Temperature effects on baroreflex control of heart rate in the toad, Rhinella schneideri

Thermal Acclimation to the Highest Natural Ambient Temperature Compromises Physiological Performance in Tadpoles of a Stream-Breeding Savanna Tree Frog

Amphibians may be more vulnerable to climate-driven habitat modification because of their complex life cycle dependence on land and water. Considering the current rate of global warming, it is critical to identify the vulnerability of a species by assessing its potential to acclimate to warming temperatures. In many species, thermal acclimation provides a reversible physiological adjustment in response to temperature changes, conferring resilience in a changing climate. Here, we investigate the effects of temperature acclimation on the physiological performance of tadpoles of a stream-breeding savanna tree frog (Bokermannohyla ibitiguara) in relation to the thermal conditions naturally experienced in their microhabitat (range: 18.8-24.6°C). We quantified performance measures such as routine and maximum metabolic rate at different test (15, 20, 25, 30, and 34°C) and acclimation temperatures (18 and 25°C). We also measured heart rate before and after autonomic blockade with atropine and sotalol at the respective acclimation temperatures. Further, we determined the critical thermal maximum and warming tolerance (critical thermal maximum minus maximum microhabitat temperature), which were not affected by acclimation. Mass-specific routine and mass-specific maximum metabolic rate, as well as heart rate, increased with increasing test temperatures; however, acclimation elevated mass-specific routine metabolic rate while not affecting mass-specific maximum metabolic rate. Heart rate before and after the pharmacological blockade was also unaffected by acclimation. Aerobic scope in animals acclimated to 25°C was substantially reduced, suggesting that physiological performance at the highest temperatures experienced in their natural habitat is compromised. In conclusion, the data suggest that the tadpoles of B. ibitiguara, living in a thermally stable environment, have a limited capacity to physiologically adjust to the highest temperatures found in their micro-habitat, making the species more vulnerable to future climate change.

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 (f_H) response of the baroreflex, and vascular modulation becomes largely responsible for arterial pressure regulation. In undisturbed reptiles, the baroreflex-related f_H alterations at the operating point (G_op) decreases at elevated body temperatures (T_b) and the vascular regulation changes accordingly. We investigated how the baroreflex of rattlesnakes, Crotalus durissus, is regulated during an activity at different T_b, 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 T_b: 15, 20 and 30 °C. Data were collected before and after activity at each T_b. Baroreflex gain (G_op) was assessed with the sequence method; the vascular limb, with the time constant of pressure decay (τ), using the two-element Windkessel equation. Both G_op and τ reduced when T_b increased. Activity also reduced G_op and τ in all T_b. 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 G_op at different T_b indicated those variables are actively adjusted to work at different f_H and pressure conditions at rest. Our data suggest that during activity, the baroreflex-related f_H 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 T_b where G_op is already low.

The baroreflex in aquatic and amphibious teleosts: Does terrestriality represent a significant driving force for the evolution of a more effective baroreflex in vertebrates?

All vertebrates have baroreflexes that provide fast regulation of arterial blood pressure (P_A) to maintain adequate tissue perfusion and avoid vascular lesions from excessive pressures. The baroreflex is a negative feedback loop, where altered P_A results in reciprocal changes in heart rate (f_H) and systemic vascular conductance to restore pressure. In terrestrial environments, gravity usually leads to blood pooling in the lower body reducing venous return, cardiac filling, cardiac output and P_A. Conversely, in aquatic environments, the hydrostatic pressure of surrounding water mitigates blood pooling and prevents vascular distensions. In this context, we aimed to test the hypothesis that vertebrate species that were exposed to gravity-induced hemodynamic disturbances throughout their evolutionary histories have a more effective barostatic reflex than those that were not. We examined the cardiac baroreflex of fish that perform (Clarias gariepinus and Hoplerythrinus unitaeniatus) and do not perform (Hoplias malabaricus and Oreochromis niloticus) voluntary terrestrial sojourns, using pharmacological manipulations of P_A to characterize reflex changes in f_H using a four-variable sigmoidal logistic function (i.e. the "Oxford technique"). Our results revealed that amphibious fish exhibit higher baroreflex gain and responsiveness to hypotension than strictly aquatic fish, suggesting that terrestriality and the gravitational circulatory stresses constitute a relevant driving force for the evolution of a more effective baroreflex in vertebrates. We also demonstrate that strictly aquatic teleosts have considerable baroreflex gain, supporting the view that the baroreflex is an ancient cardiovascular trait that appeared before vertebrates colonized the gravity-dominated realm of land.

The nonpharmacological sequence method provides a reliable evaluation of baroreflex sensitivity in fish

The most commonly used technique to study the barostatic regulation of blood pressure in ectothermic vertebrates consists of determining the heart rate response to pharmacological manipulations of blood pressure, the so-called "Oxford method." Although well established, the Oxford method has some important limitations, such as induction of hypervolemia in small animals and undesired effects of vasoactive drugs on central and peripheral baroreflex components. As an alternative, the sequence method, which consists in the computerized evaluation of naturally-occurring baroreflex adjustments of heart rate without the need for pharmacological administrations, was developed to study baroreflexes. In the present study, we compare this sequence method with the Oxford technique in two teleost species with different life styles, and we assess the optimal software configuration for the employment of the sequence method in fish. Calculation of baroreflex gain through the sequence method was adequate and reliable when the software was configured to search for baroreflex sequences with a minimum length of three cardiac cycles with a delay of one cardiac cycle between fluctuations in mean ventral aortic blood pressure and reflex changes in pulse interval. When properly configured, the sequence and the Oxford methods yielded similar determinations of the baroreflex gain in fish.

Baroreflex gain and time of pressure decay at different body temperatures in the tegu lizard, Salvator merianae

Ectotherms may experience large body temperature (T_b) variations. Higher T_b have been reported to increase baroreflex sensitivity in ectotherm tetrapods. At lower T_b, pulse interval (PI) increases and diastolic pressure decays for longer, possibly resulting in lower end-diastolic pressures and mean arterial pressures (P_m). Additionally, compensatory baroreflex-related heart rate modulation (i.e. the cardiac branch of the baroreflex response) is delayed due to increased PI. Thus, low T_b is potentially detrimental, leading to cardiovascular malfunctioning. This raises the question on how P_m is regulated in such an adverse condition. We investigated the baroreflex compensations that enables tegu lizards, Salvator merianae, to maintain blood pressure homeostasis in a wide T_b range. Lizards had their femoral artery cannulated and pressure signals recorded at 15°C, 25°C and 35°C. We used the sequence method to analyse the heart rate baroreflex-related corrections to spontaneous pressure fluctuations at each temperature. Vascular adjustments (i.e. the peripheral branch) were assessed by calculating the time constant for arterial pressure decay (τ)—resultant from the action of both vascular resistance and compliance—by fitting the diastolic pressure descent to the two-element Windkessel equation. We observed that at lower T_b, lizards increased baroreflex gain at the operating point (G_op) and τ, indicating that the diastolic pressure decays at a slower rate. G_op normalized to P_m and PI, as well as the ratio τ/PI, did not change, indicating that both baroreflex gain and rate of pressure decay are adjusted according to PI lengthening. Consequently, pressure parameters and the oscillatory power fraction (an index of wasted cardiac energy) were unaltered by T_b, indicating that both G_op and τ modulation are crucial for cardiovascular homeostasis.

Fenofibrate ameliorates neural, mechanical, chemical, and electrical alterations in the murine model of heart failure

Heart failure (HF) is a leading cause of hospitalization across the world and is known to cause ill-health and heavy economic losses. In the present study, a rat model of isoproterenol (ISO, 85 mg/kg subcutaneously for two subsequent days) induced HF was developed. ISO induces HF by its direct effect, that is, rise in left ventricular end-diastolic pressure (mechanical) and indirectly by altering the baroreflex (neural), electrocardiography (electrical), and development of oxidative stress and hyperlipidemia (chemical). Fenofibrate, a hypolipidemic drug, which ameliorates myocardial energy metabolism was seen to improve the both ISO-induced oxidative stress and lipid profile and consequently improved Baroreflex Sensitivity (BRS), partial ventricular functions, and cardiac hypertrophy. Therefore, our result suggests that fenofibrate treatment protected the heart by alleviating the ISO-induced effects, that is, neural, mechanical, electrical, and chemical alterations.

Analysis of the respiratory component of heart rate variability in the Cururu toad Rhinella schneideri

Beat-to-beat variation in heart rate (f H ) has been used as a tool for elucidating the balance between sympathetic and parasympathetic modulation of the heart. A portion of the temporal changes in f H is evidenced by a respiratory influence (cardiorespiratory interaction) on heart rate variability (HRV) with heartbeats increasing and decreasing within a respiratory cycle. Nevertheless, little is known about respiratory effects on HRV in lower vertebrates. By using frequency domain analysis, we provide the first evidence of a ventilatory component in HRV similar to mammalian respiratory sinus arrhythmia in an amphibian, the toad Rhinella schneideri. Increases in the heartbeats arose synchronously with each lung inflation cycle, an intermittent breathing pattern comprised of a series of successive lung inflations. A well-marked peak in the HRV signal matching lung inflation cycle was verified in toads whenever lung inflation cycles exhibit a regular rhythm. The cardiac beat-to-beat variation evoked at the moment of lung inflation accounts for both vagal and sympathetic influences. This cardiorespiratory interaction may arise from interactions between central and peripheral feedback mechanisms governing cardiorespiratory control and may underlie important cardiorespiratory adjustments for gas exchange improvement especially under extreme conditions like low oxygen availability.

Temperature effects on the cardiorespiratory control of American bullfrog tadpoles based on a non-invasive methodology

Temperature effects on cardiac autonomic tonus in amphibian larval stages have never been investigated. Therefore, we evaluated the effect of different temperatures (15, 25 and 30°C) on the cardiorespiratory rates and cardiac autonomic tonus of premetamorphic tadpoles of the bullfrog, Lithobates catesbeianus To this end, a non-invasive method was developed to permit measurements of electrocardiogram (ECG) and buccal movements (f_B; surface electromyography of the buccal floor). For evaluation of autonomic regulation, intraperitoneal injections of Ringer solution (control), atropine (cholinergic muscarinic antagonist) and sotalol (β-adrenergic antagonist) were performed. Ringer solution injections did not affect heart rate (f_H) or f_B across temperatures. Cardiorespiratory parameters were significantly augmented by temperature (f_H: 24.5±1.0, 54.5±2.0 and 75.8±2.8 beats min^-1 at 15, 25 and 30°C, respectively; f_B: 30.3±1.1, 73.1±4.0 and 100.6±3.7 movements min^-1 at 15, 25 and 30°C, respectively). A predominant vagal tone was observed at 15°C (32.0±3.2%) and 25°C (27.2±6.7%) relative to the adrenergic tone. At 30°C, the adrenergic tone increased relative to the lower temperature. In conclusion, the cholinergic and adrenergic tones seem to be independent of temperature for colder thermal intervals (15-25°C), while exposure to a hotter ambient temperature (30°C) seems to be followed by a significant increase in adrenergic tone and may reflect cardiovascular adjustments made to match oxygen delivery to demand. Furthermore, while excluding the use of implantable electrodes or cannulae, this study provides a suitable non-invasive method for investigating cardiorespiratory function (cardiac and respiratory rates) in water-breathing animals such as the tadpole.

Effect of temperature on chemosensitive locus coeruleus neurons of savannah monitor lizards, Varanus exanthematicus

Savannah monitor lizards (Varanus exanthematicus) are unusual among ectothermic vertebrates in maintaining arterial pH nearly constant during changes in body temperature in contrast to the typical α-stat regulating strategy of most other ectotherms. Given the importance of pH in the control of ventilation, we examined the CO₂/H⁺ sensitivity of neurons from the locus coeruleus (LC) region of monitor lizard brainstems. Whole-cell patch-clamp electrophysiology was used to record membrane voltage in LC neurons in brainstem slices. Artificial cerebral spinal fluid equilibrated with 80% O₂, 0.0-10.0% CO₂, balance N₂, was superfused across brainstem slices. Changes in firing rate of LC neurons were calculated from action potential recordings to quantify the chemosensitive response to hypercapnic acidosis. Our results demonstrate that the LC brainstem region contains neurons that can be excited or inhibited by, and/or are not sensitive to CO₂ in V. exanthematicus While few LC neurons were activated by hypercapnic acidosis (15%), a higher proportion of the LC neurons responded by decreasing their firing rate during exposure to high CO₂ at 20°C (37%); this chemosensitive response was no longer exhibited when the temperature was increased to 30°C. Further, the proportion of chemosensitive LC neurons changed at 35°C with a reduction in CO₂-inhibited (11%) neurons and an increase in CO₂-activated (35%) neurons. Expressing a high proportion of inhibited neurons at low temperature may provide insights into mechanisms underlying the temperature-dependent pH-stat regulatory strategy of savannah monitor lizards.

Baroreflex regulation affects ventilation in the Cururu toad Rhinella schneideri

Anurans regulate short-term oscillations in blood pressure through changes in heart rate (fH), vascular resistance and lymph hearts frequency. Lung ventilation in anurans is linked to blood volume homeostasis by facilitating lymph return to the cardiovascular system. We hypothesized that the arterial baroreflex modulates pulmonary ventilation in the Cururu toad Rhinella schneideri, and that this relationship is temperature-dependent. Pharmacologically induced hypotension (sodium nitroprusside) and hypertension (phenylephrine) increased ventilation (25°C: 248.7±25.7; 35°C: 351.5±50.2 ml kg−1 min−1) and decreased ventilation (25°C: 9.0±6.6; 35°C: 50.7±15.6 ml kg−1 min−1), respectively, relative to control values from Ringer's injection (25°C: 78.1±17.0; 35°C: 137.7±15.5 ml kg−1 min−1). The sensitivity of the ventilatory response to blood pressure changes was higher during hypotension than hypertension (25°C: -97.6±17.1 vs. -23.6±6.0 breaths min−1 kPa−1; 35°C: -141.0±29.5 vs. -28.7±6.4 breaths min−1 kPa−1, respectively), while temperature had no effect on those sensitivities. Hyperoxia (30%; 25°C) diminished ventilation, but did not abolish the ventilatory response to hypotension, indicating a response independent of peripheral chemoreceptors. Although there are previous data showing increased fH baroreflex sensitivity from 15 to 30°C in this species, further increases in temperature (35°C) diminished fH baroreflex gain (40.5±5.62 vs. 21.6±4.64 % kPa−1). Therefore, besides a pulmonary ventilation role in matching O2 delivery to demand at higher temperatures in anurans, it also plays a role in blood pressure regulation, independent of temperature, possibly owing to an interaction between baroreflex and respiratory areas in the brain, as previously suggested for mammals.

Winter metabolic depression does not change arterial baroreflex control of heart rate in the tegu lizard (Salvator merianae)

Baroreflex regulation of blood pressure (BP) is important for maintaining appropriate tissue perfusion. Although temperature affects heart rate (fH) reflex regulation in some reptiles and toads, no data are available on the influence of temperature-independent metabolic states on baroreflex. The South American tegu lizard Salvator merianae exhibits a clear seasonal cycle of activity decreasing fH along with winter metabolic downregulation, independent of body temperature. Through pharmacological interventions (phenylephrine and sodium nitroprusside), the baroreflex control of fH was studied at ∼25°C in spring-summer and winter-acclimated tegus.

In winter lizards, resting and minimum fH were lower than in spring-summer animals (respectively, 13.3±0.82 vs 10.3±0.81 and 11.2±0.65 vs 7.97±0.88 beats.min−1), while no acclimation differences occurred in resting BP (5.14±0.38 vs 5.06±0.56 kPa), baroreflex gain (94.3±10.7 vs 138.7±30.3 %.kPa−1) and rate-pressure product (an index of myocardial activity). Vagal tone exceeded the sympathetic tone of fH especially in the winter group. Therefore, despite the lower fH, winter acclimation does not diminish the fH baroreflex responses nor rate-pressure product possibly because of increased stroke volume that may arise due to heart hypertrophy. Independent of acclimation, fH responded more to hypotension than to hypertension. This should imply that tegus, which have no pressure separation within the single heart ventricle, must have other protection mechanisms against pulmonary hypertension or oedema, presumably through lymphatic drainage and/or vagal vasoconstriction of pulmonary artery. Such a predominant fH reflex response to hypothension, previously observed in anurans, crocodilians and mammals, may be a common feature of tetrapods.

Thermal biology of the toad Rhinella schneideri in a seminatural environment in southeastern Brazil

The toad, Rhinella schneideri, is a large-bodied anuran amphibian with a broad distribution over South America. R. schneideri is known to be active at night during the warm/rainy months and goes into estivation during the dry/cold months; however, there is no data on the range of body temperatures (Tb) experienced by this toad in the field, and how environmental factors, thermoregulatory behaviors or activity influence them. By using implantable temperature dataloggers, we provide an examination of Tb variation during an entire year under a seminatural setting (emulating its natural habitat) monitored with thermosensors. We also used data on preferred Tb, allowing us to express the effectiveness of thermoregulation quantitatively. Paralleling its cycle of activity, R. schneideri exhibited differences in its daily and seasonal profile of Tb variation. During the active season, toads spent daytime hours in shelters and, therefore, did not explore microhabitats with higher thermal quality, such as open areas in the sun. At nighttime, the thermal suitability of microhabitats shifted as exposed microhabitats experienced greater temperature drops than the more insulated shelter. As toads became active at night, they were driven to the more exposed areas and, as a result, thermoregulatory effectiveness decreased. Our results, therefore, indicate that, during the active season, a compromise between thermoregulation and nocturnal activity may be at play. During the estivation period, R. schneideri spent the entire day cycle inside the shelter. As toads did not engage in nocturnal activity in those areas with low thermal quality, the overall effectiveness of thermoregulation was, indeed, elevated. In conclusion, we showed that daily and seasonal variation in Tb of an anuran species is highly associated with their respective pattern of activity and may involve important physiological and ecological compromises.