA)-sensitive mechanisms in respiratory rhythmogenesis in rats and hamsters using the in situ arterially perfused preparation. Based on the hypothesis that respiratory networks in animals capable of autoresuscitation would have a greater prevalence of membrane mechanisms that promote endogenous bursting, we predicted that older (weaned) hamsters (a hibernating species) would be more sensitive to the blockade of RIL- and FFA-sensitive mechanisms than age-matched rats and that younger (preweaned) rats would behave more like hamsters. Consistent with this, we found that respiratory motor output in weaned hamsters [>21 days postnatal (P21)] was highly sensitive to RIL (0.2–20 μM), while in young rats (P12–14) it was less so (only affected at higher concentrations of RIL), and weaned rats were not affected at all. On the other hand, respiratory motor output was equally reduced by FFA (0.25–25 μM) in both young and weaned rats but was unaffected in weaned hamsters. Coapplication of RIL and FFA (RIL + FFA) produced greater inhibition of respiration in both young and weaned rats compared with either drug alone. In contrast, in weaned hamsters, FFA coapplication offset the inhibitory effect of RIL alone. Increasing respiratory drive with hypercapnia/acidosis ameliorated the respiratory inhibition produced by RIL + FFA in weaned rats but had no effect in young rats. Data from the present study indicate that respiratory rhythmogenesis in young rats is more dependent on excitatory RIL-sensitive and FFA-sensitive mechanisms than older rats and that fundamental differences exist in the respiratory rhythmogenic mechanisms between rats and hamsters.

Gill chemoreceptors and cardio-respiratory reflexes in the neotropical teleost pacu, Piaractus mesopotamicus

This study examined the location and distribution of O(2) chemoreceptors involved in cardio-respiratory responses to hypoxia in the neotropical teleost, the pacu (Piaractus mesopotamicus). Intact fish and fish experiencing progressive gill denervation by selective transection of cranial nerves IX and X were exposed to gradual hypoxia and submitted to intrabuccal and intravenous injections of NaCN while their heart rate, ventilation rate and ventilation amplitude were measured. The chemoreceptors producing reflex bradycardia were confined to, but distributed along all gill arches, and were sensitive to O(2) levels in the water and the blood. Ventilatory responses to all stimuli, though modified, continued following gill denervation, however, indicating the presence of internally and externally oriented receptors along all gill arches and either in the pseudobranch or at extra-branchial sites. Chemoreceptors located on the first pair of gill arches and innervated by the glossopharyngeal nerve appeared to attenuate the cardiac and respiratory responses to hypoxia. The data indicate that the location and distribution of cardio-respiratory O(2) receptors are not identical to those in tambaqui (Colossoma macropomum) despite their similar habitats and close phylogenetic lineage, although the differences between the two species could reduce to nothing more than the presence or absence of the pseudobranch.

Cardiorespiratory responses to hypercarbia in tambaquiColossoma macropomum: chemoreceptor orientation and specificity

Experiments were carried out to test the hypothesis that ventilatory and cardiovascular responses to hypercarbia (elevated water PCO2) in the tambaqui Colossoma macropomum are stimulated by externally oriented receptors that are sensitive to water CO2 tension as opposed to water pH. Cardiorespiratory responses to acute hypercarbia were evaluated in both the absence and presence of internal hypercarbia (elevated blood PCO2), achieved by treating fish with the carbonic anhydrase inhibitor acetazolamide. Exposure to acute hypercarbia (15 min at each level, final water CO2 tensions of 7.2,15.5 and 26.3 mmHg) elicited significant increases in ventilation frequency(at 26.3 mmHg, a 42% increase over the normocarbic value) and amplitude(128%), together with a fall in heart rate (35%) and an increase in cardiac stroke volume (62%). Rapid washout of CO2 from the water reversed these effects, and the timing of the changes in cardiorespiratory variables corresponded more closely to the fall in water PCO2(PwCO2) than to that in blood PCO2(PaCO2). Similar responses to acute hypercarbia (15 min,final PwCO2 of 13.6 mmHg) were observed in acetazolamide-treated (30 mg kg-1) tambaqui. Acetazolamide treatment itself, however, increased PaCO2 (from 4.81±0.58 to 13.83±0.91 mmHg, mean ± s.e.m.; N=8) in the absence of significant change in ventilation, heart rate or cardiac stroke volume. The lack of response to changes in blood PCO2 and/or pH were confirmed by comparing responses to the bolus injection of hypercarbic saline(5% or 10% CO2; 2 ml kg-1) into the caudal vein with those to the injection of CO2-enriched water (1%, 3%, 5% or 10%CO2; 50 ml kg-1) into the buccal cavity. Whereas injections of hypercarbic saline were ineffective in eliciting cardiorespiratory responses, changes in ventilation and cardiovascular parameters accompanied injection of CO2-laden water into the mouth. Similar injections of CO2-free water acidified to the corresponding pH of the hypercarbic water (pH 6.3, 5.6, 5.3 or 4.9, respectively) generally did not stimulate cardiorespiratory responses. These results are in agreement with the hypothesis that in tambaqui, externally oriented chemoreceptors that are predominantly activated by increases in water PCO2,rather than by accompanying decreases in water pH, are linked to the initiation of cardiorespiratory responses to hypercarbia.

Effect of hypothermia on respiratory rhythm generation in hamster brainstem–spinal cord preparations

This study examined the effect of hypothermia on respiratory neural output from brainstem-spinal cord preparations of a cold tolerant rodent, the Syrian hamster. Brainstem-spinal cords from neonatal hamsters (0-6 days) were placed in a recording dish and respiratory-like neural activity was recorded from roots of the first cervical nerve. The preparations were cooled and warmed in a continuous or stepwise fashion. Inputs from the pons completely inhibited neural activity under steady state conditions. With the pons removed, fictive breathing was robust. Cooling caused respiratory arrest, followed by spontaneous resumption of activity on re-warming. Preparations from older hamsters (4-6 days old) were more cold tolerant than younger preparations (0-3 days old). Motor discharge was episodic during continuous cooling, and seizure-like discharge was observed during continuous warming. These phenomena were not observed with stepwise temperature changes suggesting that transient temperature effects on membrane properties may be involved. These preparations were not as cold tolerant as hamster pups in vivo but they retained the ability to autoresuscitate at all ages studied.

A comparison of adrenergic stress responses in three tropical teleosts exposed to acute hypoxia

Experiments were performed to assess the afferent and efferent limbs of the hypoxia-mediated humoral adrenergic stress response in selected hypoxia-tolerant tropical fishes that routinely experience environmental O2depletion. Plasma catecholamine (Cat) levels and blood respiratory status were measured during acute aquatic hypoxia [water Po2(PwO2) = 10–60 mmHg] in three teleost species, the obligate water breathers Hoplias malabaricus (traira) and Piaractus mesopotamicus (pacu) and the facultative air breather Hoplerythrinus unitaeniatus (jeju). Traira displayed a significant increase in plasma Cat levels (from 1.3 ± 0.4 to 23.3 ± 15.1 nmol/l) at PwO2levels below 20 mmHg, whereas circulating Cat levels were unaltered in pacu at all levels of hypoxia. In jeju denied access to air, plasma Cat levels were increased markedly to a maximum mean value of 53.6 ± 19.1 nmol/l as PwO2was lowered below 40 mmHg. In traira and jeju, Cat release into the circulation occurred at abrupt thresholds corresponding to arterial Po2(PaO2) values of approximately 8.5–12.5 mmHg. A comparison of in vivo blood O2equilibration curves revealed low and similar P50values (i.e., PaO2at 50% Hb-O2saturation) among the three species (7.7–11.3 mmHg). Thus Cat release in traira and jeju occurred as blood O2concentration was reduced to approximately 50–60% of the normoxic value. Intravascular injections of nicotine (600 nmol/kg) elicited pronounced increases in plasma Cat levels in traira and jeju but not in pacu. Thus the lack of Cat release during hypoxia in pacu may reflect an inoperative or absent humoral adrenergic stress response in this species. When allowed access to air, jeju did not release Cats into the circulation at any level of aquatic hypoxia. The likeliest explanation for the absence of Cat release in these fish was that air breathing, initiated by aquatic hypoxia, prevented PaO2values from falling to the critical threshold required for Cat secretion. The ventilatory responses to hypoxia in each species were similar, consisting generally of increases in both frequency and amplitude. These responses were not synchronized with or influenced by plasma Cat levels. Thus the acute humoral adrenergic stress response does not appear to stimulate ventilation during acute hypoxia in these tropical species.

Respiratory chemoreflexes and effects of cortical activation state in urethane anesthetized rats

Urethane anesthetized (< 1 .3 g/kg), Sprague-Dawley (SD) rats spontaneously cycled between a cortically desynchronized state (State I) and a cortically synchronized state (State III), which were very similar to awake and slow wave sleep (SWS) states in unanesthetized animals, based on EEG criteria. These low levels of urethane anaesthesia did not cause significant respiratory depression or reductions in sensitivity to hypoxia (10% O2 in nitrogen) or hypercapnia (5% CO2 in air) in rats in either State I or State III. Thus, breathing frequency (fR), tidal volume (VT) and total ventilation (VTOT) all increased on cortical activation in urethane-anaesthetized rats whether breathing air, the hypoxic or the hypercapnic gas mixture, in a manner that was very similar to that observed in unanaesthetized animals. The relative sensitivity to hypoxia was greater in State III than State I, the relative sensitivity to CO2, overall, was equal in both states, State III occurred less often during hypoxia and hypercapnia, and hypoxic, urethane-anaesthetized rats sighed frequently, particularly in State I. This is also similar to the situation seen in unanesthetized rats. Given the similarities seen between urethane anesthetized rats in the present study and literature values for unanesthetized rats, the data suggest that urethane-anaesthetized rats provide a good model system for studying respiratory patterns and chemoreflexes as a function of cortical activation state.

Interactive effects of mechano- and chemo-receptor inputs on cardiorespiratory outputs in the toad

Arterial blood pressure (P(b)), pulmocutaneous blood flow (Q(pc)), heart rate (f(H)), and fictive ventilation (motor activity in the Vth cranial nerve, V(int)), were recorded from decerebrated, paralysed toads receiving unidirectional ventilation with experimental gas mixtures over a range of lung inflation. At the onset of spontaneous bouts of fictive ventilation, (Q(pc)) and P(b) increased immediately, often with changes in heart rate, implying central cardiorespiratory interactions. Inflation of the lungs with different gas mixtures revealed that the effect of hypercarbia on V(int) was reduced by lung inflation and that feedback from pulmonary stretch receptors may summate with central feedforward control of f(H) and (Q(pc)) in an interactive fashion. The results of bolus injections of cyanide into the carotid or the pulmonary circulations suggest there are oxygen sensitive receptors in both circuits that affect the cardiovascular system directly and respiratory activity by complex central interactions with inputs from central chemoreceptors and pulmonary stretch receptors.

The ventilatory response to environmental hypercarbia in the South American rattlesnake, Crotalus durissus

To study the effects of environmental hypercarbia on ventilation in snakes, particularly the anomalous hyperpnea that is seen when CO(2) is removed from inspired gas mixtures (post-hypercapnic hyperpnea), gas mixtures of varying concentrations of CO(2) were administered to South American rattlesnakes, Crotalus durissus, breathing through an intact respiratory system or via a tracheal cannula by-passing the upper airways. Exposure to environmental hypercarbia at increasing levels, up to 7% CO(2), produced a progressive decrease in breathing frequency and increase in tidal volume. The net result was that total ventilation increased modestly, up to 5% CO(2) and then declined slightly on 7% CO(2). On return to breathing air there was an immediate but transient increase in breathing frequency and a further increase in tidal volume that produced a marked overshoot in ventilation. The magnitude of this post-hypercapnic hyperpnea was proportional to the level of previously inspired CO(2). Administration of CO(2) to the lungs alone produced effects that were identical to administration to both lungs and upper airways and this effect was removed by vagotomy. Administration of CO(2) to the upper airways alone was without effect. Systemic injection of boluses of CO(2)-rich blood produced an immediate increase in both breathing frequency and tidal volume. These data indicate that the post-hypercapnic hyperpnea resulted from the removal of inhibitory inputs from pulmonary receptors and suggest that while the ventilatory response to environmental hypercarbia in this species is a result of conflicting inputs from different receptor groups, this does not include input from upper airway receptors.

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.

Effects of changing ambient temperature on metabolic, heart, and ventilation rates during steady state hibernation in golden-mantled ground squirrels (Spermophilus lateralis)

To determine whether metabolic rate is suppressed in a temperature-independent fashion in the golden-mantled ground squirrel during steady state hibernation, we measured body temperature and metabolic rate in ground squirrels during hibernation at different T(a)'s. In addition, we attempted to determine whether heart rate, ventilation rate, and breathing patterns changed as a function of body temperature or metabolic rate. We found that metabolic rate changed with T(a) as it was raised from 5 degrees to 14 degrees C, which supports the theory that different species sustain falls in metabolic rate during hibernation in different ways. Heart rate and breathing pattern also changed with changing T(a), while breathing frequency did not. That the total breathing frequency did not correlate closely with oxygen consumption or body temperature, while the breathing pattern did, raises important questions regarding the mechanisms controlling ventilation during hibernation.

THE effects of tonic lung inflation on ventilation in the American bullfrog Rana catesbeiana Shaw

This study was designed to determine whether lung inflation stimulates or inhibits breathing in frogs by examining the effect of tonic lung inflation on the ‘fictive’ breathing pattern of decerebrate, unidirectionally ventilated bullfrogs. Neural discharge was monitored in the trigeminal nerve as an indication of the frequency and force of contraction of the buccal pump, and in the laryngeal branch of the vagus nerve as an indication of glottal opening, and hence fictive lung ventilation. Based on the temporal coordination of discharge in the trigeminal and vagus nerves during naturally occurring breaths it was possible to characterize the fictive breaths as inflation, deflation or balanced breaths. Increasing lung inflation increased absolute breathing frequency by reducing the duration of apnea between breaths and promoting a change in breathing pattern from no breathing to single breaths, breathing episodes and, finally, to continuous breathing. Associated with this was a decrease in the amplitude and area of the integrated trigeminal electroneurogram associated with the lung breaths, indicative of a reduction in the force of the buccal pump, and a shift in the timing of the trigeminal and vagal discharge, indicative of a shift from inflation to deflation breaths. Taken together the data suggest that lung deflation produces infrequent, large-amplitude inflation breaths or cycles, but that progressive lung inflation changes the breathing pattern to one of high-frequency attempts to deflate the lungs that are largely passive, and accompanied by contractions of the buccal pump that are no larger than those associated with normal buccal oscillations.

Hypoxic metabolic response of the golden-mantled ground squirrel

We examined the magnitude of the hypoxic metabolic response in golden-mantled ground squirrels to determine whether the shift in thermoregulatory set point (T(set)) and subsequent fall in body temperature (T(b)) and metabolic rate observed in small mammals were greater in a species that routinely experiences hypoxic burrows and hibernates. We measured the effects of changing ambient temperature (T(a); 6--29 degrees C) on metabolism (O(2) consumption and CO(2) production), T(b), ventilation, and heart rate in normoxia and hypoxia (7% O(2)). The magnitude of the hypoxia-induced falls in T(b) and metabolism of the squirrels was larger than that of other rodents. Metabolic rate was not simply suppressed but was regulated to assist the initial fall in T(b) and then acted to slow this fall and stabilize T(b) at a new, lower level. When T(a) was reduced during 7% O(2), animals were able to maintain or elevate their metabolic rates, suggesting that O(2) was not limiting. The slope of the relationship between temperature-corrected O(2) consumption and T(a) extrapolated to a T(set) in hypoxia equals the actual T(b). The data suggest that T(set) was proportionately related to T(a) in hypoxia and that there was a shift from increasing ventilation to increasing O(2) extraction as the primary strategy employed to meet increasing metabolic demands under hypoxia. The animals were neither hypothermic nor hypometabolic, as T(b) and metabolic rate appeared to be tightly regulated at new but lower levels as a result of a coordinated hypoxic metabolic response.

Branchial CO(2) receptors and cardiorespiratory adjustments during hypercarbia in Pacific spiny dogfish (Squalus acanthias)

Adult Pacific spiny dogfish (Squalus acanthias) were exposed to acute (approximately 20 min) hypercarbia while we monitored arterial blood pressure, systemic vascular resistance (R(S)), cardiac output (vdot (b)) and frequency (fh) as well as ventilatory amplitude (V(AMP)) and frequency (f(V)). Separate series of experiments were conducted on control, atropinized (100 nmol kg(−)(1)) and branchially denervated fish to investigate putative CO(2)-chemoreceptive sites on the gills and their link to the autonomic nervous system and cardiorespiratory reflexes.In untreated fish, moderate hypercarbia (water CO(2)partial pressure; Pw(CO2)=6.4+/−0.1 mmHg) (1 mmHg=0.133 kPa) elicited significant increases in V(AMP) (of approximately 92 %) and f(V) (of approximately 18 %) as well as decreases in fh (of approximately 64 %), V.(b) (approximately 29 %) and arterial blood pressure (of approximately 11 %); R(S) did not change significantly.Denervation of the branchial branches of cranial nerves IX and X to the pseudobranch and each gill arch eliminated all cardiorespiratory responses to hypercarbia. Prior administration of the muscarinic receptor antagonist atropine also abolished the hypercarbia-induced ventilatory responses and virtually eliminated all CO(2)-elicited cardiovascular adjustments. Although the atropinized dogfish displayed a hypercarbic bradycardia, the magnitude of the response was significantly attenuated (36+/−6 % decrease in fh in controls versus 9+/−2 % decrease in atropinized fish; means +/− s.e.m.).Thus, the results of the present study reveal the presence of gill CO(2) chemoreceptors in dogfish that are linked to numerous cardiorespiratory reflexes. In addition, because all cardiorespiratory responses to hypercarbia were abolished or attenuated by atropine, the CO(2) chemoreception process and/or one or more downstream elements probably involve cholinergic (muscarinic) neurotransmission.

The influence of NMDA receptor-mediated processes on breathing pattern in ground squirrels

The effects of blockade of N-methyl-D-aspartate (NMDA) type glutamate receptors by a non-competitive antagonist (MK-801) on cortical arousal, breathing pattern and ventilatory responses to hypoxia (10% O2 in N2) and hypercapnia (5% CO2 in air) were assessed in anesthetized (urethane) and unanesthetized golden-mantled ground squirrels (Spermophilus lateralis). Intra-cerebroventricular administration of MK-801 did not alter ventilation during wakefulness, although it did alter the pattern (breathing frequency and tidal volume components) of the hypercapnic ventilatory response, and suppressed the ventilatory response to hypoxia. Animals did not sleep following treatment with MK-801, and intravenous administration of MK-801 prevented expression of the sleep-like state normally observed in anesthetized animals. In anesthetized animals MK-801 elevated breathing frequency to levels observed without anesthesia, and suppressed the hypoxic ventilatory response. These data suggest that NMDA-type glutamatergic receptor-mediated processes influence cortical arousal and facilitate depression of breathing frequency during anesthesia and the hypoxic ventilatory response. Such processes are not essential for the hypercapnic ventilatory response.

Vagal feedback is essential for breathing in unanesthetized ground squirrels

The roles of vagal afferent feedback in terminating inspiration and modulating breathing pattern and ventilatory responses to hypoxia and hypercapnia were assessed in the golden-mantled ground squirrel, Spermophilus lateralis, during wakefulness and urethane anesthesia. Hypoxia increased ventilation primarily through increases in breathing frequency (f(R)) while hypercapnia increased ventilation primarily through increases in tidal volume (V(T)) in both anesthetized and unanesthetized animals. Vagotomy resulted in an increase in tidal volume, a decrease in breathing frequency and ventilation, and depressed ventilatory responses to both hypoxia and hypercapnia in anesthetized animals. In unanesthetized animals vagotomy produced a transient 'gasp-like' breathing pattern that rapidly progressed to a non-obstructive central apnea. These data indicate that vagal feedback shapes ventilation on a breath-by-breath basis during anesthesia and is essential for ventilation in unanesthetized animals. The mechanisms that transform the influences of vagal input on breathing between anesthetized and unanesthetized states remain unclear. Changes in breathing pattern induced by the removal of vagal feedback compromise chemoreflexes.

Time domains of the hypoxic ventilatory response in awake ducks: episodic and continuous hypoxia

Time-dependent ventilatory responses to episodic and continuous isocapnic hypoxia were measured in unidirectionally ventilated, awake ducks. Three protocols were used: (1) ten 3-min episodes of moderate hypoxia (10% O(2)) with 5-min normoxic intervals; (2) three 3-min episodes of severe hypoxia (8% O(2)) with 5-min normoxic intervals; and (3) 30-min of continuous moderate hypoxia. Ventilation (V(I)) increased immediately within a hypoxic episode (acute response), followed by a further slow rise in V(I) (short-term potentiation). The peak V(T) response increased from the first to second moderate hypoxic episode (progressive augmentation), but was unchanged thereafter. During normoxic intervals, V(I) increased progressively (56% following the tenth episode; long term facilitation). Time-dependent changes were not observed during or following 30-min of continuous hypoxia. Although several time-dependent ventilatory responses to episodic hypoxia are observed in awake ducks, they are relatively small and biased towards facilitation versus inhibitory mechanisms.

The influence of descending inputs on breathing pattern formation in the isolated bullfrog brainstem-spinal cord

This study used in vitro brainstem-spinal cord preparations from the American bullfrog, Rana catesbeiana, to examine the influence of central descending inputs on breathing pattern formation. In preparations with an episodic pattern of fictive breathing, a transection slightly caudal to the optic chiasma produced a continuous breathing pattern and increased the overall frequency of fictive breathing. Following a transection to isolate the medulla, the frequency of fictive breathing decreased and the incidence of other forms of motor output increased. Further transections between the trigeminal and vagus nerve roots resulted in variable and asynchronous discharge from each nerve. These results suggest that a primary respiratory rhythm is produced within the medulla but descending influences stimulate breathing and promote episodic breathing. It would appear that multiple elements of the respiratory control system, including tegmental and medullary sites, play a role in shaping the burst pattern of motor output associated with each breath and that slower rhythms of longer burst duration are generated by more caudal hindbrain sites.

Pulmonary vagal modulation of ventilation in toads (Bufo marinus)

This study examined the role of pulmonary vagal feedback on hypercapnic chemosensitivity and breathing pattern formation in cane toads (Bufo marinus). Decerebrate, paralysed toads were uni-directionally ventilated with air, 2.5% CO(2) or 5.0% CO(2) with the lungs inflated or deflated, before and after pulmonary vagotomy. Motor output from the mandibular branch of the trigeminal nerve served as an index of fictive breathing. As respiratory drive was increased, breathing frequency increased and breaths were clustered into discrete episodes separated by periods of apnea. Lung deflation tended to enhance episodic breathing while inflation biased the system towards apnea at low levels of respiratory drive and a pattern of continuous, small breaths at higher levels of respiratory drive. Following bilateral pulmonary vagotomy there was no increase in ventilation during hypercapnia and lung inflation/deflation had no effect on breathing pattern. In isolated brainstem-spinal cord preparations from the same animals, all variables associated with fictive breathing were unaffected by changes in superfusate pH from 8.0 to 7.6. The breathing pattern from the in vitro preparations was highly variable. This study demonstrates a crucial role for vagal feedback in modulating respiration and the respiratory responses to hypercapnia in B. marinus.

Branchial receptors and cardiorespiratory reflexes in a neotropical fish, the tambaqui (Colossoma macropomum)

This study examined the location and physiological roles of branchial chemoreceptors involved in the cardiorespiratory responses to hypoxia and hypercarbia in a neotropical fish that exhibits aquatic surface respiration, the tambaqui (Colossoma macropomum). Fish were exposed to abrupt progressive environmental hypoxia (18. 6–1.3 kPa water P(O2)) and hypercarbia (water equilibrated with 5 % CO(2) in air, which lowered the water pH from 7.0 to 5.0). They were also subjected to injections of NaCN into the ventral aorta (to stimulate receptors monitoring the blood) and buccal cavity (to stimulate receptors monitoring the respiratory water). All tests were performed before and after selective denervation of branchial branches of cranial nerves IX and X to the gill arches. The data suggest that the O(2) receptors eliciting reflex bradycardia and increases in breathing frequency are situated on all gill arches and sense changes in both the blood and respiratory water and that the O(2) receptors triggering the elevation in systemic vascular resistance, breathing amplitude, swelling of the inferior lip and that induce aquatic surface respiration during hypoxia are extrabranchial, although branchial receptors also contribute to the latter two responses. Hypercarbia also produced bradycardia and increases in breathing frequency, as well as hypertension, and, while the data suggest that there may be receptors uniquely sensitive to changes in CO(2)/pH involved in cardiorespiratory control, this is based on quantitative rather than qualitative differences in receptor responses. These data reveal yet another novel combination for the distribution of cardiorespiratory chemoreceptors in fish from which teleologically satisfying trends have yet to emerge.

Cardiovascular and respiratory reflexes in the tropical fish, traira (Hoplias malabaricus): CO2/pH chemoresponses

To examine the distribution and physiological role of CO2/pH-sensitive chemoreceptors in the gills of the tropical fish, traira (Hoplias malabaricus), fish were exposed to acute environmental hypercarbia (1.25, 2.5 and 5.0% CO2 in air) and subjected to injections of HCl into the ventral aorta and buccal cavity. This was done before and after selective denervation of branchial branches of the IXth and Xth cranial nerves to various gills arches. Hypercarbia produced a significant decrease in heart rate, a mild hypotension as well as increases in both ventilation rate and ventilation amplitude. The data suggest that the hypercarbic bradycardia and increase in ventilation frequency arise from receptors exclusively within the gills but present on more than the first gill arch, while extra-branchial receptors may also be involved in controlling the increase in ventilation amplitude. With the exception of a decrease in heart rate in response to HCl injected into the ventral aorta, the acid injections (internal and external) did not mimic the cardiorespiratory responses observed during hypercarbia suggesting that changes in CO2 are more important than changes in pH in producing cardiorespiratory responses. Finally, the data indicate that chemoreceptors sensitive to CO2/pH and to O2 in the gills of this species involved in producing ventilatory responses are distributed in a similar fashion, but that those involved in producing the bradycardia are not.

Cardiorespiratory responses of the toad (Bufo marinus) to hypoxia at two different temperatures

Central vascular blood flows and ventilation were measured in conscious toads (Bufo marinus) at 15 and 25 degrees C. The animals were exposed to hypoxia (Fi(O)(sum)=0.10 and 0.05, where Fi(O)(sum) is the fractional oxygen concentration of inspired air) at both temperatures. In addition, the cardiorespiratory responses to hypercapnia (Fi(CO)(sum)=0.05) and atropine injection (5 mg kg(−)(1); 7.4 μmol kg(−)(1)) were studied at 25 degrees C. At 25 degrees C, systemic blood flow (q_dot (sys)) exceeded pulmocutaneous blood flow (q_dot (pc)), indicating a large net right-to-left shunt (q_dot (pc)/ q_dot (sys) was 0.39). q_dot (pc)/ q_dot (sys) was reduced significantly to 0.22 at 15 degrees C. At both temperatures, q_dot (pc) increased significantly during hypoxia (from 26.2 to 50.8 ml min(−)(1)kg(−)(1) at 25 degrees C and from 11. 2 to 18.9 ml min(−)(1)kg(−)(1) at 15 degrees C), whereas q_dot (sys) changed little (from 77.2 to 66.2 ml min(−)(1)kg(−)(1) at 25 degrees C and from 54.3 to 50.1 ml min(−)(1)kg(−)(1) at 15 degrees C). As a result, the net right-to-left shunt was greatly reduced, while total cardiac output remained almost unaffected. The ventilatory response was more pronounced during hypercapnia but, since q_dot (pc) and q_dot (sys) were affected similarly, there was no change in the shunt pattern. In undisturbed toads at 25 degrees C, atropine injection increased q_dot (pc) and eliminated the net right-to-left shunt. This is consistent with the known vagal innervation of the pulmonary artery.The present study shows that the cardiac right-to-left shunt that prevails in undisturbed and resting toads is reduced with increased temperature and during hypoxia. These findings are consistent with the general view that the cardiac right-to-left shunt is regulated and reduced whenever oxygen delivery is compromised or metabolic rate is increased.

Lung deflation stimulates fictive ventilation in decerebrated and unidirectionally ventilated toads

We describe how the degree of lung inflation and hypercapnia influenced fictive ventilation in five toads (Bufo marinus) that were decerebrated and paralysed with roccuronium. Both lungs were unidirectionally ventilated and the degree of lung inflation was determined by controlling the outflow resistance of these catheters, while ventilatory motor output was assessed on the basis of nervous activity in the mandibular branch of the Vth cranial nerve. The pattern of the recorded activity ('fictive ventilation') resembled the ventilatory patterns previously described for conscious toads. Increasing the fraction of CO2 in the gas mixture used for unidirectional ventilation from 0.00 to 0.05 stimulated fictive breathing. Fictive ventilation was also greatly stimulated, at all CO2 levels, by reduced lung volume, while complete inflation of the lungs abated fictive ventilation at all levels of CO2. Stimulation of CO2 sensitive chemoreceptors and pulmonary stretch receptors appear to have interactive effects on the central generation of ventilatory output in toads.

Regulation of cardiac rhythm in hibernating mammals

The dramatic fall in heart rate exhibited by mammals entering hibernation begins before there is any noticeable fall in body temperature. The initial, progressive decrease in heart rate is the result of a cyclic parasympathetic activation that induces skipped beats and regular asystoles as well as slows the even heart beat. As body temperature subsequently falls, the parasympathetic influence is progressively withdrawn and periods of parasympathetic and sympathetic dominance alternate and give rise to regular periods of arrhythmia (tachycardia followed by bradycardia), and occasional long asystoles or periods of highly irregular cardiac activity. Superimposed on this is a vagally-mediated, respiratory sinus arrhythmia that is accentuated in species that breathe episodically. These events give way to a uniform heart rate in deep hibernation at low temperatures where both parasympathetic and sympathetic tone appear absent. The complete absence of tone is not a function of reduced temperature but is reflective of the state of deep, steady state hibernation. The elevation in heart rate that accompanies the onset of arousal is the result of dramatic increases in sympathetic activation that precede any increases in body temperature. As body temperature then rises, sympathetic influence is slowly withdrawn. Arrhythmias are also common during natural arousals or shifts from lower to warmer hibernation temperatures as periods of parasympathetic and sympathetic dominance again alternate en route to re-establishing a steady state in euthermia. The mechanism behind, and the biological significance of, cardiac changes mediated through orchestrated arrhythmias remain unknown.

Central respiratory pattern generation in the bullfrog, Rana catesbeiana

There are two components to breathing pattern generation the production of the pattern of neural discharge associated with individual breaths, and the pattern in which breaths are produced to effect ventilation. Bullfrogs typically breathe with randomly distributed breaths. When respiratory drive is elevated, breathing becomes more regular and often episodic. Studies on in vitro brainstem-spinal cord preparations of the adult bullfrog and in situ preparations of decerebrate, paralyzed, unidirectionally ventilated animals suggest that output from the central rhythm generator in frogs is conditional on receiving some input and that a host of central inputs remain even in the most reduced preparations. There appear to be descending inputs from sites in the dorsal brainstem just caudal to the optic chiasma that cluster breaths into episodes, a strong excitatory input caudal to this site but rostral to the origin of the Vth cranial nerve and, possibly, segmental rhythm generators throughout the medulla that are normally entrained to produce the normal breathing pattern. The data also suggest that the shape of the discharge pattern (augmenting, decrementing) and timing of outputs (alternating vs synchronous) associated with motor outflow during each breath are also dependent on the interconnections between these various sites.

Cardiovascular and respiratory reflexes: the tropical fish, traira (Hoplias malabaricus) O2 chemoresponses

To determine the location and distribution of chemoreceptors involved in the cardiovascular and respiratory responses to hypoxia of traira (Hoplias malabaricus), we measured heart rate, arterial blood pressure, ventilation frequency and amplitude of opercular movements during exposure to hypoxia and application of NaCN to either water bathing the gills (external) or the ventral aortic blood (internal). This was done before and after selective denervation of branchial branches of the IXth and Xth cranial nerves to various gill arches. The data suggest that hypoxia elicits a bradycardia that arises from internal receptors located in the first gill arch. They also indicate the presence of branchial and extra branchial O2-chemoreceptors that reflexively elevate systemic vascular resistance during hypoxia. Hypoxia induced increases in ventilation frequency arose primarily from external receptors located exclusively within the gills while increases in breathing amplitude also involved extra branchial receptors. In addition, the data suggest there are O2 sensitive chemoreceptors located in the first gill arch that attenuate the respiratory responses.

The effect of isovolemic anaemia on blood O2 affinity and red cell triphosphate concentrations in the painted turtle (Chrysemys picta)

The blood oxygen affinity of vertebrates is regulated, in part, through changes in red cell phosphate levels and increased oxygen affinity during reductions in inspired oxygen and is a well-described and common feature. However, during anaemia, when oxygen delivery is compromised by a reduction in blood oxygen carrying capacity, a lowering of blood oxygen affinity will facilitate oxygen unloading in the tissues, while oxygen loading at the gas exchange organ is not impaired. The present study investigated the effects of artificially induced anaemia in vivo on the blood oxygen affinity and red cell nucleoside triphosphate (NTP) concentrations in the turtle, Chrysemys picta. Blood was obtained from conscious animals through an arterial catheter and oxygen equilibrium curves were determined using the Tucker method while NTP concentrations were analyzed spectrophotometrically. Before induction of anaemia haematocrit averaged 23% and P50 was 18.5 +/- 0.7 with a NTP/Hb of 0.20 +/- 0.01 (mmol/mmol). After the haematocrit had been reduced to approximately 10% by bleeding (48-96 h) (blood volume was maintained by re-infusion of plasma and Ringer) there were no effects on P50 or red cell NTP concentrations. Thus, in contrast to fish and mammals, turtles do not exhibit a change in blood oxygen affinity during anaemia.

Respiratory pattern formation in the isolated bullfrog (Rana catesbeiana) brainstem-spinal cord

This study characterizes various patterns of motor output obtained from cranial nerves V, VII, X, and XII of in vitro, saline-perfused, brainstem-spinal cord preparations of the American bullfrog (Rana catesbeiana). Motor output indicative of fictive breathing was present in all preparations. In 17 of 26 preparations, fictive breaths were either evenly spaced or randomly distributed, while in the remaining nine preparations fictive breaths occurred in episodes separated by relatively long periods of quiescence. With the exception of fictive breath duration in the non-episodic preparations and the instantaneous frequency of fictive breaths within episodes, all variables associated with fictive breathing were insensitive to changes in perfusion saline pH. In addition to fictive breathing, a large number of other forms of motor output were observed arising from these nerves. While the data suggest that the in vitro preparation is capable of producing a wide repertoire of motor patterns, similar to those seen in vivo, it was difficult, with the current protocol, to reliably produce any single pattern in spite of carefully regulated conditions.

Time domains of the hypoxic ventilatory response

The ventilatory response to hypoxia depends on the pattern and intensity of hypoxic exposure and involves several physiological mechanisms. These mechanisms differ in their effect (facilitation or depression) on different components of ventilation (tidal volume and frequency) and in their time course (seconds to years). Some mechanisms last long enough to affect future ventilatory responses to hypoxia, indicating 'memory' or functional plasticity in the ventilatory control system. A standard terminology is proposed to describe the different time domains of the hypoxic ventilatory response (HVR) and to promote integration of results from different experimental preparations and laboratories. In general, the neurophysiological and neurochemical basis for short time domains of the HVR (seconds and minutes) are understood better than longer time domains (days to years), primarily because short time domains are studied in the laboratory more easily. Understanding the mechanisms for different time domains of the HVR has important implications for both basic and clinical science.

Cortical activation states in sleep and anesthesia. I: Cardio-respiratory effects

Under light urethane anesthesia, animals cycle through patterns of EEG activity which superficially appear like waking, light sleep and slow-wave sleep patterns (States I, II and III, respectively) in unanesthetized animals. The present study questioned whether similar cortical activity patterns in anesthetized and unanesthetized golden mantled ground squirrels represented analogous states in terms of cardiorespiratory function. Sleep exerted a strong negative influence on breathing frequency and ventilation, but had less consistent effects on tidal volume. Urethane-anesthetized animals demonstrated exactly the same alterations in respiratory variables when switching between states with similar cortical activity. Cardiovascular function was also affected by arousal state; heart rate decreased and variation increased significantly as animals moved from wake into sleep. Although urethane anesthesia greatly increased heart rate and abolished respiratory sinus arrhythmia, state-dependent changes in heart rate were still evident. Overall, the states observed under urethane anesthesia mimicked sleep/wake in terms of their effect on cardio-respiratory function.

Cortical activation states in sleep and anesthesia. II: respiratory reflexes

Under urethane anesthesia, animals exhibit patterns of cortical activity similar to those seen in wake, drowsiness and slow-wave sleep in unanesthetized animals. In the present study, hypoxic and hypercapnic ventilatory reflexes were examined in unanesthetized and urethane-anesthetized golden mantled ground squirrels in states with similar EEG profiles. Synchronized EEG patterns occurred less frequently in both unanesthetized and anesthetized animals during hypoxic (10% O2) and hypercapnic (5% CO2) exposure. Breathing frequency fell significantly during sleep in animals breathing all gas mixtures, while the relative ventilatory sensitivity to hypoxia and hypercapnia increased during sleep. Urethane-anesthetized animals also showed significant falls in breathing frequency and ventilation and increases in relative ventilatory sensitivity to hypoxia and hypercapnia as they moved into states with synchronized EEG patterns. These data suggest that the brain activity states observed under urethane anesthesia mimic sleep/wake in terms of their effect on respiratory function and that changes in breathing pattern and the enhancement of ventilatory responses in states with a synchronized EEG is not due solely to changes in levels of behavioural stimuli.

Do descending influences alternate to produce episodic breathing?

This study examines the episodic breathing patterns of three disparate groups of vertebrates. In an in vitro bullfrog brainstem-spinal cord preparation, episodic breathing was replaced by uniformly spaced breaths following transection caudal to the optic chiasma. The same effect was produced in hibernating squirrels by inhalation of mild anesthesia. Preliminary data suggest that a similar conversion is also produced in hibernating squirrels by vagotomy, in conjunction with blockade of central NMDA-type glutamate receptors. In all cases, even though overall breathing frequency increased, due to elimination of periods of apnea, instantaneous breathing frequency slowed. Seals breathe episodically in sleep and when these animals awaken after the start of a breathing episode, breathing also immediately slows. The data presented here are consistent with the suggestion that in all vertebrates, higher centres can modulate the central rhythm generator for breathing, in both a positive and a negative fashion. During episodic breathing, in the species studied here, these modulating influences alternate in a fashion that produces periods of apnea alternating with periods of relatively high frequency ventilation.

The role of the nucleus isthmi in respiratory pattern formation in bullfrogs

The nucleus isthmi (NI) is a mesencephalic structure of the amphibian brain located between the roof of the midbrain and the cerebellum. From a neuroanatomical perspective, the NI can be compared with the pons which, in mammals, contributes to the control of breathing pattern. This study tested the hypothesis that the NI plays a critical role in breathing pattern formation in the bullfrog. More specifically, we postulated that this nucleus was the site responsible for clustering breaths into distinct episodes of breathing. This hypothesis was tested by comparing the respiratory motor output of decerebrate, paralyzed and artificially ventilated bullfrogs before and after bilateral lesions of the NI by pressure microinjections of lidocaine or kainic acid (KA) into this area. Bilateral microinjections of lidocaine or KA into the NI transformed the breathing pattern from episodic (many breaths per episode) to one of evenly spaced single breaths, without affecting the amplitude of the fictive breaths. These changes in breathing pattern were associated with an overall decrease in breathing frequency and a reduction in CO2-chemosensitivity. Breathing episodes of more than one breath reappeared during hypercarbia (3.5% CO2 in air) after KA lesioning. Bilateral lesions to the NI did not affect the changes in the timing or the amplitude of the respiratory-related bursts elicited by pulmonary stretch receptor feedback, indicating that mechanoreflexes do not require NI input. We conclude that the NI is not responsible for the genesis of breathing episodes, but provides a tonic excitatory input to respiratory centers in the lower brainstem. The NI also plays an important role in either CO2 chemodetection or, more probably, integration of CO2 chemoreceptor information. This, in turn, contributes to the production of episodes of more than one breath.

Role of pulmonary stretch receptor feedback in control of episodic breathing in the bullfrog

This study compared the "fictive" breathing patterns of decerebrate, paralyzed, unidirectionally ventilated bullfrogs in which pulmonary stretch receptor (PSR) feedback was either absent bilateral vagotomy), maintained constant at different levels (tonic) or oscillated with each fictive breath (phasic) under different levels of hypoxic or CO2-related respiratory drive. Tonic and phasic PSR feedback had identical effects on the fictive breathing pattern; decreasing PSR feedback increased the peak integrated trigeminal electroneurogram recordings and decreased breathing frequency. The effects of bilateral vagotomy and lung deflation to 0 cmH2O on breathing pattern were identical. Although hypoxia (fractional concentration of O2 in air = 0.06) had no significant effect on fictive breathing, ventilating frogs with increasing CO2 levels (fractional CO2 concentration in inspired air range: 0.00-0.03) increased the number of breaths in each fictive breathing episode, and this effect was potentiated by PSR feedback. Whenever respiratory drive was increased, regardless of the method (increase in PSR feedback or chemoreceptor drive), occasional single breaths were replaced by breathing episodes, indicating that the mechanisms responsible for the clustering of the breaths and the onset/termination of breathing episodes are not dependent on either input alone.

The role of CO2/pH chemoreceptors in ventilatory control

It now appears that at least some members of all classes of vertebrates exhibit ventilatory responses to changes in CO2/pH per se, including fishes. With the transition from aquatic to aerial respiration, there is an increase in the sensitivity of animals to this complex of stimuli, an increase in the variety of putative receptors possibly involved in eliciting ventilatory responses and an increase in the relative importance of this complex of stimuli in the genesis of resting ventilation. The variety of CO2-sensitive chemoreceptors present in air-breathing lower vertebrates adds considerable complexity to experimental studies of ventilatory responses to CO2/pH. Because of the locations, discharge characteristics and reflex effects of the different receptor groups, most air-breathing lower vertebrates show different responses to increases in CO2/[H+] due to cerebral ischemia, anoxia, metabolic acidosis and environmental hypercarbia. In some cases the differences are only quantitative, while in other cases the responses are qualitatively very different. These differences appear to reflect differences in the relative strength of the reflexes elicited by the various receptor groups and the net sum of their modulating influences when CO2/pH are altered via different routes. Although the situation is simpler in the higher vertebrates, in all cases the input from all of the CO2/[H+]-sensitive receptors appears to act as a biasing input which summates with other afferent information to modulate respiratory motor output, even in those species that breathe intermittently.

Cardio-ventilatory control in rainbow trout: II. Reflex effects of exogenous neurochemicals

The effects of various neurochemicals were examined in intact, unanesthetized rainbow trout (Oncorhynchus mykiss) to assess the role of branchial O2-sensitive chemoreceptors in the cardio-ventilatory responses to exogenous neurochemicals. cyanide stimulated ventilation and elicited bradycardia when give externally but only stimulated ventilation when injected internally. Norepinephrine increased heart rate, blood pressure and ventilatory rate but opercular pressure was not affect. Dopamine had no effect on either heart or ventilatory rate but increased blood pressure and decreased opercular pressure. Serotonin stimulated heart rate and ventilation but decreased blood pressure. Acetylcholine and nicotine stimulated all cardio-ventilatory variables. Muscarine decreased heart rate and blood pressure and had a biphasic effect on ventilation. These results, combined with the results from the preceding study, suggest that the cardio-ventilatory effects of exogenously administered (1) cyanide are entirely mediated by gill O2 receptors, (2) serotonin, and cholinergic drugs could be partly mediated by O2 receptors and (3) catecholaminergic drugs are not mediated by O2 receptors.

Characteristics of diaphragm muscle fibre types in hibernating squirrels

Muscle samples from the diaphragms of 7 full-awake (FA), 10 winter-awake (WA), and 8 hibernating (H) squirrels (Spermophilus lateralis) were quick frozen, sectioned and processed for NADH-TR reaction end-product and myofibrillar-ATPase. Both WA and H squirrels showed small increases in diaphragm weight, reductions in body weight, and hence, significant increases in the diaphragm weight to body weight ratio compared to FA squirrels. They also showed increases in muscle fibre type cross-sectional areas and in the oxidative capacity of type 2b fibres as well as a reduction in capillary density. Furthermore, there also was an increase in the proportion of type 2b fibres in the diaphragm of the H squirrels. Thus, despite the dramatically reduced ventilation associated with hibernation, H squirrels exhibited (1) hypertrophy of the diaphragm which may represent an adaptive response that enables them to work against a stiffer chest wall, and (2) an increased oxidative capacity which enables them to fuel this with fat.

Pulmonary mechanics of hibernating squirrels (Spermophilus lateralis)

In this study we examined the effects of hibernation on several aspects of pulmonary mechanics in golden mantled ground squirrels. Measurements were made on anesthetized animals that were active in late fall (FA, n = 7), maintained at 23 degrees C and normal photoperiod in mid-winter (WA, n = 0) and hibernating at 5 degrees C in mid-winter (H, n = 8). Compared with FA animals, WA animals showed an increase in inspiratory reserve volume (IRV) and in inspiratory, vital and total lung capacities (IC, VC, and TLC). Hibernating animals exhibited further increases in IRV, IC, VC and TLC, an elevated residual volume, and virtual elimination of the expiratory reserve volume. There was also a decrease in specific lung compliance and a sharp knee in the lower portion of the quasi-static volume-pressure curve. There was a significant increase in the elastic work required to ventilate the hibernating animals compared with FA animals. The data suggest that, despite an increase in compliance at TLC, there is decreased compliance at low volumes, gas trapping at functional residual capacity, and an increase in the work required to breathe in hibernating animals.