The relevance of vascular adjustments to hemodynamic control in the face of temperature change in Crotalus durissus.

The presence of cardiac shunts in ectothermic tetrapods is thought to be consistent with active vascular modulations for proper hemodynamic support. Local control of blood flow modulates tissue perfusion and thus systemic conductance (Gsys) is assumed to increase with body temperature (Tb) to accommodate higher aerobic demand. However, the general increase of Gsys presses for a higher right-to-left (R-L) shunt, which reduces arterial oxygen concentration. In contrast, Tb reduction leads to a Gsys decrease and a left-to-right shunt, which purportedly increases pulmonary perfusion and plasma filtration in the respiratory area. This investigation addressed the role of compensatory vascular adjustments in the face of the metabolic alterations caused by Tb change in the South American rattlesnake (Crotalus durissus). Cardiovascular recordings were performed in decerebrated rattlesnake preparations at 10, 20 and 30°C. The rise in Tb increased metabolic demand, and correlated with an augmentation in heart rate. Although cardiac output increased, systemic stroke volume reduced while pulmonary stroke volume remained stable. Although that resulted in a proportionally higher increase in pulmonary blood flow, the R-L shunt was maintained. While the systemic compliance of large arteries was the most relevant factor in regulating arterial systemic blood pressure, peripheral conductance of pulmonary circulation was the major factor influencing the final cardiac shunt. Such dynamic adjustment of systemic compliance and pulmonary resistance for shunt modulation has not been demonstrated before and contrasts with previous knowledge on shunt control.

Recovery of the baroreflex and autonomic modulation after anesthesia with MS-222 in bullfrogs.

The time course for recovery after anesthesia is poorly described for tricaine methanesulfonate (MS-222). We suggest that the baroreflex and the heart rate variability (HRV) could be used to index the recovery of the autonomic modulation after anesthesia. We analyzed the recovery profile of behavioral and physiological parameters over time to analyze the progression of recovery after anesthesia of American bullfrogs with MS-222. Mean heart rate stabilized after 17 h, whereas the baroreflex efficiency index took 23 h and the baroreflex operating gain, 29 h. Mean arterial pressure recovered after 26 h. Power spectral density peaked at 23 h and again after 40 h. Baroreflex was a relevant component of the first phase of HRV, while autonomic modulation for resting may take longer than 40 h. We suggest that physiological recovery is a complex phenomenon with multiple progressive phases, and the baroreflex may be a useful tool to observe the first substantial recovery of post-instrumentation capacity for autonomic modulation.

Ontogenetic scaling of the baroreflex function in the green iguana (Iguana iguana).

Large body mass (Mb) in vertebrates is associated with longer pulse intervals between heartbeats (PI) and thicker arterial walls. Longer PI increases the time for diastolic pressure decay, possibly resulting in loss of cardiac energy as "oscillatory power," whereas thicker arterial walls may affect the transmission of impulses and sensing of pressure fluctuations thus impairing baroreflex function. We aimed to investigate the effect of growth on the relative cardiac energy loss and baroreflex function. We predicted that 1) the relative use of cardiac energy should be preserved with increased time constant for pressure decay (τ = vascular resistance × compliance) and 2) if arterial circumferential distensibility does not change, baroreflex function should be unaltered with Mb. To test these hypotheses, we used green iguanas (Iguana iguana) weighing from 0.03 to 1.34 kg (43-fold increment in Mb). PI (P = 0.037) and τ (P = 0.035) increased with Mb, whereas the oscillatory power fraction (P = 0.245) was unrelated to it. Thus, the concomitant alterations of τ and PI allowed the conservation of cardiac energy in larger lizards. Larger animals had thicker arterial walls (P = 0.0007) and greater relative collagen content (P = 0.022). Area compliance scaled positively to Mb (P = 0.045), though circumferential distensibility (P = 0.155) and elastic modulus (P = 0.762) were unaltered. In addition, baroreflex sensitivity, measured by both the pharmacological (P = 0.152) and sequence methods (P = 0.088), and the baroreflex effectiveness index (P = 0.306) were also unrelated to Mb. Therefore, changes in arterial morphology did not affect circumferential distensibility and presumably sensing of pressure fluctuation, and the cardiovagal baroreflex is preserved across different Mb.

Tolerance of an acute warming challenge declines with body mass in Nile tilapia: evidence of a link to capacity for oxygen uptake.

It has been proposed that larger individuals within fish species may be more sensitive to global warming, as a result of limitations in their capacity to provide oxygen for aerobic metabolic activities. This could affect size distributions of populations in a warmer world but evidence is lacking. In Nile tilapia Oreochromis niloticus (n=18, mass range 21-313 g), capacity to provide oxygen for aerobic activities (aerobic scope) was independent of mass at an acclimation temperature of 26°C. Tolerance of acute warming, however, declined significantly with mass when evaluated as the critical temperature for fatigue from aerobic swimming (CTSmax). The CTSmax protocol challenges a fish to meet the oxygen demands of constant aerobic exercise while their demands for basal metabolism are accelerated by incremental warming, culminating in fatigue. CTSmax elicited pronounced increases in oxygen uptake in the tilapia but the maximum rates achieved prior to fatigue declined very significantly with mass. Mass-related variation in CTSmax and maximum oxygen uptake rates were positively correlated, which may indicate a causal relationship. When fish populations are faced with acute thermal stress, larger individuals may become constrained in their ability to perform aerobic activities at lower temperatures than smaller conspecifics. This could affect survival and fitness of larger fish in a future world with more frequent and extreme heatwaves, with consequences for population productivity.

Baroreflex responses of decerebrate rattlesnakes (Crotalus durissus) are comparable to awake animals.

A decerebrate rattlesnake, Crotalus durissus, has previously been used as a model Squamate for cardiovascular studies. It enabled instrumentation for concomitant recordings of diverse variables that showed autonomic responses. However, to validate the preparation and its scope for use, it is necessary to assess how close its cardiovascular variables are to non-decerebrate snakes and the effectiveness of its autonomic responses. Similarly, it is important to analyze its recovery profile after instrumentation and observe if it maintains stability throughout the duration of experimental protocol. Here we have objectively assessed these points by comparing decerebrate preparations and non-decerebrate snakes, after the occlusive cannulation of the vertebral artery. We have assessed cardiovascular variables and the baroreflex to analyze the presence, magnitude and stability of complex autonomic-controlled parameters as indicators of autonomic nervous system (ANS) functionality. After instrumentation, mean heart rates were high but recovered to stable values within 24 h. Mean arterial pressure stabilized within 24 h in control snakes and 48 h in decerebrate preparations. After that, both parameters remained stable. The operational gain and effectiveness index of the baroreflex recovered within the first 6 h after instrumentation in both experimental groups. In addition, the baroreflex capacities and its limits were also equivalent between the groups. These experiments demonstrated that decerebrate preparations and inactive, non-decerebrate snakes showed comparable recovery profiles following anesthesia and cannulation, maintained similar values of cardiovascular variables during experimental manipulation and exhibited functional, ANS modulated reflexes. Accordingly, the present results attest the relevance of this decerebrate preparation for studies on cardiovascular modulation.

Prenatal hypoxia affects scaling of blood pressure and arterial wall mechanics in the common snapping turtle, Chelydra serpentina.

In reptiles, exposure to hypoxia during embryonic development affects several cardiovascular parameters. These modifications may impose different mechanical stress to the arterial system, and we speculated that the arterial wall of major outflow vessels would be modified accordingly. Since non-crocodilian reptiles possess a partially divided ventricle, ensuing similar systemic and pulmonary systolic pressures, we investigated how morphological and mechanical properties of segments from the left aortic arch (LAo) and the proximal and distal segments of the left pulmonary artery (LPAp and LPAd, respectively) change as body mass (Mb) increases. Eggs from common snapping turtles, Chelydra serpentina, were incubated under normoxia (21% O2; N21) or hypoxia (10% O2; H10), hatched and maintained in normoxia thereafter. Turtles (0.11-6.85 kg) were cannulated to measure arterial pressures, and an injection of adrenaline was used to increase pressures. Portions of the LAo, LPAp and LPAd were fixed under physiological hydrostatic pressures for histology and mechanical assessment. Arterial pressures increased with Mb for N21 but not for H10. Although mechanical and functional characteristics from the LPAp and LPAd were similar between N21 and H10, wall thickness from LAo did not change with Mb in the H10 group, thus wall stress increased in larger turtles. This indicates that larger H10 turtles probably experience an elevated probability of arterial wall rupture without concomitant changes in the cardiovascular system to prevent it. Finally, collagen content of the LPAp and LAo was smaller than in LPAd, suggesting a more distensible arterial wall could attenuate higher pressures from larger turtles.

Pharmacodynamics of propofol and alfaxalone in rattlesnakes (Crotalus durissus).

To characterise the effect of two common induction agents, propofol and alfaxalone, on mean arterial blood pressure (MAP) and heart rate (HR), we equipped 19 adult South American rattlesnakes (Crotalus durissus) with an indwelling arterial catheter approximately 24 h prior to recording of baseline resting values. Then, seven snakes received alfaxalone (15 mg kg-1) intravascularly (IV) through the catheter, while groups two and three (both n = 6) received propofol (15 mg kg-1 IV). The first two groups were not handled, while the group 3 was manually restrained for 2 min for a mock injection of 0.2 ml saline into the ventral tail vein. Baseline HR was similar in all groups and handling caused a significant tachycardia (p = 0.031) in group three. When given IV to undisturbed animals, both propofol and alfaxalone induced a significant increase in HR (p = 0.0022 and p = 0.0045, respectively) lasting approximately 30 min, but with values only significantly exceeding baseline for the first 5 min for propofol and the first 10 min with alfaxalone. Handling caused a significant increase in MAP (p = 0.0313). Propofol did not affect MAP (p = 0.1064), while alfaxalone caused a marked hypertension (although only significant at 2 min; p = 0.031). Manual restraint significantly increases both HR and MAP, which may lead to a masking of true cardiovascular effects of anaesthetic agents.

Cholinergic regulation along the pulmonary arterial tree of the South American rattlesnake: vascular reactivity, muscarinic receptors, and vagal innervation.

Vascular tone in the reptilian pulmonary vasculature is primarily under cholinergic, muscarinic control exerted via the vagus nerve. This control has been ascribed to a sphincter located at the arterial outflow, but we speculated whether the vascular control in the pulmonary artery is more widespread, such that responses to acetylcholine and electrical stimulation, as well as the expression of muscarinic receptors, are prevalent along its length. Working on the South American rattlesnake (Crotalus durissus), we studied four different portions of the pulmonary artery (truncus, proximal, distal, and branches). Acetylcholine elicited robust vasoconstriction in the proximal, distal, and branch portions, but the truncus vasodilated. Electrical field stimulation (EFS) caused contractions in all segments, an effect partially blocked by atropine. We identified all five subtypes of muscarinic receptors (M1-M5). The expression of the M1 receptor was largest in the distal end and branches of the pulmonary artery, whereas expression of the muscarinic M3 receptor was markedly larger in the truncus of the pulmonary artery. Application of the neural tracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-carbocyanine perchlorate (DiI) revealed widespread innervation along the whole pulmonary artery, and retrograde transport of the same tracer indicated two separate locations in the brainstem providing vagal innervation of the pulmonary artery, the medial dorsal motor nucleus of the vagus and a ventro-lateral location, possibly constituting a nucleus ambiguus. These results revealed parasympathetic innervation of a large portion of the pulmonary artery, which is responsible for regulation of vascular conductance in C. durissus, and implied its integration with cardiorespiratory control.

Heart rate variability in the tegu lizard, Salvator merianae, its neuroanatomical basis and role in the assessment of recovery from experimental manipulation.

Using long-term, remote recordings of heart rate (fH) on fully recovered, undisturbed lizards, we identified several components of heart rate variability (HRV) associated with respiratory sinus arrhythmia (RSA): 1.) A peak in the spectral representation of HRV at the frequency range of ventilation. 2.) These cardiorespiratory interactions were shown to be dependent on the parasympathetic arm of the autonomic nervous system. 3.) Vagal preganglionic neurons are located in discrete groups located in the dorsal motor nucleus of the vagus and also, in a ventro-lateral group, homologous to the nucleus ambiguus of mammals. 4.) Myelinated nerve fibers in the cardiac vagus enabling rapid communication between the central nervous system and the heart. Furthermore, the study of the progressive recovery of fH in tegu following anesthesia and instrumentation revealed that 'resting' levels of mean fH and reestablishment of HRV occurred over different time courses. Accordingly, we suggest that, when an experiment is designed to study a physiological variable reliant on autonomic modulation at its normal, resting level, then postsurgical reestablishment of HRV should be considered as the index of full recovery, rather than mean fH.

Respiratory sinus arrhythmia is a major component of heart rate variability in undisturbed, remotely monitored rattlesnakes, Crotalus durissus.

ECG recordings were obtained using an implanted telemetry device from the South American rattlesnake, Crotalus durissus, held under stable conditions without restraining cables or interaction with researchers. Mean heart rate (fH) recovered rapidly (<24 h) from anaesthesia and operative procedures. This preceded a more gradual development of heart rate variability (HRV), with instantaneous fH increasing during each lung ventilation cycle. Atropine injection increased mean fH and abolished HRV. Complete autonomic blockade revealed a cholinergic tonus on the heart of 55% and an adrenergic tonus of 37%. Power spectral analysis of HRV identified a peak at the same frequency as ventilation. This correlation was sustained after temperature changes and it was more evident, marked by a more prominent power spectrum peak, when ventilation is less episodic. This HRV component is homologous to that observed in mammals, termed respiratory sinus arrhythmia (RSA). Evidence for instantaneous control of fH indicated rapid conduction of activity in the cardiac efferent nervous supply, as supported by the description of myelinated fibres in the cardiac vagus. Establishment of HRV 10 days after surgical intervention seems a reliable indicator of the re-establishment of control of integrative functions by the autonomic nervous system. We suggest that this criterion could be applied to other animals exposed to natural or imposed trauma, thus improving protocols involving animal handling, including veterinarian procedures.

CARDIOVASCULAR EFFECTS OF ALFAXALONE AND PROPOFOL IN THE BULLFROG, LITHOBATES CATESBEIANUS.

Alfaxalone is becoming a popular anesthetic for nonmammalian vertebrates, but the physiological effects of its administration remain largely unknown in these taxa. Therefore, the cardiovascular responses to a clinically relevant dose of alfaxalone (10 mg/kg) are reported in the bullfrog ( Lithobates catesbeianus), following intramuscular (IM) and intravascular (IV) administration (via a femoral artery catheter) and compared with an IV dose of propofol, another parenteral GABA (γ-aminobutyric acid) agonist in common veterinary use as an induction agent. Heart rate (HR) and mean arterial blood pressure (MAP) (assessed by direct measurement from the catheter) are reported from under undisturbed conditions to assess both the direct effects of the drugs and the interaction with the stress of handling associated with IM injection of alfaxalone where IM administration is possible. Alfaxalone caused HR to increase significantly for over 45 min in both groups from a baseline of approximately 30 beats/min. This was significantly different from the lack of significant HR response on the IV administration of propofol. MAP increased in the peri-injection period with both routes of administration for alfaxalone but after IV use decreased significantly from 10 min following administration. Propofol did not affect blood pressure after 5 min from injection. Assessment of immobilization following intramuscular injection of alfaxalone in a pilot study was in accordance with the literature, as it provided no antinociception as a sole agent but did produce sedation and loss of righting reflex.

When local anesthesia becomes universal: Pronounced systemic effects of subcutaneous lidocaine in bullfrogs (Lithobates catesbeianus).

Sodium channel blockers are commonly injected local anesthetics but are also routinely used for general immersion anesthesia in fish and amphibians. Here we report the effects of subcutaneous injection of lidocaine (5 or 50mgkg-1) in the hind limb of bullfrogs (Lithobates catesbeianus) on reflexes, gular respiration and heart rate (handled group, n=10) or blood pressure and heart rate via an arterial catheter (catheterized group n=6). 5mgkg-1 lidocaine did not cause loss of reflexes or change in heart rate in the handled group, but was associated with a reduction in gular respiratory rate (from 99±7 to 81±17breathsmin-1). 50mgkg-1 lidocaine caused a further reduction in respiratory rate to 59±15breathsmin-1, and led to a progressive loss of righting reflex (10/10 loss by 40min), palpebral reflex (9/10 loss at 70min), and contralateral toe pinch withdrawal (9/10 loss at 70min). Reflexes were regained over 4h. Systemic sedative effects were not coupled to local anti-nociception, as a forceps pinch test at the site of injection provoked movement at the height of the systemic effect (tested at 81±4min). Amphibians are routinely subject to general anesthesia via exposure to sodium channel blockers such as MS222 or benzocaine, however caution should be exercised when using local injectable lidocaine in amphibians, as it appears to dose-dependently cause sedation, without necessarily preventing local nociception for the duration of systemic effects.

Improved cardiac filling facilitates the postprandial elevation of stroke volume in Python regius.

To accommodate the pronounced metabolic response to digestion, pythons increase heart rate and elevate stroke volume, where the latter has been ascribed to a massive and fast cardiac hypertrophy. However, numerous recent studies show that heart mass rarely increases, even upon ingestion of large meals, and we therefore explored the possibility that a rise in mean circulatory filling pressure (MCFP) serves to elevate venous pressure and cardiac filling during digestion. To this end, we measured blood flows and pressures in anaesthetized Python regius The anaesthetized snakes exhibited the archetypal tachycardia as well as a rise in both venous pressure and MCFP that fully account for the approximate doubling of stroke volume. There was no rise in blood volume and the elevated MCFP must therefore stem from increased vascular tone, possibly by means of increased sympathetic tone on the veins. Furthermore, although both venous pressure and MCFP increased during volume loading, there was no evidence that postprandial hearts were endowed with an additional capacity to elevate stroke volume. In vitro measurements of force development of paced ventricular strips also failed to reveal signs of increased contractility, but the postprandial hearts had higher activities of cytochrome oxidase and pyruvate kinase, which probably serves to sustain the rise in cardiac work during digestion.

The progressive onset of cholinergic and adrenergic control of heart rate during development in the green iguana, Iguana iguana.

The autonomic control of heart rate was studied throughout development in embryos of the green iguana, Iguana iguana by applying receptor agonists and antagonists of the parasympathetic and sympathetic systems. Acetylcholine (Ach) slowed or stopped the heart and atropine antagonized the response to Ach indicating the presence of muscarinic cholinoceptors on the heart of early embryos. However, atropine injections had no impact on heart rate until immediately before hatching, when it increased heart rate by 15%. This cholinergic tonus increased to 34% in hatchlings and dropped to 24% in adult iguanas. Although epinephrine was without effect, injection of propranolol slowed the heart throughout development, indicating the presence of ß-adrenergic receptors on the heart of early embryos, possibly stimulated by high levels of circulating catecholamines. The calculated excitatory tonus varied between 33% and 68% until immediately before hatching when it fell to 25% and 29%, a level retained in hatchlings and adults. Hypoxia caused a bradycardia in early embryos that was unaffected by injection of atropine indicating that hypoxia has a direct effect upon the heart. In later embryos and hatchlings hypoxia caused a tachycardia that was unaffected by injection of atropine. Subsequent injection of propranolol reduced heart rate both uncovering a hypoxic bradycardia in late embryos and abolishing tachycardia in hatchlings. Hypercapnia was without effect on heart rate in late stage embryos and in hatchlings.

Assessing the influence of mechanical ventilation on blood gases and blood pressure in rattlesnakes.

OBJECTIVE: To characterize the impact of mechanical positive pressure ventilation on heart rate (HR), arterial blood pressure, blood gases, lactate, glucose, sodium, potassium and calcium concentrations in rattlesnakes during anesthesia and the subsequent recovery period. STUDY DESIGN: Prospective, randomized trial. ANIMALS: Twenty one fasted adult South American rattlesnakes (Crotalus durissus terrificus). METHODS: Snakes were anesthetized with propofol (15 mg kg(-1)) intravenously, endotracheally intubated and assigned to one of four ventilation regimens: Spontaneous ventilation, or mechanical ventilation at a tidal volume of 30 mL kg(-1) at 1 breath every 90 seconds, 5 breaths minute(-1), or 15 breaths minute(-1). Arterial blood was collected from indwelling catheters at 30, 40, and 60 minutes and 2, 6, and 24 hours following induction of anesthesia and analyzed for pH, PaO2, PaCO2, and selected variables. Mean arterial blood pressure (MAP) and HR were recorded at 30, 40, 60 minutes and 24 hours. RESULTS: Spontaneous ventilation and 1 breath every 90 seconds resulted in a mild hypercapnia (PaCO2 22.4 ± 4.3 mmHg [3.0 ± 0.6 kPa] and 24.5 ± 1.6 mmHg [3.3 ± 0.2 kPa], respectively), 5 breaths minute(-1) resulted in normocapnia (14.2 ± 2.7 mmHg [1.9 ± 0.4 kPa]), while 15 breaths minute(-1) caused marked hypocapnia (8.2 ± 2.5 mmHg [1.1 ± 0.3 kPa]). Following recovery, blood gases of the four groups were similar from 2 hours. Anesthesia, independent of ventilation was associated with significantly elevated glucose, lactate and potassium concentrations compared to values at 24 hours (p < 0.0001). MAP increased significantly with increasing ventilation frequency (p < 0.001). HR did not vary among regimens. CONCLUSIONS AND CLINICAL RELEVANCE: Mechanical ventilation had a profound impact on blood gases and blood pressure. The results support the use of mechanical ventilation with a frequency of 1-2 breaths minute(-1) at a tidal volume of 30 mL kg(-1) during anesthesia in fasted snakes.

The phylogeny and ontogeny of autonomic control of the heart and cardiorespiratory interactions in vertebrates.

Heart rate in vertebrates is controlled by activity in the autonomic nervous system. In spontaneously active or experimentally prepared animals, inhibitory parasympathetic control is predominant and is responsible for instantaneous changes in heart rate, such as occur at the first air breath following a period of apnoea in discontinuous breathers like inactive reptiles or species that surface to air breathe after a period of submersion. Parasympathetic control, exerted via fast-conducting, myelinated efferent fibres in the vagus nerve, is also responsible for beat-to-beat changes in heart rate such as the high frequency components observed in spectral analysis of heart rate variability. These include respiratory modulation of the heartbeat that can generate cardiorespiratory synchrony in fish and respiratory sinus arrhythmia in mammals. Both may increase the effectiveness of respiratory gas exchange. Although the central interactions generating respiratory modulation of the heartbeat seem to be highly conserved through vertebrate phylogeny, they are different in kind and location, and in most species are as yet little understood. The heart in vertebrate embryos possesses both muscarinic cholinergic and ß-adrenergic receptors very early in development. Adrenergic control by circulating catecholamines seems important throughout development. However, innervation of the cardiac receptors is delayed and first evidence of a functional cholinergic tonus on the heart, exerted via the vagus nerve, is often seen shortly before or immediately after hatching or birth, suggesting that it may be coordinated with the onset of central respiratory rhythmicity and subsequent breathing.

Ablation of the ability to control the right-to-left cardiac shunt does not affect oxygen uptake, specific dynamic action or growth in the rattlesnake Crotalus durissus.

The morphologically undivided ventricle of the heart in non-crocodilian reptiles permits the mixing of oxygen-rich blood returning from the lungs and oxygen-poor blood from the systemic circulation. A possible functional significance for this intra-cardiac shunt has been debated for almost a century. Unilateral left vagotomy rendered the single effective pulmonary artery of the South American rattlesnake, Crotalus durissus, unable to adjust the magnitude of blood flow to the lung. The higher constant perfusion of the lung circulation and the incapability of adjusting the right-left shunt in left-denervated snakes persisted over time, providing a unique model for investigation of the long-term consequences of cardiac shunting in a squamate. Oxygen uptake recorded at rest and during spontaneous and forced activity was not affected by removing control of the cardiac shunt. Furthermore, metabolic rate and energetic balance during the post-prandial metabolic increment, plus the food conversion efficiency and growth rate, were all similarly unaffected. These results show that control of cardiac shunting is not associated with a clear functional advantage in adjusting metabolic rate, effectiveness of digestion or growth rates.

A comparison of four common mathematical models to assess baroreflex sensitivity.

Different methods have been used to assess baroreflex gain in experiments where changes in the carotid sinus pressure or the arterial blood pressure using different techniques provoke a baroreflex response, usually a rapid variation of heart rate. Four mathematical models are most used in the literature: the linear regression, the piecewise regression, and two different four-parameter logistic equations: equation 1, Y = (A1-D1)/[1 + eB1(X - C1) ] + D1; equation 2, Y = (A2-D2)/[1 + (X/C2)B2 ] + D2. We compared the four models regarding the best fit to previously published data in all vertebrate classes. The linear regression had the worst fit in all cases. The piecewise regression generally exhibited a better fit than the linear regression, though it returned a similar fit when no breakpoints were found. The logistic equations showed the best fit among the tested models and were similar to each other. We demonstrate that equation 2 is asymmetric and the level of asymmetry is accentuated according to B2. This means that the baroreflex gain calculated when X = C2 is different from the actual maximum gain. Alternatively, the symmetric equation 1 returns the maximum gain when X = C1. Furthermore, the calculation of baroreflex gain using equation 2 disregards that baroreceptors may reset when individuals experience different mean arterial pressures. Finally, the asymmetry from equation 2 is a mathematical artifact inherently skewed to the left of C2, thus bearing no biological meaning. Therefore, we suggest that equation 1 should be used instead of equation 2.

An overview of the phylogeny of cardiorespiratory control in vertebrates with some reflections on the 'Polyvagal Theory'.

Mammals show clear changes in heart rate linked to lung ventilation, characterized as respiratory sinus arrhythmia (RSA). These changes are controlled in part by variations in the level of inhibitory control exerted on the heart by the parasympathetic arm of the autonomic nervous system (PNS). This originates from preganglionic neurons in the nucleus ambiguous that supply phasic, respiration-related activity to the cardiac branch of the vagus nerve, via myelinated, efferent fibres with rapid conduction velocities. An elaboration of these central mechanisms, under the control of a 'vagal system' has been endowed by psychologists with multiple functions concerned with 'social engagement' in mammals and, in particular, humans. Long-term study of cardiorespiratory interactions (CRI) in other major groups of vertebrates has established that they all show both tonic and phasic control of heart rate, imposed by the PNS. This derives centrally from neurones located in variously distributed nuclei, supplying the heart via fast-conducting, myelinated, efferent fibres. Water-breathing vertebrates, which include fishes and larval amphibians, typically show direct, 1:1 CRI between heart beats and gill ventilation, controlled from the dorsal vagal motor nucleus. In air-breathing, ectothermic vertebrates, including reptiles, amphibians and lungfish, CRI mirroring RSA have been shown to improve oxygen uptake during phasic ventilation by changes in perfusion of their respiratory organs, due to shunting of blood over across their undivided hearts. This system may constitute the evolutionary basis of that generating RSA in mammals, which now lacks a major physiological role in respiratory gas exchange, due to their completely divided systemic and pulmonary circulations.

Baroreflex responses to activity at different temperatures in the South American rattlesnake, Crotalus durissus.

In humans, physical exercise imposes narrower limits for the heart rate (fH) response of the baroreflex, and vascular modulation becomes largely responsible for arterial pressure regulation. In undisturbed reptiles, the baroreflex-related fH alterations at the operating point (Gop) decreases at elevated body temperatures (Tb) and the vascular regulation changes accordingly. We investigated how the baroreflex of rattlesnakes, Crotalus durissus, is regulated during an activity at different Tb, expecting that activity would reduce the capacity of the cardiac baroreflex neural pathway to buffer arterial pressure fluctuations while being compensated by the vascular neural pathway regulation. Snakes were catheterized for blood pressure assessment at three different Tb: 15, 20 and 30 °C. Data were collected before and after activity at each Tb. Baroreflex gain (Gop) was assessed with the sequence method; the vascular limb, with the time constant of pressure decay (τ), using the two-element Windkessel equation. Both Gop and τ reduced when Tb increased. Activity also reduced Gop and τ in all Tb. The relationship between τ and pulse interval (τ/PI) was unaffected by the temperature at resting snakes, albeit it reduced after activity at 20 °C and 30 °C. The unchanged τ/PI and normalized Gop at different Tb indicated those variables are actively adjusted to work at different fH and pressure conditions at rest. Our data suggest that during activity, the baroreflex-related fH response is attenuated and hypertension is buffered by a disproportional increase in the rate which pressure decays during diastole. This compensation seems especially important at higher Tb where Gop is already low.