The neurogenic constrictor response of isolated rat saphenous artery is reduced after 2-week tail suspension

Under real or simulated microgravity conditions the control of arterial vascular tone is greatly disturbed. The low arterial vessel reactivity to sympathetic influences may be the cause of an increase in flow in hind limb skeletal muscles in tail-suspended (TS) rats. Our previous experiments with constant pressure perfusion of rat hind limb demonstrated the reduced vasoconstrictor responses to sympathetic nerve stimulation in TS rats. Responses to exogenous noradrenaline depended on the perfusion conditions. It is known that the vessels of various branching orders noticeably differ in nerve density and in sensitivity to vasoconstrictor agonists. So under neurogenic or exogenous noradrenaline influences the vascular resistance may be increased at different levels of vascular bed, thus making the data analysis seriously complicated. This uncertainty may be overcome by investigation of a single vessel isolated from hind limb vascular bed. The saphenous artery, a resistance artery with dense innervation, is a very convenient object for this purpose. Thus, this study was aimed at comparing the effects of 2-week tail suspension upon the constrictor responses of isolated saphenous artery to neurogenic and exogenous noradrenaline stimuli in rats.     

Microcirculation characteristics in apparently healthy subjects during acute hypoxia and intermittent hypoxic training

The characteristics of the microcirculation in the forearm skin of 29 apparently healthy male volunteers were studied in acute hypoxia and during intermittent hypoxic training (IHT) using computer laser Doppler flowmetry. It was shown that short-term exposure of apparently healthy subjects to simulated acute hypoxia optimized the microcirculation owing to sympathetic innervation and concomitant rearrangement of microvascular tone regulation (activation of skin perfusion and reduction of blood flow through arteriovenular shunts when the neurogenic tone component increases). A second (placebo-controlled) series of experiments showed that long-term hypoxic preconditioning (20 IHT sessions) facilitated fixing of the adaptive dynamic rearrangements aimed at microcirculation improvement. The microvasculature response during acute hypoxia and a course of IHT depends on the initial sensitivity of subjects to simulated hypoxia. Significant perfusion enhancement in the tissues studied and microvascular tone normalization were observed in the subjects that were initially sensitive to hypoxia.     

Evidence for nitroxidergic innervation in monkey ophthalmic arteries in vivo and in vitro

In anesthetized monkeys, electrical stimulation (ES) of the pterygopalatine or geniculate ganglion dilated the ipsilateral ophthalmic artery (OA). The induced vasodilatation was unaffected by phentolamine but potentiated by atropine. Intravenous N(G)-nitro-L-arginine (L-NNA) abolished the response, which was restored by L-arginine. Hexamethonium-abolished vasodilator responses induced solely by geniculate ganglionic stimulation. The L-NNA constricted OA; L-arginine reversed the effect. Destruction of the pterygopalatine ganglion constricted the ipsilateral artery. Helical strips of OA isolated under deep anesthesia from monkeys, denuded of endothelium, responded to transmural ES with relaxations, which were abolished by tetrodotoxin and L-NNA but were potentiated by atropine. It is concluded that neurogenic vasodilatation of monkey OA is mediated by nerve-derived nitric oxide (NO), and the nerve is originated from the ipsilateral pterygopalatine ganglion that is innervated by cholinergic neurons from the brain stem via the geniculate ganglion. The OA appears to be dilated by mediation of NO continuously liberated from nerves that receive tonic discharges from the vasomotor center. Acetylcholine liberated from postganglionic cholinergic nerves would impair the release of neurogenic NO.     

Neurogenic inflammation, vascular permeability, and mast cells

Electrical stimulation (ES) of sensory nerves causes increased vascular permeability and vasodilatation, a process known as neurogenic inflammation. The purpose of this study was to assess the role of mast cells in neurogenic inflammation induced by ES of sensory nerves. ES of the rat saphenous nerve for 1, 3, 5, 15, or 30 min induced a 166 to 436% increase in the amount of 125I-albumin deposited in the skin. Through the initial 15 min of ES, the histamine content of the skin remained unchanged. However, 30 min of ES caused a 22.1% decrease in skin histamine (p less than 0.05). ES for 5 min followed by measurement of vascular permeability from 0 to 30 min thereafter resulted in maximal increases in 125I-albumin in the skin immediately after cessation of the pulse of ES. When skin histamine was measured at various intervals after a 5-min pulse of ES, no change in the histamine content was observed through the subsequent 30 min. When mast cell degranulation was assessed histologically, 5 min of ES failed to stimulate mast cell degranulation. However, 30 min of ES caused a significant increase in the proportion of degranulating mast cells. When draining venous plasma histamine was monitored before, during and after ES, no change in plasma histamine was observed. In contrast, the intradermal injection of 5 micrograms of compound 48/80 produced a significant increase in plasma histamine. In order to examine the possibility that histamine might be released but remain in the skin after ES, skin "blisters" were developed by intradermal injections of saline. There was a significant increase in the amount of 125I-albumin extravasated into blister fluid measured after 3, 5, and 10 min of ES and a significant increase in histamine after 5 or 10 min. Therefore, prolonged ES of sensory nerves can cause mast cell degranulation. However, ES causes increased vascular permeability at times when no mast cell activation can be observed. These data suggest that the initial phases of neurogenic inflammation are independent of mast cell activation.     

Skeletal muscle vascular responses in human limbs to isometric handgrip

Studies of whole limb blood flow have shown that static handgrip elicits a vasodilatation in the resting forearm and vasoconstriction in the resting leg. We asked if these responses occur in the skeletal muscle vascular bed, and if so, what is the relative contribution of local metabolic versus other mechanisms to these vascular responses. Blood flow recordings were made simultaneously in the skeletal muscle of the resting arm and leg using the Xenon-washout method in ten subjects during 3 min of isometric handgrip at 30% of maximal voluntary contraction. In the arm, skeletal muscle vascular resistance (SMVR) decreased transiently at the onset of exercise followed by a return to baseline levels at the end of exercise. In the leg SMVR remained unchanged during the 1st min of handgrip, but had increased to exceed baseline levels by the end of exercise. During exercise electromyography (EMG) recordings from nonexercising limbs demonstrated a progressive 20-fold increase in activity in the arm, but remained at baseline in the leg. During EMG-signal modelled exercise performed to mimic the inadvertent muscle activity, decreases in forearm SMVR amounted to 57% of the decrease seen with controlateral handgrip. The present study would seem to indicate that vascular tone in nonexercising skeletal muscle in the arm and leg are controlled differently during the early stages of static handgrip. Metabolic vasodilatation due to involuntary contraction could significantly modulate forearm skeletal muscle vascular responses, but other factors, most likely neural vasodilator mechanisms, must make major contributions. During the later stages of contralateral sustained handgrip, vascular adjustments in resting forearm skeletal muscle would seem to be the final result of reflex sympathetic vasoconstrictor drive, local metabolic vasodilator forces and possibly neurogenic vasodilator mechanisms.     

Raised tone reveals ATP as a sympathetic neurotransmitter in the porcine mesenteric arterial bed

The relative importance of ATP as a functional sympathetic neurotransmitter in blood vessels has been shown to be increased when the level of preexisting vascular tone or pressure is increased, in studies carried out in rat mesenteric arteries. The aim of the present study was to determine whether tone influences the involvement of ATP as a sympathetic cotransmitter with noradrenaline in another species. We used the porcine perfused mesenteric arterial bed and porcine mesenteric large, medium and small arteries mounted for isometric tension recording, because purinergic cotransmission can vary depending on the size of the blood vessel. In the perfused mesenteric bed at basal tone, sympathetic neurogenic vasocontractile responses were abolished by prazosin, an α₁-adrenoceptor antagonist, but there was no significant effect of α,β-methylene ATP, a P2X receptor-desensitizing agent. Submaximal precontraction of the mesenteric arterial bed with U46619, a thromboxane A₂ mimetic, augmented the sympathetic neurogenic vasocontractile responses; under these conditions, both α,β-methylene ATP and prazosin attenuated the neurogenic responses. In the mesenteric large, medium and small arteries, prazosin attenuated the sympathetic neurogenic contractile responses under conditions of both basal and U46619-raised tone. α,β-Methylene ATP was effective in all of these arteries only under conditions of U46619-induced tone, causing a similar inhibition in all arteries, but had no significant effect on sympathetic neurogenic contractions at basal tone. These data show that ATP is a cotransmitter with noradrenaline in porcine mesenteric arteries; the purinergic component was revealed under conditions of partial precontraction, which is more relevant to physiological conditions.     

Prolonged head-down tilt exposure reduces maximal cutaneous vasodilator and sweating capacity in humans.

Cutaneous vasodilation and sweat rate are reduced during a thermal challenge after simulated and actual microgravity exposure. The effects of microgravity exposure on cutaneous vasodilator capacity and on sweat gland function are unknown. The purpose of this study was to test the hypothesis that simulated microgravity exposure, using the 6 degrees head-down tilt (HDT) bed rest model, reduces maximal forearm cutaneous vascular conductance (FVC) and sweat gland function and that exercise during HDT preserves these responses. To test these hypotheses, 20 subjects were exposed to 14 days of strict HDT bed rest. Twelve of those subjects exercised (supine cycle ergometry) at 75% of pre-bed rest heart rate maximum for 90 min/day throughout HDT bed rest. Before and after HDT bed rest, maximal FVC was measured, via plethysmography, by heating the entire forearm to 42 degrees C for 45 min. Sweat gland function was assessed by administering 1 x 10(-6) to 2 M acetylcholine (9 doses) via intradermal microdialysis while simultaneously monitoring sweat rate over the microdialysis membranes. In the nonexercise group, maximal FVC and maximal stimulated sweat rate were significantly reduced after HDT bed rest. In contrast, these responses were unchanged in the exercise group. These data suggest that 14 days of simulated microgravity exposure, using the HDT bed rest model, reduces cutaneous vasodilator and sweating capacity, whereas aerobic exercise training during HDT bed rest preserves these responses.     

Neurogenic vasodilation and release of calcitonin gene-related peptide (CGRP) from perivascular nerves in the rat mesenteric artery

The effect of perivascular nerve stimulation (PNS) on the release of calcitonin gene-related peptide (CGRP) was examined by radioimmunoassay (RIA) in isolated, perfused rat mesenteric arteries. The released CGRP-like immunoreactivity (CGRP-LI) was identified to be CGRP itself and its oxidized form by combined analysis with RIA and high performance liquid chromatography. CGRP-LI was localized in the perivascular nerves of the large mesenteric artery and its branches. In the preparation precontracted by methoxamine, and perfused with a solution containing guanethidine, an adrenergic neuron blocker, PNS induced vasodilator responses and an increase of CGRP-LI in the perfusate in a frequency-dependent manner. Both the responses were attenuated by tetrodotoxin (10(-6) M), suggesting that they were neurogenic in origin. Removal of Ca2+ from the perfusing solution also abolished the PNS-induced release of CGRP-LI. These findings suggest that CGRP plays a transmitter role in the neurogenic vasodilation in the rat mesenteric vascular bed.     

Comparative proteomic analysis reveals differential expression of Hsp25 following the directed differentiation of mouse embryonic stem cells

Murine embryonic stem (ES) cells can be committed to neural differentiation with high efficiency in culture through the use of feeder- and serum-free media. This system is proving to be an excellent model to study processes involved in ES cell commitment to neural cell fate. We used this approach to generate neurogenic embryoid bodies (NEBs) in a serum-free culture system to perform proteomic analysis of soluble fractions and identify early changes in protein expression as ES cells differentiate. Ten candidate proteins were altered significantly in expression levels. One of the most significant alterations was for the small heat shock protein Hsp25. Three species of Hsp25 are detected in ES cells, and this expression pattern changes during the first 24 h of differentiation until expression is decreased to levels that are barely detectable at 4 days following differentiation. We used immunofluorescence studies to confirm that following ES cell differentiation, expression of Hsp25 becomes excluded from neural precursors as well as other differentiating cells, making it a potentially useful marker of early ES cell differentiation.     

Effects of polyester jerseys on psycho-physiological responses during exercise in a hot and moist environment

Gonzales, BR, Hagin, V, Guillot, R, Placet, V, and Groslambert, A. Effects of polyester jerseys on psycho-physiological responses during exercise in a hot and moist environment. J Strength Cond Res 25(12): 3432-3438, 2011-With the general acceptance that extreme environments have a detrimental effect on thermoregulation and human performance, the aim of this study was to investigate the influence of 3 polyester jerseys with knits of different sizes on physiological and perceptual responses in trained cyclists during exercise performed in a hot and moist environment. Ten trained male cyclists (mean ± SD, age: 29.1 ± 8 years, height: 177.12 ± 5 cm, body mass: 70.10 ± 6 kg), performed 3 tests of 15 minutes at 150 W on a calibrated home trainer by randomly wearing jerseys with small knits (SK), medium knits, and large knits (LK). While exercising, the jersey and torso skin temperatures, perceived exertion and hotness, and heart rate (HR) were continuously recorded. The major results of this study showed that perceived hotness with LK was significantly lower (p < 0.05) than with SK at minutes 10 (effect size [ES] = 1.18) and 12 (ES = 1.04) of exercise. The torso skin temperature with LK was significantly lower (p < 0.05) than with SK at minute 10 (ES = 0.84) and at minute 14 (ES = 0.81) of exercise, and the LK jersey temperature was significantly lower (p < 0.05) than with SK jerseys at minutes 12 (ES = 0.83) and 14 (ES = 0.90) of exercise. However, no significant difference was found in perceived exertion or HR. These results suggest that the use of polyester jerseys with larger knits could limit the drift of skin temperature and therefore increase the thermal comfort of cyclists during exercise performed in a hot and moist environment. Therefore, coaches are encouraged to take particular care that their athletes wear exercise-appropriate clothing in hot temperatures.     

Neutral endopeptidase and neurogenic inflammation in rats with respiratory infections

Neuropeptides such as substance P are implicated in inflammation mediated by sensory nerves (neurogenic inflammation), but the roles in disease of these peptides and the peptidases that degrade them are not understood. It is well established that inflammation is a prominent feature of several airway diseases, including viral infections, asthma, bronchitis, and cystic fibrosis. These diseases are characterized by cough, airway edema, and abnormal secretory and bronchoconstrictor responses, all of which can be elicited by substance P. The effects of substance P and other peptides that may be involved in inflammation are decreased by endogenous neutral endopeptidase (NEP; also called enkephalinase, EC 3.4.24.11), which is a peptidase that degrades substance P and other peptides. In the present study, we report that rats with histories of infections caused by common respiratory tract pathogens (parainfluenza virus type 1, rat corona-virus, and Mycoplasma pulmonis) not only have greater susceptibility to neurogenic inflammatory responses than do pathogen-free rats but also have a lower activity of NEP in the trachea. This reduction in NEP activity may cause the increased susceptibility to neurogenic inflammation by allowing higher concentrations of substance P to reach tachykinin receptors in the trachea. Thus decreased NEP activity may exacerbate some of the pathological responses in animals with respiratory tract infections.     

Biomechanical outcomes and neural correlates of cutaneous reflexes evoked during rhythmic arm cycling

During walking cutaneous stimulation of the foot yields neural and mechanical reflexes that serve a functional purpose to correct or assist the ongoing movement. Concurrently, while cutaneous stimulation of the hand during rhythmic arm movement parallel the neural responses observed in the legs, studies of rhythmic arm movement have only limited mechanical measurements. Therefore it is difficult to determine whether reflex responses in the arms during rhythmic arm movement serve a functional purpose similar to those seen in the lower limbs. The purpose of this study was to explore the mechanical outcomes of stimulating a cutaneous nerve innervating the hand during arm cycling. We hypothesized that there would be measurable mechanical effects to cutaneous stimulation during arm cycling that function to correct or assist the task of arm cycling. Specifically, kinetic responses measured at the handle would be considered assistive if they were tangential to the arm cycling path in the direction of forward progression. Also, limb kinematic responses would be considered corrective if they allowed limb movement that would result in removal of the limb from stimulus while not altering the kinetic profile at the handle necessary for arm cycling progression. Participants performed seated arm cycling while EMG was recorded from the arm and trunk muscles, kinematic data was recorded from the right arm, and kinetic data was recorded from the handle. Cutaneous reflexes were evoked by stimulating the superficial radial nerve. The results show that there are observable mechanical responses to cutaneous stimulation of the hand during arm cycling. Subjects responded to cutaneous stimulation of the hand during arm cycling with significant changes in backward and lateral forces at the handle as well as wrist abduction/adduction and wrist flexion/extension kinematics. These responses, related to the task and phase of movement, are consistent with the anatomical location of the stimulus and are correlated to the neural responses. Therefore, these responses are comparable to functionally relevant responses in the legs during rhythmic movement. However, while there is a single observation of a kinematic corrective strategy, the kinetics measured at the handle are not tangential to the arm cycling path and therefore not considered an assistive response. Therefore, unlike the observations in the lower limbs, the mechanical responses during arm cycling are not clearly related to the functional context of the ongoing task.     

Roles of ES Cell-Derived Gliogenic Neural Stem/Progenitor Cells in Functional Recovery after Spinal Cord Injury

Transplantation of neural stem/progenitor cells (NS/PCs) following the sub-acute phase of spinal cord injury (SCI) has been shown to promote functional recovery in rodent models. However, the types of cells most effective for treating SCI have not been clarified. Taking advantage of our recently established neurosphere-based culture system of ES cell-derived NS/PCs, in which primary neurospheres (PNS) and passaged secondary neurospheres (SNS) exhibit neurogenic and gliogenic potentials, respectively, here we examined the distinct effects of transplanting neurogenic and gliogenic NS/PCs on the functional recovery of a mouse model of SCI. ES cell-derived PNS and SNS transplanted 9 days after contusive injury at the Th10 level exhibited neurogenic and gliogenic differentiation tendencies, respectively, similar to those seen in vitro. Interestingly, transplantation of the gliogenic SNS, but not the neurogenic PNS, promoted axonal growth, remyelination, and angiogenesis, and resulted in significant locomotor functional recovery after SCI. These findings suggest that gliogenic NS/PCs are effective for promoting the recovery from SCI, and provide essential insight into the mechanisms through which cellular transplantation leads to functional improvement after SCI.     

Effects of simulated microgravity on embryonic stem cells

There have been many studies on the biological effects of simulated microgravity (SMG) on differentiated cells or adult stem cells. However, there has been no systematic study on the effects of SMG on embryonic stem (ES) cells. In this study, we investigated various effects (including cell proliferation, cell cycle distribution, cell differentiation, cell adhesion, apoptosis, genomic integrity and DNA damage repair) of SMG on mouse embryonic stem (mES) cells. Mouse ES cells cultured under SMG condition had a significantly reduced total cell number compared with cells cultured under 1 g gravity (1G) condition. However, there was no significant difference in cell cycle distribution between SMG and 1G culture conditions, indicating that cell proliferation was not impaired significantly by SMG and was not a major factor contributing to the total cell number reduction. In contrast, a lower adhesion rate cultured under SMG condition contributed to the lower cell number in SMG. Our results also revealed that SMG alone could not induce DNA damage in mES cells while it could affect the repair of radiation-induced DNA lesions of mES cells. Taken together, mES cells were sensitive to SMG and the major alterations in cellular events were cell number expansion, adhesion rate decrease, increased apoptosis and delayed DNA repair progression, which are distinct from the responses of other types of cells to SMG.     

Vasomotor control: functional hyperemia and beyond

Historically, functional hyperemia has been viewed largely as an interaction between a parenchymal cell and its associated microvasculature. Locally released metabolites have been thought to produce relaxation of the smooth muscle and a vasodilation that increases blood flow in proportion to metabolic need. This symposium report presents evidence from a variety of disciplines and a number of different types of biological preparations that demonstrates that functional hyperemia is a complex process involving several classes of microvessels including capillaries, arterioles, and small arteries. These vessels do not function independently but are coordinated by a complex set of interrelations involving at least three different modes of interaction between parenchymal cells and the various segments of the vascular bed. These are local metabolic effects, propagated effects extending over long segments of the vasculature, and flow-dependent vasodilation induced by local changes in blood flow. In addition to these acute responses to metabolic demand it appears that tissues may be capable of more long-term structural alterations of the arterial and arteriolar network in response to sustained changes in the relationship between supply and demand. The vascular bed appears to be able to adapt either by increasing the maximal anatomic diameter of the large arteries or by inserting new arterioles into the parenchyma. Thus, classical functional hyperemia appears to be but one manifestation of a multifaceted process leading to highly coordinated responses of many vascular elements, resulting finally in vascular patterns that are optimized to meet parenchymal cell demands.     

Trunk Muscle Activation at the Initiation and Braking of Bilateral Shoulder Flexion Movements of Different Amplitudes

The aim of this study was to investigate if trunk muscle activation patterns during rapid bilateral shoulder flexions are affected by movement amplitude. Eleven healthy males performed shoulder flexion movements starting from a position with arms along sides (0°) to either 45°, 90° or 180°. EMG was measured bilaterally from transversus abdominis (TrA), obliquus internus (OI) with intra-muscular electrodes, and from rectus abdominis (RA), erector spinae (ES) and deltoideus with surface electrodes. 3D kinematics was recorded and inverse dynamics was used to calculate the reactive linear forces and torque about the shoulders and the linear and angular impulses. The sequencing of trunk muscle onsets at the initiation of arm movements was the same across movement amplitudes with ES as the first muscle activated, followed by TrA, RA and OI. All arm movements induced a flexion angular impulse about the shoulders during acceleration that was reversed during deceleration. Increased movement amplitude led to shortened onset latencies of the abdominal muscles and increased level of activation in TrA and ES. The activation magnitude of TrA was similar in acceleration and deceleration where the other muscles were specific to acceleration or deceleration. The findings show that arm movements need to be standardized when used as a method to evaluate trunk muscle activation patterns and that inclusion of the deceleration of the arms in the analysis allow the study of the relationship between trunk muscle activation and direction of perturbing torque during one and the same arm movement.     

Noggin and chordin have distinct activities in promoting lineage commitment of mouse embryonic stem (ES) cells

To examine the role of secreted signaling molecules and neurogenic genes in early development, we have developed a culture system for the controlled differentiation of mouse embryonic stem (ES) cells. In the current investigation, two of the earliest identified BMP antagonists/neural-inducing factors, noggin and chordin, were expressed in pluripotent mouse ES cells. Neurons were present as early as 24 h following transfection of ES cells with a pCS2/noggin expression plasmid, with differentiation peaking at 72 h. With neuronal differentiation, stem cell marker genes were down-regulated and neural determination genes expressed. Coculture experiments and exposure to noggin-conditioned medium produced similar neuronal differentiation of control ES cells, while addition of BMP-4 to noggin expressants strikingly inhibited neuronal differentiation. Transfection of ES cells with a pCS2/chordin expression vector or exposure to chordin-conditioned medium produced a more complex pattern of differentiation; ES cells formed neurons, mesenchymal cells as well as N-CAM-positive, nestin-positive neuroepithelial progenitors. These data suggest that, consistent with their different expression fields, noggin and chordin may play distinct roles in patterning the early mouse embryo.     

Interactions of neurogenic responses of longitudinal and circular muscle in the guinea-pig ileum

The relationship between neurogenic responses of longitudinal and circular muscle was studied by measuring contractions and EMG or nonadrenergic, non-cholinergic (NANC) relaxations and NANC inhibitory junction potentials in different preparations of the guinea-pig ileum. NANC relaxation of longitudinal muscle was observed also without any preceding or concomitant circular muscle contraction ruling out the possibility that the latter might be the cause of the NANC relaxation. Circular muscle twitches or powerful contractions were absent if there was no preceding neurogenic or myogenic excitation of longitudinal muscle; in preparations with myenteric plexus-longitudinal muscle layers removed only small residual responses were seen although still under neurogenic influences. Thus excitation of longitudinal muscle seemed a prerequisite for synchronized and powerful contractions of circular muscle to occur. Cholinergic contraction and NANC relaxation of longitudinal muscle evoked by field stimulation were partly inhibited if the submucous plexus was also present suggesting the involvement of a more complex neuronal circuitry in these responses.     

Thermoregulatory responses to upper body exercise

The purpose of this study was to compare thermoregulatory responses between upper body and lower body exercise. Nine male subjects performed 60 min of arm crank (AC) and cycle (CY) exercise at the same absolute intensity (oxygen uptake = 1.61 X min-1) and at the same relative intensity (60% of ergometer specific peak oxygen uptake) in a temperate (24 degrees C, 20% rh) environment. During the absolute intensity experiments, rectal temperature and sweating rate responses were essentially the same for both modes of exercise. In addition, no differences were found for chest, back, arm, or thigh skin temperatures, but calf skin temperature was significantly (P less than 0.05) lower during arm crank than cycle exercise. During the relative intensity experiments, thermoregulatory responses were lower during arm crank than cycle exercise. In addition, we found no difference between esophageal and rectal temperature values elicited by arm crank exercise. These results indicate that the examined thermoregulatory responses are independent of the skeletal muscle mass employed and dependent upon the absolute metabolic intensity.     

Responses of muscle sympathetic nerve activity to static handgrip exercise after 14 days of exposure to simulated microgravity

Decrease in muscle perfusion affects on cardiovascular response to exercise. Muscle hypoperfusion enhances the increase in blood pressure responses to exercise. Muscle perfusion depends not only on central blood pressure but also how fit the active muscle is above or below the heart level; muscle perfusion decreases as arm is elevated. Static exercise increases muscle sympathetic nerve activity (MSNA) innervating vessels in non-active muscles. The exercise-induced increase in MSNA is mainly mediated by stimulating chemosensitive muscle afferents in active muscles. However, the effect of arm elevation on MSNA during forearm exercise is not examined. On the other hand, space flight and simulated microgravity exposure causes reduction in muscle blood flow, suggesting chronic muscle hypoperfused condition during simulated microgravity. Therefore, there is a possibility that arm elevation after microgravity exposure alters MSNA responsiveness during exercise. However, arm elevation effect after exposure to simulated microgravity is not examined.