Kênia C. Bícego

TRPV4 activates autonomic and behavioural warmth‐defence responses in Wistar rats

In this study, we aimed at investigating the involvement of the warmth‐sensitive channel – TRPV4 (in vitro sensitive to temperatures in the range of approx. 24–34 °C) – on the thermoregulatory mechanisms in rats.

Cardiorespiratory Effects of Gap Junction Blockade in the Locus Coeruleus in Unanesthetized Adult Rats

The locus coeruleus (LC) plays an important role in central chemoreception. In young rats (P9 or younger), 85% of LC neurons increase firing rate in response to hypercapnia vs. only about 45% of neurons from rats P10 or older. Carbenoxolone (CARB - gap junction blocker) does not affect the % of LC neurons responding in young rats but it decreases the % responding by half in older animals. We evaluated the participation of gap junctions in the CO2 ventilatory response in unanesthetized adult rats by bilaterally microinjecting CARB (300μM, 1mM or 3mM/100nL), glycyrrhizic acid (GZA, CARB analog, 3mM) or vehicle (aCSF - artificial cerebrospinal fluid) into the LC of Wistar rats. Bilateral gap junction blockade in LC neurons did not affect resting ventilation; however, the increase in ventilation produced by hypercapnia (7% CO2) was reduced by ∼25% after CARB 1mM or 3mM injection (1939.7±104.8mLkg(-1)min(-1) for the aCSF group and 1468.3±122.2mLkg(-1)min(-1) for 1mM CARB, P<0.05; 1939.7±104.8mLkg(-1)min(-1) for the aCSF group and 1540.9±68.4mLkg(-1)min(-1) for the 3mM CARB group, P<0.05) due largely to a decrease in respiratory frequency. GZA injection or CARB injection outside the LC (peri-LC) had no effect on ventilation under any conditions. The results suggest that gap junctions in the LC modulate the hypercapnic ventilatory response of adult rats.

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

For an adequate blood supply to support metabolic demands, vertebrates regulate blood pressure to maintain sufficient perfusion to avoid ischemia and other tissue damage like edema. Using a pharmacological approach (phenylephrine and sodium nitroprusside) we investigated baroreflex sensitivity at 15, 25, and 30°C in toads Rhinella schneideri. Baroreflex sensitivity presented a high thermal dependence (Q10=1.9-4.1), and the HR-baroreflex curve was shifted up and to the right as temperature increased from 15 to 30°C. Baroreflex variables, namely, HR range, gain50 (maximal gain) and normalized gain50 increased 206, 235, and 160% from 15 to 30°C, respectively. The cardiac limb of the baroreflex response to pharmacological treatments was significantly blunted after full autonomic blockade. In addition, there was a clear baroreflex-HR response mainly to hypotension at all three temperatures tested. These findings indicate that toads present temperature dependence for cardiac limb of the barostatic response and the cardiac baroreflex response in R. schneideri is primarily hypotensive rather than hypertensive as well as crocodilians and mammals. Thus, the cardiac baroreflex compensation to changes in arterial pressure might present different patterns among amphibian species, since the previously reported bradycardic compensation to hypertension in some anurans was not observed in the toad used in the present study.

ATP in the locus coeruleus as a modulator of cardiorespiratory control in unanaesthetized male rats

•u2002 What is the central question of this study? Does the ATP‐mediated purinergic signalling within locus coeruleus neurons modulate the cardiorespiratory responses to hypercapnia in unanaesthetized rats? •u2002 What is the main finding and its importance? This study demonstrates that P2X receptor agonist (α,β‐methylene ATP) increases hypercapnia‐induced hyperventilation, and this response is blocked by a non‐selective P2 receptor antagonist (pyridoxalphosphate‐6‐azophenyl‐2′,4′‐disulfonic acid). Until the present study, there was no evidence in the literature of ATP‐mediated respiratory mechanisms in the locus coeruleus region. We added to this scenario the participation of purinergic signalling in the locus coeruleus in the modulation of respiration in unanaesthetized animals.

Thermoregulatory consequences of salt loading in the lizard Pogona vitticeps.

ABSTRACT Previous research has demonstrated that dehydration increases the threshold temperature for panting and decreases the thermal preference of lizards. Conversely, it is unknown whether thermoregulatory responses such as shuttling and gaping are similarly influenced. Shuttling, as an active behavioural response, is considered one of the most effective thermoregulatory behaviours, whereas gaping has been proposed to be involved in preventing brain over-heating in lizards. In this study we examined the effect of salt loading, a proxy for increased plasma osmolality, on shuttling and gaping in Pogona vitticeps. Then, we determined the upper and lower escape ambient temperatures (UETa and LETa), the percentage of time spent gaping, the metabolic rate (V̇O2), the evaporative water loss (EWL) during gaping and non-gaping intervals and the evaporative effectiveness (EWL/V̇O2) of gaping. All experiments were performed under isotonic (154u2005mmolu2005l−1) and hypertonic saline injections (625, 1250 or 2500u2005mmolu2005l−1). Only the highest concentration of hypertonic saline altered the UETa and LETa, but this effect appeared to be the result of diminishing the animals propensity to move, instead of any direct reduction in thermoregulatory set-points. Nevertheless, the percentage of time spent gaping was proportionally reduced according to the saline concentration; V̇O2 was also decreased after salt loading. Thermographic images revealed lower head than body surface temperatures during gaping; however this difference was inhibited after salt loading. Our data suggest that EWL/V̇O2 is raised during gaping, possibly contributing to an increase in heat transfer away from the lizard, and playing a role in head or brain cooling. Summary: Gaping is proposed to be an important evaporative water loss response aimed at preventing overheating of the head and brain in bearded dragons.

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.

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

ABSTRACT Baroreflex regulation of blood pressure 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-u2009and winter-acclimated tegus. In winter lizards, resting and minimum fH were lower than in spring–summer animals (respectively, 13.3±0.82 versus 10.3±0.81 and 11.2±0.65 versus 7.97±0.88 beats min−1), while no acclimation differences occurred in resting blood pressure (5.14±0.38 versus 5.06±0.56 kPa), baroreflex gain (94.3±10.7 versus 138.7±30.3% kPa−1) or 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 or rate-pressure product, possibly because of increased stroke volume that may arise because of 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 hypotension, previously observed in anurans, crocodilians and mammals, may be a common feature of tetrapods. Summary: Winter acclimation is compatible with lower resting heart rate but unchanged baroreflex sensitivity in the lizard Salvator merianae; independent of acclimation, heart rate responds more to hypotension than to hypertension.

Role of brain nitric oxide in the cardiovascular control of bullfrogs

The goal of the present study was to determine if nitric oxide (NO) acting on the brain of bullfrog (Lithobates catesbeianus) is involved in arterial pressure and heart rate (HR) control by influencing sympathetic activity. We investigated the effect of intracerebroventricular injections of L-NMMA (a nonselective NO synthase inhibitor) on mean arterial blood pressure (MAP), HR and cutaneous vascular conductance (CVC) of pelvic skin after intravenous injection of α or β adrenergic blockers, prazosin or sotalol, respectively. Arterial pressure was directly measured by a telemetry sensor inserted in the aortic arch of animals. L-NMMA increased MAP, but did not change HR. This hypertensive response was inhibited by the pre-treatment with prazosin, but accentuated by sotalol. The effect of L-NMMA on MAP was also inhibited by i.v. injections of the ganglionic blocker, hexamethonium. Thus, NO acting on the brain of bullfrog seems to present a hypotensive effect influencing the sympathetic activity dependent on α and β adrenergic receptors in the periphery.

Baroreflex regulation affects ventilation in the Cururu toad Rhinella schneideri

ABSTRACT Anurans regulate short-term oscillations in blood pressure through changes in heart rate (fH), vascular resistance and lymphatic fH. 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 ml kg−1 min−1; 35°C: 351.5±50.2u2005mlu2005kg−1 min−1) and decreased ventilation (25°C: 9.0±6.6 ml kg−1 min−1; 35°C: 50.7±15.6u2005mlu2005kg−1 min−1), respectively, relative to control values from Ringer solution injection (25°C: 78.1±17.0 ml kg−1 min−1; 35°C: 137.7±15.5u2005mlu2005kg−1 min−1). The sensitivity of the ventilatory response to blood pressure changes was higher during hypotension than during hypertension [25°C: −97.6±17.1 versus −23.6±6.0 breaths min−1 kPa−1; 35°C: −141.0±29.5 versus −28.7±6.4 breaths min−1 kPa−1, respectively; negative values indicate an inverse relationship between blood pressure and ventilation (or breathing frequency), i.e. as blood pressure increases, ventilation decreases, and vice versa], while temperature had no effect on these 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 versus 21.6±4.64% kPa−1). Therefore, besides an involvement of pulmonary ventilation 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. Summary: Anurans integrate multiple mechanisms to maintain cardiovascular homeostasis; influences of the baroreflex on lung ventilation in the toad Rhinella schneideri correlate with function of the lymphatic system.

Thermal tachypnea in avian embryos

ABSTRACT Many adult mammals and birds respond to high surrounding temperatures with thermal tachypnea – an increase in breathing frequency accompanied by shallow tidal volume, with minimal increase in oxygen consumption (V̇O2). This pattern favors heat dissipation by evaporative water loss (EWL) through the respiratory tract. We asked to what extent this response was apparent at the earliest stages of development, when pulmonary ventilation initiates. Measurements of pulmonary ventilation (V̇E; barometric technique), V̇O2 (open-flow methodology) and EWL (water scrubbers) were performed on chicken embryos at the earliest appearance of pulmonary ventilation, during the internal pipping stage. Data were collected, first, at the normal incubation temperature (37.5°C); then, ambient and egg temperatures were increased to approximately 44°C over a 2u2005h period. Other embryos of the same developmental stage (controls) were maintained in normothermia for the whole duration of the experiment. During heat exposure, the embryos V̇O2 and carbon dioxide production increased little. In contrast, V̇E more than doubled (∼128% increase), entirely because of the large rise in breathing frequency (∼132% increase), with no change in tidal volume. EWL did not change significantly, probably because, within the egg, the thermal and water vapor gradients are almost nonexistent. We conclude that chicken embryos respond to a major heat load with tachypnea, like many adult mammals and birds do. Its appearance so early in development, although ineffective for heat loss, signifies that thermal tachypnea represents an important breathing response necessary to be functional from hatching. Summary: Chicken embryos during the internal pipping phase (at a time when pulmonary ventilation initiates) are capable of responding to heat exposure by tachypnea.