Vascular reactivity and baroreflex function during hyperthermia in conscious rats

Infrared light elicits endothelium-dependent vasodilation in isolated occipital arteries of the rat via soluble guanylyl cyclase-dependent mechanisms

The left and right occipital arteries provide blood supply to afferent cell bodies in the ipsilateral nodose and petrosal ganglia. This supply is free of an effective blood-ganglion barrier, so changes in occipital artery blood flow directly affect the access of circulating factors to the afferent cell bodies. The application of infrared (IR) light to modulate neural and other cell processes has yielded information about basic biological processes within tissues and is gaining traction as a potential therapy for a variety of disease processes. To address whether IR can directly modulate vascular function, we performed wire myography studies to determine the actions of IR on occipital arteries isolated from male Sprague-Dawley rats. Based on our previous research that functionally-important differences exist between occipital artery segments close to their origin at the external carotid artery (ECA) and those closer to the nodose ganglion, the occipital arteries were dissected into two segments, one closer to the ECA and the other closer to the nodose ganglion. Segments were constricted with 5-hydroxytryptamine to a level equal to 50% of the maximal response generated by the application of a high (80 mM) concentration of K+ ions. The direct application of pulsed IR (1,460 nm) for 5 s produced a rapid vasodilation in occipital arteries that was significantly more pronounced in segments closest to the ECA, although the ECA itself was minimally responsive. The vasodilation remained for a substantial time (at least 120 s) after cessation of IR application. The vasodilation during and following cessation of the IR application was markedly diminished in occipital arteries denuded of the endothelium. In addition, the vasodilation elicited by IR in endothelium-intact occipital arteries was substantially reduced in the presence of a selective inhibitor of the nitric oxide-sensitive guanylate cyclase, 1H-[1,2,4]oxadiazolo [4,3-a]quinoxalin-1-one (ODQ). It appears that IR causes endothelium-dependent, nitric-oxide-mediated vasodilation in the occipital arteries of the rat. The ability of IR to generate rapid and sustained vasodilation may provide new therapeutic approaches for restoring or improving blood flow to targeted tissues.

Effect of thermal preconditioning on Hsp70 expression in the medulla oblongata and on hemodynamics during passive hyperthermia

A short-term episode of elevated core body temperature that induces Hsp70 expression (thermal preconditioning) may protect against heatstroke during subsequent hyperthermia. The protective effects of thermal preconditioning may involve several cellular and immunological mechanisms and improvements in baroreflex sensitivity. To substantiate the hypothesis that the protective effect of thermal preconditioning also occurs in conditions with intact thermoregulation, we examined the evolution of spontaneous cardiovagal baroreflex sensitivity and the protective effect of Hsp70 expression after thermal preconditioning in nonanesthetized Wistar-Kyoto rats with implanted telemetric transmitters. In the baroreflex centers of the medulla oblongata, thermal preconditioning induced Hsp70 in perineuronal and perivascular oligodendrocytes, microglia, and endothelial cells but not in neurons. The maximal Hsp70 expression was detected 4 h after preconditioning, but a significant number of Hsp70-positive cells was still present 72 h after preconditioning. Increased c-Fos expression in the neurons of baroreflex centers was detectable only 4 h after preconditioning. The mean values of cardiovagal baroreflex sensitivity did not show significant differences during the 72-hour follow-up period after thermal preconditioning. Similarly, cardiovascular variability measures of the autonomic nervous system activity were also not significantly affected by thermal preconditioning. During passive hyperthermia, thermal preconditioning had no statistically significant effect on thermoregulation and the onset of arterial pressure decline. Our data suggest that thermal preconditioning induces a glial type of Hsp70 expression in the baroreflex centers of the medulla oblongata. However, this response was not associated with cardiovagal baroreflex sensitization and protection against hemodynamic instability during passive hyperthermia.

Biphasic changes in spontaneous cardiovagal baroreflex sensitivity during passive hyperthermia

Successful adaptation to passive hyperthermia requires continual adjustment of circulation, which is mediated mainly by the autonomic nervous system. The goal of this study was to explore the alterations in spontaneous cardiovagal baroreflex sensitivity (BRS) during exposure to a hot environment. To continuously follow changes in core body temperature (Tc), haemodynamics, and BRS, male Wistar-Kyoto rats were implanted with telemetric transmitters. BRS at an ambient temperature of 23 °C was not steady but oscillated with a maximum power in the range of 0.02–0.2 Hz. Exposure to hot air immediately shifted the distribution of BRS to higher values, although Tc remained unchanged (37.2 (0.3) °C), and the average BRS changed from 1.3 (0.3) to 3 (1.4) ms.mmHg⁻¹, p < 0.0001. The degree of initial cardiovagal baroreflex sensitization explained 57% of the variability in the time to the onset of arterial pressure decline (p = 0.0114). With an increasing Tc (>38.8 (0.6) °C), BRS non-linearly declined, but haemodynamic parameters remained stable even above a Tc of 42 °C when the cardiovagal baroreflex was virtually non-operative. Abrupt full desensitization of the cardiovagal baroreflex with a muscarinic blocker did not induce arterial pressure decline. Our data indicate that a progressive decrease in BRS during passive hyperthermia does not induce haemodynamic instability. The positive association between initial cardiovagal baroreflex sensitization and the time to the onset of arterial pressure decline may reflect the potential protective role of parasympathetic activation during exposure to a hot environment.

Whole body heat stress attenuates the pressure response to muscle metaboreceptor stimulation in humans

The effects of whole body heat stress on sympathetic and cardiovascular responses to stimulation of muscle metaboreceptors and mechanoreceptors remains unclear. We examined the muscle sympathetic nerve activity (MSNA), blood pressure, and heart rate in 14 young healthy subjects during fatiguing isometric handgrip exercise, postexercise circulatory occlusion (PECO), and passive muscle stretch during PECO. The protocol was performed under normothermic and whole body heat stress (increase internal temperature ~0.6°C via a heating suit) conditions. Heat stress increased the resting MSNA and heart rate. Heat stress did not alter the mean blood pressure (MAP), heart rate, and MSNA responses (i.e., changes) to fatiguing exercise. During PECO, whole body heat stress accentuated the heart rate response [change (Δ) of 5.8 ± 1.5 to Δ10.0 ± 2.1 beats/min, P = 0.03], did not alter the MSNA response (Δ16.4 ± 2.8 to Δ17.3 ± 3.8 bursts/min, P = 0.74), and lowered the MAP response (Δ20 ± 2 to Δ12 ± 1 mmHg, P < 0.001). Under normothermic conditions, passive stretch during PECO evoked significant increases in MAP and MSNA (both P < 0.001). Of note, heat stress prevented the MAP and MSNA responses to stretch during PECO (both P > 0.05). These data suggest that whole body heat stress attenuates the pressor response due to metaboreceptor stimulation, and the sympathetic nerve response due to mechanoreceptor stimulation.

Human cardiovascular responses to passive heat stress

Heat stress increases human morbidity and mortality compared to normothermic conditions. Many occupations, disease states, as well as stages of life are especially vulnerable to the stress imposed on the cardiovascular system during exposure to hot ambient conditions. This review focuses on the cardiovascular responses to heat stress that are necessary for heat dissipation. To accomplish this regulatory feat requires complex autonomic nervous system control of the heart and various vascular beds. For example, during heat stress cardiac output increases up to twofold, by increases in heart rate and an active maintenance of stroke volume via increases in inotropy in the presence of decreases in cardiac preload. Baroreflexes retain the ability to regulate blood pressure in many, but not all, heat stress conditions. Central hypovolemia is another cardiovascular challenge brought about by heat stress, which if added to a subsequent central volumetric stress, such as hemorrhage, can be problematic and potentially dangerous, as syncope and cardiovascular collapse may ensue. These combined stresses can compromise blood flow and oxygenation to important tissues such as the brain. It is notable that this compromised condition can occur at cardiac outputs that are adequate during normothermic conditions but are inadequate in heat because of the increased systemic vascular conductance associated with cutaneous vasodilation. Understanding the mechanisms within this complex regulatory system will allow for the development of treatment recommendations and countermeasures to reduce risks during the ever-increasing frequency of severe heat events that are predicted to occur.

Chronic fetal exposure to caffeine altered resistance vessel functions via RyRs-BKCa down-regulation in rat offspring

Caffeine modifies vascular/cardiac contractility. Embryonic exposure to caffeine altered cardiac functions in offspring. This study determined chronic influence of prenatal caffeine on vessel functions in offspring. Pregnant Sprague-Dawley rats (5-month-old) were exposed to high dose of caffeine, their offspring (5-month-old) were tested for vascular functions in mesenteric arteries (MA) and ion channel activities in smooth muscle cells. Prenatal exposure to caffeine increased pressor responses and vasoconstrictions to phenylephrine, accompanied by enhanced membrane depolarization. Large conductance Ca2⁺-activated K⁺ (BK_Ca) channels in buffering phenylephrine-induced vasoconstrictions was decreased, whole cell BK_Ca currents and spontaneous transient outward currents (STOCs) were decreased. Single channel recordings revealed reduced voltage/Ca²⁺ sensitivity of BK_Ca channels. BK_Ca α-subunit expression was unchanged, BK_Ca β1-subunit and sensitivity of BK_Ca to tamoxifen were reduced in the caffeine offspring as altered biophysical properties of BK_Ca in the MA. Simultaneous [Ca²⁺]_i fluorescence and vasoconstriction testing showed reduced Ca²⁺, leading to diminished BK_Ca activation via ryanodine receptor Ca²⁺ release channels (RyRs), causing enhanced vascular tone. Reduced RyR1 was greater than that of RyR3. The results suggest that the altered STOCs activity in the caffeine offspring could attribute to down-regulation of RyRs-BK_Ca, providing new information for further understanding increased risks of hypertension in developmental origins.

Cardiovascular and thermoregulatory dysregulation over 24 h following acute heat stress in rats

The influences of severe heat stroke (HS) on cardiovascular function during recovery are incompletely understood. We hypothesized that HS would elicit a heart rate (HR) increase persisting through 24 h of recovery due to hemodynamic, thermoregulatory, and inflammatory events, necessitating tachycardia to support mean arterial pressure (MAP). Core temperature (Tc), HR, and MAP were measured via radiotelemetry in conscious male Fischer 344 rats (n = 22; 282.4 ± 3.5 g) during exposure to 37°C ambient temperature until a maximum Tc of 42.0°C, and during recovery at 20°C ambient temperature through 24 h. Rats were divided into Mild, Moderate, and Severe groups based on pathophysiology. HS rats exhibited hysteresis relative to Tc with HR higher for a given Tc during recovery compared with heating (P < 0.0001). "Reverse" hysteresis occurred in MAP with pressure during cooling lower than heating per degree Tc (P < 0.0001). Mild HS rats showed tachycardia [P < 0.01 vs. control (Con)] through 8 h of recovery, elevated MAP (P < 0.05 vs. Con) for the initial 5 h of recovery, with sustained hyperthermia (P < 0.05 vs. Con) through 24 h. Moderate HS rats showed significant tachycardia (P < 0.01 vs. Con), normal MAP (P > 0.05 vs. Con), and rebound hyperthermia from 4 to 24 h post-HS (P < 0.05 vs. Con). Severe HS rats showed tachycardia (P < 0.05 vs. Con), hypotension (P < 0.01 vs. Con), and hypothermia for 24 h (P < 0.05 vs. Con). Severe HS rats showed 14- and 12-fold increase in heart and liver inducible nitric oxide synthase expression, respectively. Hypotension and hypothermia in Severe HS rats was consistent with inducible nitric oxide synthase-mediated systemic vasodilation. These findings provide mechanistic insight into hemodynamic and thermoregulatory impairments during 24 h of HS recovery.

Cardiovascular and thermoregulatory biomarkers of heat stroke severity in a conscious rat model

Multiorgan failure is a catastrophic consequence of heat stroke (HS) and considered the underlying etiology of mortality. Identifying novel biomarkers capable of predicting the extent of HS-induced organ damage will enhance point-of-care triage and treatment. Conscious male F344 rats ( n = 32) were radiotelemetered for continuous core temperature (Tc), heart rate, and arterial pressure measurement. Twenty-two animals were exposed to ambient temperature of 37°C to a maximum Tc of 41.9 ± 0.1°C. Rats were euthanized at 24 h of recovery for analysis of plasma biomarkers [cardiac troponin I (cTnI), blood urea nitrogen (BUN), alanine aminotransferase (ALT), albumin, glucose] and histology. Tc profiles observed during recovery stratified HS severity into Mild, Moderate, and Severe. Eleven (50%) animals exhibited an acute compensatory hemodynamic response to heat exposure and a monophasic Tc profile consisting of sustained hyperthermia (∼1°C). Five (23%) rats displayed hemodynamic challenge and a biphasic Tc profile with rapid return to baseline followed by rebound hyperthermia. All biomarkers were significantly altered from control values ( P < 0.05). Four (18%) animals exhibited significant hemodynamic compromise during heat and a triphasic profile characterized by rapid cooling to baseline Tc, rebound hyperthermia, and subsequent hypothermia (∼35°C) through 24 h. cTnI showed a 40-fold increase over CON ( P < 0.001) and correlated with BUN ( r = 0.912) consistent with cardiorenal failure. Hypoglycemia correlated with ALT ( r = 0.824) suggestive of liver dysfunction. Histology demonstrated myocardial infarction, renal tubular necrosis, and acute liver necrosis. Two (9%) animals succumbed during HS recovery. This study identified novel biomarkers that predict HS severity and organ damage during acute recovery that could provide clinical significance for identifying key biomarkers of HS pathogenesis.

Heat and α1-adrenergic responsiveness in human skeletal muscle feed arteries: the role of nitric oxide

Increased local temperature exerts a sympatholytic effect on human skeletal muscle feed arteries. We hypothesized that this attenuated α(1)-adrenergic receptor responsiveness may be due to a temperature-induced increase in nitric oxide (NO) bioavailability, thereby reducing the impact of the α(1)-adrenergic receptor agonist phenylephrine (PE). Thirteen human skeletal muscle feed arteries were harvested, and wire myography was used to generate PE concentration-response curves at 37 °C and 39 °C, with and without the NO synthase (NOS) inhibitor N(G)-monomethyl-L-arginine (L-NMMA). A subset of arteries (n = 4) were exposed to 37 °C or 39 °C, and the protein content of endothelial NOS (eNOS) and α(1)-adrenergic receptors was determined by Western blot analysis. Additionally, cultured bovine endothelial cells were exposed to static or shear stress conditions at 37 °C and 39 °C and assayed for eNOS activation (phosphorylation at Ser(1177)), eNOS expression, and NO metabolites [nitrate + nitrite (NOx)]. Maximal PE-induced vasocontraction (PE(max)) was lower at 39 °C than at 37 °C [39 ± 10 vs. 84 ± 30% maximal response to 100 mM KCl (KCl(max))]. NO blockade restored vasocontraction at 39 °C to that achieved at 37 °C (80 ± 26% KCl(max)). Western blot analysis of the feed arteries revealed that heating increased eNOS protein, but not α(1)-adrenergic receptors. Heating of bovine endothelial cells resulted in greater shear stress-induced eNOS activation and NOx production. Together, these data reveal for the first time that, in human skeletal muscle feed arteries, NO blockade can restore the heat-attenuated α(1)-adrenergic receptor-mediated vasocontraction and implicate endothelium-derived NO bioavailability as a major contributor to heat-induced sympatholysis. Consequently, these findings highlight the important role of vasodilators in modulating the vascular response to vasoconstrictors.

Anandamide modulates carotid sinus nerve afferent activity via TRPV1 receptors increasing responses to heat

Abnormal respiratory chemosensitivity is implicated in recurrent apnea syndromes, with the peripheral chemoreceptors, the carotid bodies, playing a particularly important role. Previous work suggests that supraphysiological concentrations of the endocannabinoid endovanilloid and TASK channel blocker anandamide (ANA) excite carotid bodies, but the mechanism(s) and physiological significance are unknown. Given that carotid body output is temperature-sensitive, we hypothesized that ANA stimulates carotid body chemosensory afferents via temperature-sensitive vanilloid (TRPV1) receptors. To test this hypothesis, we used the dual-perfused in situ rat preparation to confirm that independent perfusion of carotid arteries with supraphysiological concentrations of ANA strongly excites carotid sinus nerve afferents and that this activity is sufficient to increase phrenic activity. Next, using ex vivo carotid body preparations, we demonstrate that these effects are mediated by TRPV1 receptors, not CB1 receptors or TASK channels: in CB1-null mouse preparations, ANA increased afferent activity across all levels of Po(2), whereas in TRPV1-null mouse preparations, the stimulatory effect of ANA was absent. In rat ex vivo preparations, ANA's stimulatory effects were mimicked by olvanil, a nonpungent TRPV1 agonist, and suppressed by the TRPV1 antagonist AMG-9810. The specific CB1 agonist oleamide had no effect. Physiological levels of ANA had no effect alone but increased sensitivity to mild hyperthermia. AMG-9810 blocked ANA's effect on the temperature response. Immunolabeling and RT-PCR demonstrated that TRPV1 receptors are not expressed in carotid body glomus cells but reside in petrosal sensory afferents. Together, these results suggest that ANA plays a physiological role in augmenting afferent responses to mild hyperthermia by activating TRPV1 receptors on petrosal afferents.

Muscle sympathetic responses during orthostasis in heat-stressed individuals

PURPOSE

Whole-body heat stress compromises the control of blood pressure during an orthostatic challenge, although the extent to which this occurs can vary greatly between individuals. The mechanism(s) responsible for these varying responses remain unclear. This study tested the hypothesis that the individuals who are best able to tolerate an orthostatic challenge while heat stressed are the ones with the largest increase in sympathetic activity during orthostasis, indexed from recordings of muscle sympathetic nerve activity (MSNA).

METHODS

MSNA, arterial blood pressure, and heart rate were recorded from 11 healthy volunteers throughout passive whole-body heating and during 15 min of 60° head-up tilt (HUT) or until the onset of pre-syncopal symptoms.

RESULTS

Whole-body heating significantly increased core temperature (~0.9°C), supine heart rate and MSNA. Eight of 11 subjects developed pre-syncopal symptoms resulting in early termination of HUT. The HUT tolerance time was positively correlated (R = 0.82, P = 0.01) with the increase in MSNA by HUT.

CONCLUSION

These data suggest that the individuals with the largest increase in MSNA during upright tilt have the greatest capacity to withstand the orthostatic challenge while heat stressed.

Human skeletal muscle feed arteries studied in vitro: the effect of temperature on α(1)-adrenergic responsiveness

Heat and cold exposure can decrease and increase total peripheral resistance, respectively, in humans. With unique access to human skeletal muscle feed arteries, we sought both to characterize these vessels and to determine the interaction between temperature and α(1)-adrenergic receptor responsiveness. We hypothesized that α(1)-mediated vasocontraction of human feed arteries would be attenuated in response to 39 or 35°C. Skeletal muscle feed arteries were harvested from thirty-two human volunteers and studied using isometric techniques. Vessel function was assessed using KCl, sodium nitroprusside (SNP), phenylephrine (PE) and ACh dose-response curves to characterize non-receptor- and receptor-mediated vasocontraction and vasorelaxation. Single doses of PE (1 mm) and KCl (100 mm) were administered at 37°C and then, in a balanced design, repeated at both 35 and 39°C. The KCl and PE dose-response curves elicited significant vasocontraction (2009 ± 407 and 1974 ± 508 mg developed tension, respectively), whereas SNP and ACh induced the expected vasorelaxation (102 ± 6 and 73 ± 10% relaxation, respectively). Altering the temperature had no effect on inherent smooth muscle function (KCl response), but both a reduction (35°C) and an increase in temperature (39°C) decreased the vasocontractile response to 1 mm PE (37°C, 1478 ± 338 mg; 35°C, 546 ± 104 mg; and 39°C, 896 ± 202 mg; P < 0.05) or across PE dose (P < 0.05, 35 and 39 versus 37°C). Despite clear heterogeneity between both the human volunteers and the feed arteries themselves, this novel approach to the procurement of human vessels revealed a robust 'inverted U' response to altered temperature, such that α(1)-mediated vasocontraction was attenuated with either warming or cooling.

α-Adrenergic vasoconstrictor responsiveness is preserved in the heated human leg

This study tested the hypothesis that passive leg heating attenuates α-adrenergic vasoconstriction within that limb. Femoral blood flow (FBF, femoral artery ultrasound Doppler) and femoral vascular conductance (FVC, FBF/mean arterial blood pressure), as well as calf muscle blood flow (CalfBF, ¹³³xenon) and calf vascular conductance (CalfVC) were measured during intra-arterial infusion of an α₁-adrenoreceptor agonist, phenylephrine (PE, 0.025 to 0.8 μg kg₋₁ min₋₁) and an α₂-adrenoreceptor agonist, BHT-933 (1.0 to 10 μg kg₋₁ min₋₁) during normothermia and passive leg heating (water-perfused pant leg). Passive leg heating (∼46◦C water temperature) increased FVC from 4.5 ± 0.5 to 11.9 ± 1.3 ml min₋₁ mmHg₋₁ (P < 0.001). Interestingly, CalfBF (1.8±0.2 vs. 2.8±0.3mlmin₋₁ (100 g)₋₁) and CalfVC (2.0±0.3 vs. 3.9±0.5mlmin₋₁ (100 g)₋₁ mmHg₋₁ ×100) were also increased by this perturbation (P <0.05 for both). Infusion of PE and BHT-933 resulted in greater absolute decreases in FVC during leg heating compared to normothermic conditions (maximal decreases in FVC during heating vs. normothermia: PE: 7.8 ± 1.1 vs. 2.8 ± 0.5 ml min₋₁ mmHg₋₁; BHT-933: 8.6 ± 1.7 vs. 2.1 ± 0.4 ml min₋₁ mmHg₋₁; P < 0.01 for both). However, the nadir FVC during drug infusion was higher during passive leg heating compared to normothermic conditions (FVC at highest dose of respective drugs during heating vs. normothermic conditions: PE: 3.7 ± 0.4 vs. 2.0 ± 0.3 ml min₋₁ mmHg₋₁; BHT-933: 3.8 ± 0.2 vs. 2.1 ± 0.3 ml min₋₁ mmHg₋₁; P < 0.001 for both). Leg heating did not alter the responsiveness of CalfBF or CalfVC to either PE or BHT-933. Taken together, these observations suggest that local heating does not decrease α-adrenergic responsiveness.However, heat-induced vasodilatation opposes α-adrenergic vasoconstriction. Furthermore, passive heating of a limb causes not only an increase in skin blood flow but also in muscle blood flow.

Heat stress attenuates the increase in arterial blood pressure during the cold pressor test

The mechanisms by which heat stress impairs the control of blood pressure leading to compromised orthostatic tolerance are not thoroughly understood. A possible mechanism may be an attenuated blood pressure response to a given increase in sympathetic activity. This study tested the hypothesis that whole body heating attenuates the blood pressure response to a non-baroreflex-mediated sympathoexcitatory stimulus. Ten healthy subjects were instrumented for the measurement of integrated muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP), heart rate, sweat rate, and forearm skin blood flow. Subjects were exposed to a cold pressor test (CPT) by immersing a hand in an ice water slurry for 3 min while otherwise normothermic and while heat stressed (i.e., increase core temperature ~0.7°C via water-perfused suit). Mean responses from the final minute of the CPT were evaluated. In both thermal conditions CPT induced significant increases in MSNA and MAP without altering heart rate. Although the increase in MSNA to the CPT was similar between thermal conditions (normothermia: Δ14.0 ± 2.6; heat stress: Δ19.1 ± 2.6 bursts/min; P = 0.09), the accompanying increase in MAP was attenuated when subjects were heat stressed (normothermia: Δ25.6 ± 2.3, heat stress: Δ13.4 ± 3.0 mmHg; P < 0.001). The results demonstrate that heat stress can attenuate the pressor response to a sympathoexcitatory stimulus.

Heat stress and baroreflex regulation of blood pressure

In healthy, noninjured, individuals, passive (i.e., nonexercising) whole-body heating has the potential to cause significant cardiovascular stress that may be second only to the cardiovascular stress associated with exercise. For example, such a heat stress can increase heart rate to well over 100 beats min(-1) with cardiac output increasing upward to 13 L min(-1). This increase in cardiac output is necessary to maintain blood pressure due to profound reductions in total vascular conductance associated with cutaneous vasodilation. These responses are accompanied with elevations in sympathetic activity and reductions in vascular conductance (i.e., increased vascular resistance) from noncutaneous beds. While heat-stressed, blood pressure control is compromised resulting in orthostatic intolerance. A plausible explanation for such an event is that heat stress impairs baroreflex responsiveness perhaps due to the reduced range by which baroreflexes can increase heart rate, cardiac output, sympathetic activity, and vascular resistance during a hypotensive challenge. Given that dynamic exercise has the potential to cause large increases in internal temperature, possibly a component of the response to exercise, with respect to baroreflex control of blood pressure, may be affected by the thermal load during the exercise bout. Within this context, the purpose of this review was to summarize findings investigating the effects of heat stress on baroreflex regulation of blood pressure.

Effects of a manually assisted mechanical force on cutaneous temperature

OBJECTIVE

Digitized infrared segmental thermometry (DIST) is a tool used for measuring cutaneous temperature (CT). This project ascertains the effect of a manually assisted mechanical force producing a chiropractic adjustment in the lumbar spine after the Activator Methods Chiropractic Technique on CT during 2 different time recording periods (TRPs).

METHODS

Sixty-six healthy subjects (36 women and 30 men) without acute low back conditions or symptoms were recruited. Subjects were randomly divided into 2 groups based on the length of the acclimatization period (8 or 30 minutes; TRP(8) and TRP(30), respectively). In turn, each recording period group was divided into 3 subgroups (n = 11 per subgroup): treatment, sham, and control subgroups. Bilateral DIST was conducted at L-4 (TRP(30)) and L-5 (TRP(8)) using infrared cameras (Subluxation Station Insight 7000; Chiropractic Leadership Alliance, Mahwah, NJ).

RESULTS

Before treatment (t(-0.5)), the TRP(8) CT was significantly different between the ipsilateral and the contralateral sides for all subgroups. At 10 minutes (t(10)) after intervention, CT increased significantly (P < .05) for the treatment group but not for the sham and control groups. In contrast, there were no significant differences in the TRP(30) CT before treatment between the ipsilateral and the contralateral sides; but at t(10), CT was significantly (P < .05) greater for all 3 subgroups compared with preintervention CT.

CONCLUSION

Contacting the skin with the instrument with (treatment group TRP(30)) or without (sham group TRP(30)) a thrust with a sustained pressure stronger than the loading principle taught in the Activator Methods Chiropractic Technique protocol or a thrust respecting the standard loading principle (treatment group TRP(8)) of the instrument produced a CT cooling immediately after the adjustment. Furthermore, we observed that when contacting the skin with the instrument with a thrust respecting the standard loading principle (treatment group TRP(8)) of the instrument, it produced a secondary cooling at t(5) followed by a rewarming at t(10). Finally, contacting the skin with the instrument without a thrust and respecting the standard loading principle (sham TRP(8)) of the instrument did not produce a CT change.

Prenatal gender-related nicotine exposure increases blood pressure response to angiotensin II in adult offspring

Epidemiological studies suggest that maternal cigarette smoking is associated with an increased risk of elevated blood pressure (BP) in postnatal life. The present study tested the hypothesis that prenatal nicotine exposure causes an increase in BP response to angiotensin II (Ang II) in adult offspring. Nicotine was administered to pregnant rats via subcutaneous osmotic minipumps throughout the gestation. BP and vascular responses to Ang II were measured in 5-month-old adult offspring. Prenatal nicotine had no effect on baseline BP but significantly increased Ang II-stimulated BP in male but not female offspring. The baroreflex sensitivity was significantly decreased in both male and female offspring. Prenatal nicotine significantly increased arterial media thickness in male but not female offspring. In male offspring, nicotine exposure significantly increased Ang II-induced contractions of aortas and mesenteric arteries. These responses were not affected by inhibition of endothelial NO synthase activity. Losartan blocked Ang II-induced contractions in both control and nicotine-treated animals. In contrast, PD123319 had no effect on Ang II-induced contractions in control but inhibited them in nicotine-treated animals. Nicotine significantly increased Ang II type 1 receptor but decreased Ang II type 2 receptor protein levels, resulting in a significant increase in the ratio of Ang II type 1 receptor/Ang II type 2 receptor in the aorta. Furthermore, the increased contractions of mesenteric arteries were mediated by increases in intracellular Ca(2+) concentrations and Ca(2+) sensitivity. These results suggest that prenatal nicotine exposure alters vascular function via changes in Ang II receptor-mediated signaling pathways in adult offspring in a gender-specific manner, which may lead to an increased risk of hypertension in male offspring.

Effect of hypothermia on baroreflex control of heart rate and renal sympathetic nerve activity in anaesthetized rats

The present study investigated the effect of acute hypothermia on baroreflex control of heart rate (HR) and renal sympathetic nerve activity (RSNA) by generating baroreflex logistic function curves, using bolus doses of phenylephrine and sodium nitroprusside, in anaesthetized male Wistar rats at a core temperature (T(b)) of 37 degrees C, during acute severe hypothermia at T(b)= 25 degrees C and on rewarming to 37 degrees C. Comparisons were made between rats without (euthermic, n= 6) and with (acclimated, n= 7) prior exposure to lower ambient temperatures and shorter photoperiod, simulating adaptation to winter conditions. In both groups of rats, acute hypothermia to T(b)= 25 degrees C shifted the baroreflex-RSNA curve slightly leftwards and downwards with decreases in the setpoint pressure and maximal gain, whereas it markedly impaired the baroreflex-HR curve characterized by decreases in response range by approximately 90% (P < 0.001), minimum response by approximately 10% (P < 0.05) and maximum gain by approximately 95% (P < 0.001), from that at T(b)= 37 degrees C. All parameters were restored to precooling levels on rewarming. Electrical stimulation of cardiac vagal efferents induced a voltage-related bradycardia, the magnitude of which was partially reduced during acute hypothermia, and there was a significant prolongation of the electrocardiogram intervals indicating a delay in cardiac conduction. Mild suppression of baroreflex control of RSNA could contribute to hypothermic hypotension and may primarily reflect an effect of T(b) on central drive. The marked attenuation of the baroreflex control of HR during hypothermia was likely to be due to an impairment of both the central and peripheral components of the reflex arc. Baroreflex control of RSNA and HR was similar between both groups of rats, which implied that the control was non-adaptive on chronic cold exposure.

Change in spontaneous baroreflex control of pulse interval during heat stress in humans

Spontaneous baroreflex control of pulse interval (PI) was assessed in healthy volunteers under thermoneutral and heat stress conditions. Subjects rested in the supine position with their lower legs in a water bath at 34 degrees C. Heat stress was imposed by increasing the bath temperature to 44 degrees C. Arterial blood pressure (Finapres), PI (ECG), esophageal and skin temperature, and stroke volume were continuously collected during each 5-min experimental stage. Spontaneous baroreflex function was evaluated by multiple techniques, including 1) the mean slope of the linear relationship between PI and systolic blood pressure (SBP) with three or more simultaneous increasing or decreasing sequences, 2) the linear relationship between changes in PI and SBP (deltaPI/DeltaSBP) derived by using the first differential equation, 3) the linear relationship between changes in PI and SBP with simultaneously increasing or decreasing sequences (+deltaPI/+deltaSBP or -deltaPI/-deltaSBP), and 4) transfer function analysis. Heat stress increased esophageal temperature by 0.6 +/- 0.1 degrees C, decreased PI from 1,007 +/- 43 to 776 +/- 37 ms and stroke volume by 16 +/- 5 ml/beat. Heat stress reduced baroreflex sensitivity but increased the incidence of baroreflex slopes from 5.2 +/- 0.8 to 8.6 +/- 0.9 sequences per 100 heartbeats. Baroreflex sensitivity was significantly correlated with PI or vagal power (r2 = 0.45, r2 = 0.71, respectively; P < 0.05). However, the attenuation in baroreflex sensitivity during heat stress appeared related to a shift in autonomic balance (shift in resting PI) rather than heat stress per se.

Heat-induced increases in endothelial NO synthase expression and activity and endothelial NO release

Endothelial nitric oxide (NO) synthase (eNOS) is regulated by heat shock protein 90 (HSP90), a heat-inducible protein; however, the effect of heat shock on eNOS expression and eNO release is unknown. Bovine aortic endothelial cells were incubated for 1 h at 37 degrees C, 42 degrees C, or 45 degrees C and cell lysates were evaluated with the use of Western blotting. We observed a 2.1 +/- 0.1-fold increase in eNOS protein content, but no change in HSP90 content, HSP70 content, or HSP90/eNOS association, 24 h after heat shock at 42 degrees C. We also observed a 7.7 +/- 1.5-fold increase in HSP70 protein content, but did not observe a change in eNOS or HSP90 24 h after heat shock at 45 degrees C. eNOS activity and maximal bradykinin-stimulated NO release was significantly increased 24 h after heat shock at 42 degrees C. Heat shock in rats (core temperature: 42 degrees C, 15 min) resulted in a significant increase in aortic eNOS, HSP90, and HSP70 protein content. The aorta from heat-shocked rats exhibited a decreased maximal contractile response to phenylephrine, which was abolished by preincubation with NG-nitro-l-arginine. We conclude that prior heat shock is a physical stimulus of increased eNOS expression and is associated with an increase in eNOS activity, agonist-stimulated NO release, and a decreased vasoconstrictor response.

Phenylephrine-induced elevations in arterial blood pressure are attenuated in heat-stressed humans

To test the hypothesis that phenylephrine-induced elevations in blood pressure are attenuated in heat-stressed humans, blood pressure was elevated via steady-state infusion of three doses of phenylephrine HCl in 10 healthy subjects in both normothermic and heat stress conditions. Whole body heating significantly increased sublingual temperature by ~0.5 degrees C, muscle sympathetic nerve activity (MSNA), heart rate, and cardiac output and decreased total peripheral vascular resistance (TPR; all P < 0.005) but did not change mean arterial blood pressure (MAP; P > 0.05). At the highest dose of phenylephrine, the increase in MAP and TPR from predrug baselines was significantly attenuated during the heat stress [DeltaMAP 8.4 +/- 1.2 mmHg; DeltaTPR 0.96 +/- 0.85 peripheral resistance units (PRU)] compared with normothermia (DeltaMAP 15.4 +/- 1.4 mmHg, DeltaTPR 7.13 +/- 1.18 PRU; all P < 0.001). The sensitivity of baroreflex control of MSNA and heart rate, expressed as the slope of the relationship between MSNA and diastolic blood pressure, as well as the slope of the relationship between heart rate and systolic blood pressure, respectively, was similar between thermal conditions (each P > 0.05). These data suggest that phenylephrine-induced elevations in MAP are attenuated in heat-stressed humans without affecting baroreflex control of MSNA or heart rate.

Baroreflex modulation of sympathetic nerve activity to muscle in heat-stressed humans

To identify whether whole body heating alters arterial baroreflex control of muscle sympathetic nerve activity (MSNA), MSNA and beat-by-beat arterial blood pressure were recorded in seven healthy subjects during acute hypotensive and hypertensive stimuli in both normothermic and heat stress conditions. Whole body heating significantly increased sublingual temperature (P < 0.01), MSNA (P < 0.01), heart rate (P < 0.01), and skin blood flow (P < 0.001), whereas mean arterial blood pressure did not change significantly (P > 0.05). During both normothermic and heat stress conditions, MSNA increased and then decreased significantly when blood pressure was lowered and then raised via intravenous bolus infusions of sodium nitroprusside and phenylephrine HCl, respectively. The slope of the relationship between MSNA and diastolic blood pressure during heat stress (-128.3 +/- 13.9 U x beats(-1) x mmHg(-1)) was similar (P = 0.31) with normothermia (-140.6 +/- 21.1 U x beats(-1) x mmHg(-1)). Moreover, no significant change in the slope of the relationship between heart rate and systolic blood pressure was observed. These data suggest that arterial baroreflex modulation of MSNA and heart rate are not altered by whole body heating, with the exception of an upward shift of these baroreflex curves to accommodate changes in these variables that occur with whole body heating.