The authors have declared that no competing interests exist.
Conceived and designed the experiments: WP SPW MRML NRVL. Performed the experiments: WP MRML IATF. Analyzed the data: WP SPW MRML IATF UF ASH CCC NRVL. Contributed reagents/materials/analysis tools: UF. Wrote the manuscript: WP SPW NRVL.
The present study aimed to investigate whether running performance in different environments is dependent on intact arterial baroreceptor reflexes. We also assessed the exercise-induced cardiovascular and thermoregulatory responses in animals lacking arterial baroafferent signals. To accomplish these goals, male Wistar rats were subjected to sinoaortic denervation (SAD) or sham surgery (SHAM) and had a catheter implanted into the ascending aorta to record arterial pressure and a telemetry sensor implanted in the abdominal cavity to record core temperature. After recovering from these surgeries, the animals were subjected to constant- or incremental-speed exercises performed until the voluntary interruption of effort under temperate (25° C) and warm (35° C) conditions. During the constant-speed exercises, the running time until the rats were fatigued was shorter in SAD rats in both environments. Although the core temperature was not significantly different between the groups, tail skin temperature was higher in SAD rats under temperate conditions. The denervated rats also displayed exaggerated increases in blood pressure and double product compared with the SHAM rats; in particular, in the warm environment, these exaggerated cardiovascular responses in the SAD rats persisted until they were fatigued. These SAD-mediated changes occurred in parallel with increased variability in the very low and low components of the systolic arterial pressure power spectrum. The running performance was also affected by SAD during the incremental-speed exercises, with the maximal speed attained being decreased by approximately 20% in both environments. Furthermore, at the maximal power output tolerated during the incremental exercises, the mean arterial pressure, heart rate and double product were exaggerated in the SAD relative to SHAM rats. In conclusion, the chronic absence of the arterial baroafferents accelerates exercise fatigue in temperate and warm environments. Our findings also suggest that an augmented cardiovascular strain accounted for the early interruption of exercise in the SAD rats.
The physical exercise-induced increase in the demand of contracting muscles for oxygen and energetic substrates is a major challenge to body homeostasis and encompasses coordinated responses from the cardiovascular, ventilatory, hormonal, and thermoregulatory systems. To match the higher metabolic demands, landmark physiological responses, such as increases in heart rate (HR), mean arterial pressure (MAP), and the resetting of baroreflexes (which allows simultaneous increases in the HR and MAP), are usually observed immediately after exercise initiation [
The carotid and aortic baroreceptors buffer short-term fluctuations of blood pressure by modulating the brain stem-mediated autonomic outflow to the heart and blood vessels. These baroreceptors are involved in the cardiac and hemodynamic responses to exercise [
Although there is substantial evidence demonstrating the role of the arterial baroreceptors in generating adequate autonomic-cardiovascular responses to exercise, no study has systematically investigated the effects of cardiovascular alterations induced by arterial barodenervation on prolonged physical performance. A theoretical model that has been recently used to explain exercise fatigue suggests that the interaction between an anticipatory feed-forward control and the afferent signals provided by peripheral receptors generates a conscious perception of effort, which regulates skeletal muscle recruitment and, consequently, exercise intensity [
In response to exercises performed in a warm environment, the rates of heat dissipation must be greatly increased to avoid the occurrence of exertional hyperthermia, which may threaten survival. Therefore, aside from the high amounts of oxygenated blood and nutrients that are required in the working skeletal muscles, a higher percentage of the cardiac output is directed to the cutaneous vessels to dissipate the body heat [
Adult male Wistar rats weighing 280-350 g were used in all experiments. The animals were housed in individual cages under controlled light (lights on from 0500 until 1900 hours) and temperature (24 ± 1°C) conditions, with water and rat chow provided
Three sets of experiments were conducted to achieve the goals of the present study. The first set was performed to investigate the impact of SAD on the cardiovascular and thermoregulatory responses during passive heating, an experimental approach in which hyperthermia is a consequence of the passive heat gained from the environment. The rats were subjected to a SAD or sham-denervation surgery (SHAM), and after recovering from these procedures for approximately three weeks, they were familiarized with the experimental setup and then underwent implantation of a temperature sensor in the abdominal cavity and an arterial catheter into the ascending aorta. Each animal was subjected to two experimental trials: exposure to temperate (25° C) and warm (35° C) environments.
The SAD-induced effects on the running performance and cardiovascular and thermoregulatory adjustments during constant-speed exercises (18 m/min, 5% inclination) were evaluated in the second set of experiments. Constant-speed exercises were conducted with the objective of promoting the same power output in both experimental groups. The rats were subjected to a SAD or SHAM surgery, and after recovering from these procedures, they were familiarized with running on a treadmill (five-day protocol) and then underwent implantation of a temperature sensor and an arterial catheter. Each animal was subjected to two exercise trials in the temperate and warm environments.
The third set was performed to determine the SAD effects on the maximal treadmill speed achieved during incremental exercises and to investigate whether the arterial baroreceptors influence cardiovascular responses at the maximal power output tolerated. The rats were initially familiarized with exercising on the treadmill and then subjected to an incremental-speed exercise to evaluate their innate running capacity and divide them into groups. On the following day, the animals were subjected to either a SAD or SHAM surgery. After recovering from these procedures, a second incremental exercise was performed. These two initial incremental exercises were performed at 25° C. Next, the animals underwent implantation of an arterial catheter and were again subjected to incremental exercises under the temperate and warm conditions.
This study was divided into three sets of experiments because the quality of the arterial pressure recording worsened after a few days. Each rat had two days to recover from the implantation of the arterial catheter [
All surgical procedures were performed under ketamine-xylazine anesthesia (90 and 10.5 mg/kg body mass, respectively, i.p.). Moreover, immediately after the surgeries, the rats received an intramuscular prophylactic dose of antibiotics (pentabiotic, 48,000 IU/kg) and a subcutaneous injection of analgesic medication (flunixin meglumine, 1.1 mg/kg).
The sinoaortic denervation was performed according to the method described by Krieger [
At the end of all experimental trials, a venous catheter was implanted into the right jugular vein for the administration of vasoactive drugs with the objective of testing the effectiveness of barodenervation. Baroreflex sensitivity was assessed by examining the cardiac reflex response to increases and decreases in MAP induced by bolus intravenous injections of phenylephrine (1.0-2.5 µg/mL in 0.1 mL of saline) and sodium nitroprusside (2-5 µg/mL in 0.1 mL of saline), respectively. Sinoaortic denervation reduced baroreflex sensitivity to both phenylephrine and sodium nitroprusside by approximately 95% (
Parameters | SHAM ( |
SAD ( |
---|---|---|
Mean arterial pressure (mmHg) | 113 ± 3 | 112 ± 5 |
Heart rate (bpm) | 358 ± 12 | 426 ± 13** |
Δ mean arterial pressure (mmHg) | 36 ± 3 | 41 ± 4 |
Reflex bradycardia (bpm) | -92 ± 12 | -5 ± 2** |
Baroreflex sensitivity (bpm-1∙mmHg) | -2.51 ± 0.14 | -0.12 ± 0.06** |
Δ mean arterial pressure (mmHg) | -26 ± 2 | -46 ± 6* |
Reflex tachycardia (bpm) | 68 ± 7 | 3 ± 2** |
Baroreflex sensitivity (bpm-1∙mmHg) | -2.77 ± 0.32 | -0.10 ± 0.06** |
Values are means ± SEM. *
The rats were gradually encouraged to exercise on a treadmill designed for small animals (Modular Treadmill, Columbus Instruments, OH, USA) by light electrical stimulation (0.5 mA). After resting for 5 min on the treadmill belt, the rats were made to run at a constant speed of 18 m/min at a 5% inclination for 5 min. This familiarization protocol was conducted across five consecutive days [
Following the last familiarization exercise session, a catheter was surgically implanted in the rats for measurement of the pulsatile arterial pressure. A polyethylene catheter (PE-10 connected to a PE-50; Becton Dickinson, Franklin Lakes, NJ, USA), filled with heparin diluted in isotonic saline, was inserted into the left common carotid artery. The free end of the PE-50 tubing was tunneled subcutaneously and exteriorized at the cervical dorsal area [
On the day of the experiments, each rat was weighed, a thermocouple (YSI Inc., Dayton, OH, USA) was fixed to its tail surface, the arterial cannula was connected to a pressure transducer (Biopac Systems, Santa Barbara, CA, USA), and the rat was placed inside an acrylic chamber (49.5 cm long x 14 cm wide x 13.5 cm high). The pressure transducer was coupled to an A/D Data Acquisition System (MP100, Biopac Systems). An electrical fan positioned at one end of the chamber generated an airflow rate of 2.0-2.5 m/min. The animals were allowed to move freely in their home cages for 60 min in a temperate environment (25° C). After Tcore and tail skin temperature (Tskin) values had stabilized, the rats were kept in the temperate environment for an additional 60 min or were passively heated. To heat the environment inside the chamber (35° C), an electrical heater (Britânia model AB 1100; Curitiba, PR, Brazil) was positioned at the same level, 20-30 cm from the fan, and turned on at 1200 W [
The ambient temperature (Ta) inside the treadmill chamber was set at 25 or 35°C. The thermocouple was fixed to the rat’s tail with tape, and the arterial catheter was connected to the pressure transducer. Then, the animals were subjected to treadmill running at a constant speed of 18 m/min and an inclination of 5%. The exercise was performed until the animals were fatigued, which was defined as the point at which the animals were no longer able to keep pace with the treadmill for at least 10 s, even when being stimulated by the light electrical stimuli [
The experimental procedures were similar to those described in the previous section. However, the thermoregulatory parameters were not measured and, instead of running at a constant speed of 18 m/min, the rats were subjected to incremental speed-exercises. During the first 2 min, the rats ran at 10 m/min, followed by increments of 1 m/min every 2 min until they were fatigued [
The intraperitoneal temperature was established as the Tcore index and was measured by telemetry. Tskin was measured using a thermocouple attached to the lateral surface ≈1 cm from the base of the tail. To measure Ta, a thermocouple was fixed to the ceiling of the treadmill chamber. Tcore values were recorded every 10 s, whereas Tskin and Ta inside the treadmill were measured every minute during the experimental trials. The HR, MAP, systolic arterial pressure (SAP), and diastolic arterial pressure (DAP) values were obtained from pulsatile arterial pressure recordings with the AcqKnowledge 3.7.0 software (Biopac Systems). The double product, an index of the myocardial work, was calculated by multiplying SAP by HR. To analyze the lability of Tskin and MAP, we calculated their average deviation values throughout the resting experiments under temperate conditions.
The tape-recorded arterial pressure signal was sampled at 2 kHz. The SAP values were identified beat by beat, and the pulse interval was computed as the interval between two consecutive systolic peaks using a customized routine (MATLAB 7.8, Mathworks, Natick, MA, USA). Time- and frequency-domain analyses were evaluated during the passive heating protocol using a 30-min period selected from continuous recording after the stabilization of the cardiovascular parameters. In the exercising rats, we analyzed the recording during the 6-min period that preceded the interruption of the effort. This shorter period was selected because the SAD rats presented a short running time to fatigue in the heat (only 14 ± 1 min). The power spectral density was obtained by fast Fourier transformation and Hanning windows (512) with 50% overlap. The spectral power components for very low- (VLF, from 0.0195 to 0.25 Hz), low- (LF, from 0.27 to 0.74 Hz), and high-frequency (HF, from 0.76 to 5 Hz) bands were evaluated. These bandwidths were previously used to analyze the spectrum of the blood pressure and HR variability in chronic SAD rats [
The data are expressed as the means ± SEM. The baroreflex sensitivity was compared between experimental groups (SAD vs. SHAM) using unpaired Student’s
Under resting conditions in the temperate environment, the mean values of the MAP were not altered by denervation (
Effects of the sinoaortic denervation (SAD; n = 6) or sham surgery (SHAM; n = 6) on the mean arterial pressure (
The 60 min of heat exposure increased the MAP, Tskin and Tcore in both experimental groups (
The cutaneous heat loss through the tail vessels was not different between the experimental groups (
Temperate (25°C) |
Warm (35°C) |
|||
---|---|---|---|---|
Parameters in freely moving rats | SHAM (n = 6) | SAD (n = 6) | SHAM (n = 6) | SAD (n = 6) |
Systolic pressure (mmHg) | 128 ± 6 | 127 ± 6 | 141 ± 4 | 160 ± 10+* |
S.D. (mmHg) | 4.8 ± 0.4 | 19.1 ± 3.8* | 7.4 ± 1.1+ | 24.7 ± 2.0* |
VLF component (mmHg2) | 7.8 ± 0.6 | 34.1 ± 8.6* | 10.6 ± 2.3 | 46.7 ± 6.3* |
LF component (mmHg2) | 1.9 ± 0.6 | 3.2 ± 0.7 | 5.8 ± 1.1+ | 10.6 ± 1.6+* |
HF component (mmHg2) | 1.3 ± 0.2 | 1.9 ± 0.1 | 2.6 ± 0.8 | 12.6 ± 6.7+* |
Pulse interval (ms) | 175 ± 11 | 134 ± 5* | 151 ± 5 | 122 ± 6* |
S.D. (ms) | 6.5 ± 1.0 | 5.9 ± 0.6 | 10.1 ± 1.3+ | 7.4 ± 1.0 |
VLF component (ms2) | 8.4 ± 2.0 | 1.2 ± 0.3* | 5.4 ± 1.4 | 2.2 ± 0.4* |
LF component (ms2) | 1.4 ± 0.3 | 0.5 ± 0.2* | 1.3 ± 0.3 | 0.7 ± 0.1 |
HF component (ms2) | 8.0 ± 1.4 | 5.1 ± 0.6 | 11.7 ± 1.7 | 9.3 ± 0.6 |
These parameters were calculated in rats resting in a temperate environment (25° C) or exposed to heat (35° C).
S.D. = standard deviation; VLF = very low frequency; LF = low frequency; HF = high frequency
Values are means ± SEM
*
+ P < 0.05 compared with the temperate environment (for the same experimental group).
As shown in
Effects of the sinoaortic denervation (SAD; n = 8) or sham surgery (SHAM; n = 8) on running time until the voluntary interruption of the effort (A and B) during constant-velocity exercises (18 m/min). The exercises were performed in temperate (25° C) and warm (35° C) environments. The values represent the means ± SEM. *
During the constant-speed exercise at 25° C, SAD enhanced the exercise-induced increases in MAP (142 ± 3 mmHg vs. 123 ± 2 mmHg;
Temporal profile of the exercise-induced changes in the MAP (
Regarding the thermoregulatory responses at 25° C, Tskin was significantly higher in the SAD rats compared with the SHAM rats from the 12th to the 20th min of exercise (
Temporal profile of exercise-induced changes in the tail skin temperature (
As shown in
Temperate (25°C) |
Warm (35°C) |
|||
---|---|---|---|---|
Parameters in running rats | SHAM (n = 8) | SAD (n = 8) | SHAM (n = 8) | SAD (n = 8) |
Systolic pressure (mmHg) | 142 ± 2 | 138 ± 6 | 178 ± 6+ | 190 ± 10+ |
S.D. (mmHg) | 4.1 ± 0.2 | 8.2 ± 1.0* | 8.1 ± 0.7+ | 14.8 ± 1.1+* |
VLF component (mmHg2) | 2.7 ± 0.4 | 20.3 ± 4.1* | 5.9 ± 0.8+ | 26.9 ± 6.2* |
LF component (mmHg2) | 3.9 ± 0.6 | 7.3 ± 1.2* | 5.9 ± 1.1 | 8.6 ± 1.5 |
HF component (mmHg2) | 7.2 ± 1.2 | 11.8 ± 3.3 | 15.1 ± 3.3+ | 17.0 ± 4.3+ |
Pulse interval (ms) | 124 ± 3 | 110 ± 4* | 103 ± 4+ | 107 ± 4 |
S.D. (ms) | 4.8 ± 0.2 | 5.6 ± 0.3 | 5.6 ± 0.7 | 7.0 ± 1.2 |
VLF component (ms2) | 0.7 ± 0.2 | 1.0 ± 0.3 | 0.6 ± 0.2 | 0.4 ± 0.2 |
LF component (ms2) | 0.6 ± 0.2 | 0.8 ± 0.1 | 0.3 ± 0.1 | 0.5 ± 0.2 |
HF component (ms2) | 16.3 ± 3.8 | 22.3 ± 2.8 | 12.6 ± 3.1 | 18.6 ± 4.0 |
The parameters were calculated in rats subjected to constant-velocity exercises in temperate (25° C) and warm (35° C) environments.
S.D. = standard deviation; VLF = very low frequency; LF = low frequency; HF = high frequency
Values are means ± SEM
*
+ P < 0.05 compared with the temperate environment (for the same experimental group).
SAD significantly reduced the maximal speed achieved at 25° C by approximately 20% (25 ± 1 m/min after SAD vs. 31 ± 1 m/min before SAD;
Maximal treadmill speed (m/min) |
||
---|---|---|
Experimental conditions | SHAM (n = 6) | SAD (n = 6) |
Before SAD or SHAM surgery at 25°C | 30 ± 1 | 31 ± 1 |
After SAD or SHAM surgery at 25°C | 30 ± 1 | 25 ± 1* |
After arterial and vein cannulations | 29 ± 2 | 24 ± 1* |
Values represent the means ± SEM. After the rats had recovered from the SAD or SHAM surgery, the body masses of the SAD and SHAM rats were 311 ± 14 and 329 ± 13 g, respectively. *
Effects of sinoaortic denervation (SAD; n = 6) or sham surgery (SHAM; n = 6) on the maximal speed achieved by rats during the incremental-velocity exercises. The exercises were performed in temperate (25° C) and warm (35° C) environments. The values are means ± SEM. +
Representative recordings of the MAP and HR of rats during the incremental-speed exercises in temperate (25° C) and warm (35° C) environments. The recordings were obtained from rats that were previously subjected to the SHAM or SAD surgery.
Changes in the systolic arterial pressure (
At 25° C, the increase in the HR was more pronounced in the SAD animals from 50% of the maximal speed achieved during the incremental exercise to the maximal speed (maximal HR: 593 ± 5 bpm for SAD rats vs. 533 ± 13 bpm for SHAM rats;
In summary, our results demonstrated that SAD rats subjected to treadmill running presented: 1) accelerated fatigue during constant- and incremental-speed exercises in both temperate and warm environments; 2) exaggerated cardiovascular responses (MAP, HR and double product) for given absolute or relative exercise intensities and at the maximal power output tolerated; 3) increased cutaneous heat loss during the constant-speed exercise at 25° C; and 4) increased SAP variability without changes in the HR variability.
A novel finding of this study was that SAD shortened the running time until the voluntary interruption of effort by 20 to 56% in all four experimental conditions studied (
We hypothesized that SAD would produce greater impacts on running performance and cardiovascular regulation in the warm compared with the temperate environment. In contrast to our hypothesis, there was no interaction between Ta and the SAD-mediated decrease in running time, regardless of the exercise protocol. Moreover, in both environments, the SAD-induced effects on the cardiovascular parameters occurred in the same direction; specifically, the increases in MAP, HR, and the double product were always enhanced in denervated rats. However, at 35° C, these cardiovascular differences between groups persisted until the interruption of exercise, even during the incremental-speed protocol. These long-lasting cardiovascular differences in the heat may be due to the larger blood flow supply to the cutaneous vessels, which would increase the strain on the heart and other organs involved in blood pressure regulation.
The present experiments do not allow us to precisely describe the mechanisms underlying the increased cardiovascular responses in the SAD rats. It is reasonable that a higher sympathetic outflow caused by less baroreflex inhibition may have enhanced the visceral vasoconstriction, increasing the peripheral resistance and blood pressure in both exercise protocols. Corroborating this hypothesis, a previous study reported augmented plasma norepinephrine concentrations, as well as mesenteric and renal vascular resistances, in SAD rats exposed to passive heating [
Another mechanism that may underlie the augmented pressor response of the SAD rats is a more pronounced secretion of vasopressin, as evidenced by a previous report showing that SAD rats subjected to chronic stress exhibit increased plasma concentrations of vasopressin [
The surgical withdrawal of the baroafferents increased the maximal HR achieved during incremental exercises (
The rate of increase in Tcore during the constant-speed exercises was not modified by SAD. Interestingly, at the end of effort in the heat, the SAD rats exhibited Tcore values 1° C lower than those of SHAM rats (
During the submaximal, constant-speed exercises, the denervation increased the SAP variability and the power spectral density in the VLF and LF ranges, without affecting the HR variability (
It is important to note that our sinoaortic denervation procedure disrupted the afferent signals from the arterial baroreceptors and peripheral chemoreceptors. These chemoreceptors are the major oxygen sensors, and their stimulation increases the sympathetic vasoconstriction outflow to several vascular beds, including the skeletal muscles and visceral vessels, in rats [
In conclusion, the running performance is dependent on intact arterial baroreflexes in both temperate and warm environments. In the absence of the afferent signaling from the arterial baroreceptors, the rats exhibited exaggerated increases in the HR and double product while they were running, including at the maximal power output tolerated. These findings suggest that an enhanced cardiovascular strain caused by the barodenervation at least partially accounts for the early exercise interruption in the SAD rats.
The present results suggest that an impaired baroreflex and the consequent exaggerated cardiovascular strain may contribute to the lower aerobic performance in patients with diabetes, metabolic syndrome and hypertension. However, caution must be exercised before applying our findings in laboratory rats to human subjects because the two species exhibit different thermoregulatory and cardiovascular responses during passive heat exposure and during physical exercise in warm conditions. For example, humans have a greater density of eccrine sweat glands and consequently have a greater ability to dissipate heat by evaporative means while exercising [
In the present study, fatigue was accelerated in the SAD rats during constant and incremental-speed exercises. This early interruption of effort was associated with exacerbated increases in the MAP, HR, and VLF and LF components of SAP variability. Despite the above-mentioned interspecies differences in cardiovascular adjustments, there is evidence that enhanced cardiovascular strain also impairs human performance during prolonged exercise in the heat [
Another relevant limitation for translating the present findings to human physiology is the fact that the rats were encouraged to run on the treadmill by light electrical stimulation (0.5 mA). We did not measure any objective parameters to ensure that the electrical stimulation similarly motivated the SAD and SHAM rats to maintain physical efforts. Although we cannot rule out that an interaction between the electrical stimulation and SAD may have confounded our results, previous studies observed similar changes in physical performance induced by increased central dopamine availability, regardless of whether the rats were subjected to treadmill running with [