Subcutaneous and skeletal muscle vascular responses in human limbs to lower body negative pressure

Jacobsen , T. N., Nielsen , H. V., Kassis , E. & Amtorp o S. 1992. Subcutaneous and skeletal muscle vascular responses in human limbs to lower body negative pressure. Acta Physiol Scand 144 , 247–252. Received 8 March 1991, accepted 7 Novcmber 1991. ISSN 0001–6772. Department of Cardiology, Gentofte Hospital, University of Copenhagen, Denmark Cardiopulmonary baroreceptor unloading in humans comparably increases sympathetic discharge to skeletal muscle in the forearm and calf, but blood flow studies have disclosed differential rather than uniform vasomotor responses in the extremities. The aim of the present study was to address the issue of differential effects of orthostatic stress on forearm and calf vascular adjustment and to extend previous studies by determining changes in vascular responses separately in various vascular beds of the limbs. The local [¹³³Xenon] washout method was used for recording blood flow rates in subcutaneous tissue and skeletal muscle. Simultaneous recordings from the forearm and calf were performed in 11 healthy young males during lower body negative pressure at —10 mmHg. Heart rate, arterial mean and pulse pressures did not change during lower body negative pressure. In the forearm blood flow rates decreased significantly, in subcutaneous tissue by 16 ± 2% (mean ± SEM) and in skeletal muscle by 16 ± l%. In the calf lower body negative pressure induced a significant decrease in blood flow rates of 17 ± 3% in subcutaneous tissue and of 30 ± 2% in skeletal muscle. This vasoconstriction in calf skeletal muscle was consistently disclosed in both legs and was about the same magnitude in each calf when studied with the one leg exposed to lower body negative pressure and the other outside the lower body negative pressure chamber. These findings suggest that during unloading of cardiopulmonary afferents, reflex sympathetic activation as an important autonomic adjustment to orthostatic stress is accompanied by uniform vasoconstriction in subcutaneous and skeletal muscle vascular beds of human limbs.     

Effect of orthostatic blood pressure changes upon capillary filtration-absorption rate in the human calf

The effect of postural changes upon fluid filtration rate in the calf was studied in 3 healthy subjects placed in supine position. Volume changes in the calf were measured by a water plethysmograph. Relative changes in blood flow in subcutaneous tissue and anterior tibial muscle were measured by the local ¹³³Xe washout technique. Lowering the calf from 5–25 cm caused a linear increase in filtration rate whereas blood flow in subcutaneous tissue of the segment studied and anterior tibial muscle remained constant. Based on this average CFC was about 0.0012 ml min^-1 100 g^-1 mmHg^-1 Further lowering of the calf caused a decrease in blood flow in subcutaneous tissue and skeletal muscle by about 50% corresponding to an increase in total vascular resistance by about 100%. During these circumstances the increase in filtration rate was reduced by about 33%. The results indicate that the local veno-arteriolar reflex responsible for the observed vasoconstriction in the tissue under study together with intrinsic vascular mechanisms reduces transcapillary fluid filtration in the lowered position. The mechanism appears to be mainly due to a reduction of the increase in mean capillary pressure whereas CFC seems to remain almost constant.     

Blood flow in skin, subcutaneous adipose tissue and skeletal muscle in the forearm of normal man during an oral glucose load

Blood flow to the forearm, and the subcutaneous tissue and skin in the forearm were measured by strain gauge plethysmography, 133Xe-elimination and Laser Doppler flowmetry during an oral glucose load (I g glucose kg-1 lean body mass) and during control conditions. The forearm blood flow remained constant during both experiments. Glucose induced a two-fold vasodilatation in subcutaneous tissue. In skin, glucose induced a relative vasodilatation and later a relative vasoconstriction compared with control experiments. When estimated from forearm blood flow and subcutaneous and skin blood flows, muscle blood flow decreased about 20-30% during both experiments. Proximal nervous blockade did not abolish the glucose-induced vasodilatation in subcutaneous tissue. In the glucose experiment, arterial glucose concentration increased to 7.8 +/- 1.17 mmol l-1 30 min after the load was given and then decreased to 4.5 +/- 0.34 mmol l-1 at the end of the experiment. In the control experiments glucose concentration was constant. Arterial noradrenaline concentration increased significantly from 1.0 +/- 0.13 to about 1.5 +/- 0.3 nmol l-1 120 min after glucose and remained at this level during the experiment. Similarly adrenaline increased from 0.16 +/- 0.11 to about 0.4 +/- 0.16 nmol l-1 180 min after glucose. It is hypothesized that the vasodilating effect of glucose in subcutaneous tissue is secondary to metabolic events connected to glucose uptake and energy deposition in adipose tissue.     

Near-infrared spectroscopy provides an index of blood flow and vasoconstriction in calf skeletal muscle during lower body negative pressure

Aim: Near‐infrared spectroscopy (NIRS) has been used previously for forearm blood flow estimation at rest and during exercise. In this study we applied NIRS to selectively monitor deep calf oxygenated haemoglobin (Hb) responses in order to estimate blood flow changes in the calf muscle during lower body negative pressure (LBNP). The purpose of this study was to test the hypothesis that changes in calf skeletal muscle oxygenated‐Hb, after the removal of superficial tissue responses, were related to blood flow changes during orthostatic stress, and to determine the efficacy of using NIRS measurements as an index of vasoconstriction. Methods: Twenty‐nine subjects participated in this study. All attempted a graded LBNP trial from baseline (0 mmHg) to ?60 mmHg LBNP in 10 mmHg steps at 5‐min intervals. Calf blood flow changes were estimated by oxygenated‐Hb responses in relation to changes in mercury strain gauge plethysmography and muscle sympathetic nerve activity (MSNA). Results: Calf selective deep oxygenated‐Hb decreased continuously from ?10 mmHg LBNP. Regression analysis showed that oxygenated‐Hb was significantly related to declines in plethysmography evaluations of blood flow [oxygenated‐Hb = (?1.57 ± 0.26) + (1.86 ± 0.49) plethysmography, r² = 0.87 ± 0.09]. Changes in MSNA (total activity) were also inversely related to oxygenated‐Hb (slope < 0, P = 0.037; r² = 0.52 ± 0.15). Conclusion: These results suggest that changes in selective deep calf oxygenated‐Hb can be utilized to estimate calf muscle blood flow changes that are most likely caused by vasoconstriction during graded LBNP.     

Forearm vascular and neuroendocrine responses to graded water immersion in humans

The hypothesis that graded expansion of central blood volume by water immersion to the xiphoid process and neck would elicit a graded decrease in forearm vascular resistance was tested. Central venous pressure increased (P < 0.05) by 4.2 +/- 0.4 mmHg (mean +/- SEM) during xiphoid immersion and by 10.4 +/- 0.5 mmHg during neck immersion. Plasma noradrenaline was gradually suppressed (P < 0.05) by 62 +/- 8 and 104 +/- 11 pg mL-1 during xiphoid and neck immersion, respectively, indicating a graded suppression of sympathetic nervous activity. Plasma concentrations of arginine vasopressin were suppressed by 1.5 +/- 0.5 pg mL-1 (P < 0.05) during xiphoid immersion and by 2.0 +/- 0.5 pg mL-1 during neck immersion (P < 0.05 vs. xiphoid immersion). Forearm subcutaneous vascular resistance decreased to the same extent by 26 +/- 9 and 28 +/- 4% (P < 0.05), respectively, during both immersion procedures, whereas forearm skeletal muscle vascular resistance declined only during neck immersion by 27 +/- 6% (P < 0.05). In conclusion, graded central blood volume expansion initiated a graded decrease in sympathetic nervous activity and AVP-release. Changes in forearm subcutaneous vascular resistance, however, were not related to the gradual withdrawal of the sympathetic and neuroendocrine vasoconstrictor activity. Forearm skeletal muscle vasodilatation exhibited a more graded response with a detectable decrease only during immersion to the neck. Therefore, the forearm subcutaneous vasodilator response reaches saturation at a lower degree of central volume expansion than that of forearm skeletal muscle.     

Vascular Resistance in Peripheral Blood Vessels at Normotension and at Local Orthostatic Hypertension in Healthy Humans

The effect of changes in orthostatic pressure on vascular resistance in subcutaneous adipose tissue was examined in the forearm at normal tension and in the leg at local orthostatic hypertension in three normal subjects. Blood flow in subcutaneous tissue was measured by the local xenon-133 washout technique. Changes in orthostatic pressure were achieved by postural changes of the extremity. At levels above the heart where venous pressure is constant, there was autoregulation of blood flow over a wider arterial blood pressure range in leg than in forearm. When transmural pressure increased 25 mm Hg or more vascular resistance increased about 50 per cent in the dependent forearm and 300 per cent in the dependent leg. The effect of ischemia on vascular resistance was investigated in cutaneous tissue of hand and foot by means of the ¹³¹I-Antipyrine initial slope technique. Vascular resistance after maximum dilatation achieved after20, 30, and 35 min of ischemia was less in vessels of the hand than in vessels of the foot. Thus, adaptive structural changes of vessels subjected to increased blood pressure are present in cutaneous and subcutaneous adipose tissues in normal subjects.     

Limb neurovascular control during altered otolithic input in humans

Head-down rotation (HDR), which activates the vestibulosympathetic reflex, increases leg muscle sympathetic nerve activity (MSNA) and produces calf vasoconstriction with no change in either cardiac output or arterial blood pressure. Based on animal studies, it was hypothesized that differential control of arm and leg MSNA explains why HDR does not alter arterial blood pressure. Fifteen healthy subjects were studied. Heart rate, arterial blood pressure, forearm and calf blood flow, and leg MSNA responses were measured during HDR in these subjects. Simultaneous recordings of arm and leg MSNA were obtained from five of the subjects. Forearm and calf blood flow, vascular conductances, and vascular resistances were similar before HDR, as were arm and leg MSNA. HDR elicited similar significant increases in leg (Δ6 ± 1 bursts min⁻¹; 59 ± 16 % from baseline) and arm MSNA (Δ5 ± 1 bursts min⁻¹; 80 ± 28 % from baseline). HDR significantly decreased calf (−19 ± 2 %) and forearm vascular conductance (−12 ± 2 %) and significantly increased calf (25 ± 4 %) and forearm vascular resistance (15 ± 2 %), with ∼60 % greater vasoconstriction in the calf than in the forearm. Arterial blood pressure and heart rate were not altered by HDR. These results indicate that there is no differential control of MSNA in the arm and leg during altered feedback from the otolith organs in humans, but that greater vasoconstriction occurs in the calf than in the forearm. These findings indicate that vasodilatation occurs in other vascular bed(s) to account for the lack of increase in arterial blood pressure during HDR.     

Evidence that the arterial baroreceptors influence muscle blood flow and not subcutaneous flow in man

Studies in man indicate that skeletal muscle blood flow is modulated via high pressure baroreceptors located in the carotid body. However, whether subcutaneous blood flow is influenced in this way remains controversial. Therefore, the aim of the present study was to determine whether subcutaneous as well as skeletal muscle blood flow was influenced by direct carotid body stimulation; this was done by means of neck suction. Six young healthy males were investigated. Subcutaneous and skeletal muscle blood flow in the forearm was measured by the local 133Xe washout technique. Neck suction of -20 mmHg was applied through a tight helmet covering the head and neck. In the supine position heart rate increased about 10%, mean arterial pressure was unchanged, subcutaneous blood flow was unchanged, but skeletal muscle blood flow increased about 70%. Measurements were repeated during neck suction, -20 mmHg, and 45 degrees head-up tilt (which is known to decrease subcutaneous blood flow and skeletal muscle blood flow by increasing sympathetic vasoconstrictor activity). Heart rate increased by about 20%, mean arterial pressure was unchanged, subcutaneous blood flow was unchanged and skeletal muscle blood flow increased about 100%. These results indicate that skeletal muscle blood flow is modulated through carotid baroreceptors. Subcutaneous blood flow seems not to be influenced by high pressure baroreceptor modulation, although a vasoconstrictor response to head-up tilt was demonstrated in this as well as other recent studies.     

Effect of chronic sympathetic denervation upon the transcapillary filtration rate induced by venous stasis

The effect of venous pressure elevation upon capillary filtration rate in the limb was studied in 6 chronically sympathectomized patients. Five healthy subjects served as controls. Volume changes of the forearm or calf were recorded by a strain-gauge plethysmograph. Relative blod flow in subcutaneous and muscle tissue during venous stasis was measured by the local ¹³³Xe washout technique. In the denervated limbs there was a linear relationship between net capillary filtration rate and venous pressure elevation. In the controls a non-linear relationship was seen as venous pressure elevation of 40 mmHg only caused an increase in net filtration rate of about 66% of that expected from a linear relationship. In the denervated limbs blood flow in muscle and subcutaneous tissue remained constant during venous pressure elevation of more than 30 mmHg whereas in the non-denervated limbs blood flow decreased by about 50% in both tissues. The results suggest that a local sympathetic veno-arteriolar (axon) reflex plays a dominant role for the reduced increase in net capillary filtration rate during large increases in venous pressure. The local axon reflex may therefore act as an edema protecting factor.     

Effects of small changes of plasma vasopressin on subcutaneous and skeletal muscle blood flow in man

The effect of vasopressin (AVP) on subcutaneous blood flow was studied by the 133Xenon wash-out method in 13 healthy subjects during three consecutive infusions of synthetic AVP, using increasing infusion rates. In seven of them, both subcutaneous and skeletal muscle blood flows were measured during the first infusion. The preinfusion, and infusion pAVP levels were 1.6 +/- 0.4, 3.4 +/- 0.4, 4.9 +/- 0.5 and 8.8 +/- 0.7 pg ml-1, respectively (mean +/- SE). The values are within the range normally found during dehydration. During the AVP infusions, the blood flow in subcutaneous tissues decreased 30-40% and the vascular resistance increased 60-80%. Neither heart rate nor blood pressure change significantly during the infusions. Plasma renin activity (PRA) decreased significantly. After cessation of the infusions, blood flow and vascular resistance rapidly returned to preinfusion values, while PRA increased very slowly. Skeletal muscle and subcutaneous tissues blood flows were found to be equally sensitive to small changes in the pAVP level. The present study has demonstrated that even minor increments of pAVP levels, as seen during dehydration, can significantly alter the regional blood flow in subcutaneous and skeletal muscle tissues in man.     

Role of vascular bed compliance in vasomotor control in human skeletal muscle

The current view of neurogenic vasomotor control in skeletal muscle is based largely on changes in vascular bed resistance. The purpose of this study was to determine to what extent vascular bed compliance may also play a role in this regulation. For this purpose, pressure waveforms (Millar and Finometer) and flow waveforms (Doppler ultrasound) were measured simultaneously in the brachial artery of seven healthy individuals during physiological manoeuvres which were expected to produce non-neurogenic changes in resistance (wrist-cuff occlusion; n = 5) or compliance (arm elevation; n = 6) of the forearm vascular bed. Vascular resistance (R) was calculated from the average flow and pressure values. A lumped Windkessel model was used to obtain vascular bed compliance (C) from these concurrently measured waveforms. Compared with baseline (3.81 +/- 1.59 ml min(-1) mmHg(-1)), wrist occlusion increased R (65 +/- 75%; P < 0.05) with minimal change in C (-15 +/- 16%; n.s.). Compared with the arm in neutral position (0.0075 +/- 0.003 ml mmHg(-1)), elevation of the arm above heart level produced a 86 +/- 41% increase in C (P < 0.05) with little change in R (-5 +/- 11%). In addition, neurogenic changes were assessed during lower body negative pressure (LBNP) and a cold pressor test (CPT; n = 7). Lower body negative pressure induced a 29 +/- 24% increase in R and a 26 +/- 12% decrease in C (both P < 0.05). The CPT induced no consistent change in R but a 22 +/- 7% reduction in C (P < 0.05). It was concluded that vascular bed compliance is an independent variable which should be considered along with vascular bed resistance in the mechanics of vasomotor regulation in skeletal muscle.     

Effect of Carotid Sinus Stimulation on Cardiac Output and Peripheral Vascular Resistance during Changes in Blood Volume Distribution in Man

In ten healthy subjects (mean age 29.6 years) the hemodynamic response to carotid sinus stimulation (neck suction - 40 mmHg) was studied under control conditions and during peripheral pooling of blood (lower body negative pressure). Heart rate, arterial and central venous pressure, cardiac output and forearm blood flow were measured. The time sequence of the heart rate response was studied separately in six healthy subjects. During control conditions, carotid sinus stimulation induced a significant decrease in arterial pressure and heart rate. The blood pressure decrease mainly reflected a reduction in cardiac output, total peripheral vascular resistance being essentially unchanged. However, in the skeletal muscle, represented by a forearm segment, vascular resistance decreased significantly. During lower body negative pressure (LBNP) the same stimulation of the carotid sinus induced a significantly greater fall in mean arterial pressure even though the reduction in cardiac output was slightly smaller on the average than in the control condition. The heart rate increased, probably secondary to a time dependent increase in heart rate elicited by the continuous LBNP stimulus. Total peripheral vascular resistance decreased significantly during LBNP, the reaction likewise differing significantly from that in the control condition. Thus the augmented blood pressure response was due to a more pronounced vasodilatation when the carotid sinus was stimulated during lower body negative pressure. The results indicate that the hemodynamic changes elicited by carotid sinus stimulation are modified by changes in the distribution of blood volume and in the tone of resistance vessels.     

Local and central sympathetic reflex control of blood flow in skeletal muscle and subcutaneous tissue in normal man

The effect of age and sex on relative changes in blood flow and vascular resistance in skeletal muscle and subcutaneous tissue during postural changes and during local increase in transmural pressure was studied in 33 healthy subjects. The intra-individual variation was studied in five subjects. Blood flow was measured by the local 133Xenon wash-out method. No relation to age or sex was seen in the centrally elicited sympathetic vasoconstrictor responses in subcutaneous tissue and skeletal muscle and in the locally elicited vasoconstriction in subcutaneous tissue. A small, but statistically significant, correlation to sex and age was found in the local sympathetic vasoconstrictor response in skeletal muscle. The age correlation was caused only by an attenuated response in the young subjects below 40 years of age and may be fortuitous. The intra-individual variation was acceptably small. Based on the present results, a reduction in blood flow in skeletal muscle and subcutaneous tissue during centrally or locally elicited sympathetic vasoconstriction of 10% or less should be considered abnormal. The local 133Xenon wash-out method is of value in examining patients suspected of dysfunction in the sympathetic part of the autonomic nervous system.     

Forearm vascular responses to combined muscle metaboreceptor activation in the upper and lower limbs in humans

Our previous studies showed that venous occlusion or passive stretch of the lower limb, assuming a mechanical stimulus, attenuates the vasoconstriction in the non-exercised forearm during postexercise muscle ischaemia (PEMI) of the upper limb. In this study, we investigated whether a metabolic stimulus to the lower limb induces a similar response. Eight subjects performed a 2 min static handgrip exercise at 30% maximal voluntary contraction (MVC) followed by 3 min PEMI of the upper limb, concomitant with or without 2 min static ankle dorsiflexion at 30% MVC followed by 2 min PEMI of the lower limb. During PEMI of the upper limb alone, forearm blood flow (FBF) and forearm vascular conductance (FVC) in the non-exercised arm decreased significantly, whereas during combined PEMI of the upper and lower limbs, the decreases in FBF and FVC produced by PEMI of the upper limb was attenuated. Forearm blood flow and FVC were significantly greater during combined PEMI of the upper and lower limbs than during PEMI of the upper limb alone. When PEMI of the lower limb was released after combined PEMI of the upper and lower limbs (only PEMI of the upper limb was maintained continuously), the attenuated decreases in FBF and FVC observed during combined PEMI of the upper and lower limbs was not observed. Thus, forearm vascular responses differ when muscle metaboreceptors are activated in the upper limb and when there is combined activation of muscle metaboreceptors in both the upper and lower limbs.     

Effect of Spinal Sympathetic Blockade upon Local Regulation of Blood Flow in Subcutaneous Tissue

The influence of spinal sympathetic blockade upon local regulation of blood flow in subcutaneous adipose tissue was investigated in six subjects. The effect of changes in orthostatic pressure on blood flow in subcutaneous tissue in the crus and distal forearm was measured before and after sympathetic blockade obtained by epidural anesthesia in 4 subjects and by a bilateral sympathectomy in 2 patients suffering from manual hyperhidrosis. Blood flow in subcutaneous tissue measured by ¹³³Xe washout technique decreased by about 40 per cent when the limb was lowered, and remained constant during 30 cm elevation. This was found both before and after the blockade, though in one of the patients, the orthostatic decrease in blood flow was less pronounced 24 h after sympathectomy. Hence central sympathetic reflexes do not alter local orthostatic changes of blood flow in subcutaneous tissue. These changes therefore are most likely due to local mechanisms.     

Carotid baroreceptor reflexes in humans during orthostatic stress

Orthostatic stress, including standing, head-up tilting and lower body suction, results in increases in peripheral vascular resistance but little or no change in mean arterial pressure. This study was undertaken to determine whether the sensitivity of the carotid baroreceptor reflex was enhanced during conditions of decreased venous return. We studied eight healthy subjects and determined responses of pulse interval (ECG) and forearm vascular resistance (mean finger blood pressure divided by Doppler estimate of brachial artery blood velocity) to graded increases and decreases in carotid transmural pressure, effected by a neck suction/pressure device. Responses were determined with and without the application of lower body negative pressure (LBNP) at -40 mmHg. Stimulus-response curves were determined as the responses to graded neck pressure changes and the differential of this provided estimates of reflex sensitivity. Changes in carotid transmural pressure caused graded changes in R-R interval and vascular resistance. The cardiac responses were unaffected by LBNP. Vascular resistance responses, however, were significantly enhanced during LBNP and the peak gain of the reflex was increased from 1.2 +/- 0.3 (mean +/- S.E.M.) to 2.2 +/- 0.3 units (P < 0.05). The increased baroreflex gain may contribute to maintenance of blood pressure during orthostatic stress and limit the pressure decreases during prolonged periods of such stress.     

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.     

Arterial pressure-blood flow relations during limb elevation in man

The effect of local arterial hypotension upon blood flow was studied in 13 healthy humans. Blood flow was measured by the local 133Xe washout technique in the calf or foot region. Changes in arterial pressure was achieved by elevation of the leg above heart level, while the subject rested in supine position. In papaverine-infiltrated subcutaneous tissue, local arterial pressure and blood flow was linearly related. In subcutaneous and skeletal muscle tissue with normal vasomotor tone, blood flow remained constant during a decrease in regional arterial pressure of 30 mmHg--autoregulation of blood flow. Blood flow ceased in vasoparalysed tissue as well as in tissue with normal vasomotor tone, when the local diastolic arterial pressure was reduced to zero. The elevated foot showed signs of vascular ischaemia during these conditions. When the vessels in the ischaemic foot were distended by inflating a femur cuff to a pressure level above the diastolic blood pressure beneath the cuff, the colour of the foot changed into bluish-red and a substantial blood flow was recorded. It is concluded, that the arteries-arterioles seemed to be collapsed with blood flow cessation, when the diastolic portion of a pulsatile inflow pressure was reduced to zero. Vascular distension seemed to reopen the arterial-arteriolar collapse, and during these conditions the vascular bed offered only little resistance to the systolic peak injection.     

Effect of lower body negative pressure upon local regulation of blood flow in human subcutaneous tissue

Local regulation of subcutaneous blood flow in the forearm was studied during lower body negative pressure (LBNP) in 7 young healthy male subjects in supine position. Blood flow was measured on the forearm by the local ¹³³Xe washout technique. LBNP of -40 and -60 mmHg induced a decrease in the ¹³³Xe washout rate of 34 and 50% respectively. This response to LBNP could be blocked by proximal nervous blockade indicating that the vasoconstriction observed was due to a central sympathetic reflex mechanism. The vasoconstrictor response to increase in venous transmural pressure induced by lowering the arm (veno-arteriolar reflex mechanism) could not be demonstrated during 40 mmHg LBNP. The abolishment of this reflex is most likely due to centrally elicited increase in sympathetic activity as a normal veno-arteriolar reflex was elicited following proximal nervous blockade.     

Comparison of the effects of stimulation of the sympathetic vasomotor nerves and the adrenal medullary catecholamines on the hind limb blood vessels of the rabbit

1. The responses to sympathetic nerve stimulation and to the adrenal medullary hormones have been studied in the hind limb vascular beds of the anaesthetized rabbit.2. Simultaneous measurements of femoral arterial blood pressure and of femoral venous blood flow indicate that stimulation of the sympathetic nerves decreases the calculated vascular conductance in both the intact and skinned hind limbs. Evidence is presented to show that these changes are due to vasoconstriction.3. The vasoconstriction in both skin and muscle vascular beds reaches a maximum at frequencies of stimulation around 15 Hz. No vasodilatation is obtained at any frequency of stimulation.4. The rabbit adrenal gland secretes only adrenaline during splanchnic nerve stimulation at frequencies between 3 and 60 Hz. The amounts liberated from both glands over this frequency range are 25-500 ng.kg body wt.(-1) min(-1).5. Intravenous infusions of adrenaline in concentrations similar to those liberated by the adrenal glands during splanchnic nerve stimulation, and of noradrenaline, cause only vasoconstrictor responses in skin and muscle.6. Simultaneous stimulation of the sympathetic nerves to the hind limb and infusion of adrenaline in quantities that could be liberated by splanchnic nerve stimulation at equivalent frequencies shows that the vasoconstrictor effects exerted by the individual components are additive, though the effects produced by the direct sympathetic nerve supply overshadow those produced by the catecholamine.7. The results are discussed in the context of the possible vascular role of the adrenal medullary hormones in the rabbit.