The overall goal of this research is to improve the use of seated tilt to increase function, health and quality of life for people using power wheelchairs. Specifically, the objective of this dissertation is to evaluate the biomechanical responses to seated full body tilt and their relationships to the actual use of tilt-in-space wheelchairs. In the first phase of this study, researchers remotely monitored how 45 fulltime power wheelchair users used their tilt-in-space systems. Participants spent an average of 12.1 hours in their wheelchair each day. They spent more than 2 hours seated at positions greater than 15° and performed tilts of 5° or greater every 27 minutes, but rarely performed tilts past 30°. Two distinct types of tilt behavior were identified: uni-modal (staying at a single position more than 80% of the time) and multi-modal (staying at a single position less than 80% of the time). Participants in the multi-modal group tilted significantly more frequently (4 times per hour) than the uni-modal group, and did not have a single typical position. Participants without sensation were more likely to exhibit uni-modal behavior. In the second phase of this study, researchers used interface pressure measurements and laser Doppler flowmetry to study changes in localized loading and superficial blood flow at the ischial tuberosities across different amounts of tilt. Eleven participants with spinal cord injuries were studied in a laboratory setting. Results showed that biomechanical responses to tilt were highly variable. Pressure reduction at the ischial tuberosity was not present at 15°, but did occur with tilts to 30° and greater, and could be explained by the tilt position and upright pressure. Unlike pressure, blood flow increased with all tilts from an upright position, but did not increase when tilting from 15° to 30°. Only 4 of 11 participants had a considerable increase (≥10%) in blood flow at 30° tilt, whereas 9 participants did during maximum tilt (i.e., 45°-60°). Based on the results of this study, tilting for pressure reliefs as far as the seating system permits is recommended to maximize the potential for significant blood flow increases and pressure relief.

Introduction: Oxygen delivery, or utilization, is a function of retinal blood flow and blood oxygen saturation. The retinal pigment epithelium (RPE), in particular, has been shown to have the highest levels of metabolic activity within the human body. Oxygen delivery is therefore of extreme importance to the maintenance of the health and integrity of the retina. Animal models presuppose that the oxygen tension in the retina is highest in the innermost layers at the level of the choriocapillaris, less in the photoreceptors and further decreases throughout the outer retinal structures. The choroid provides by far the largest component of the oxygen for consumption by the photoreceptors. A lack of oxygen stores in the inner retina therefore makes a constant supply crucial for its normal functioning. Blood flow dysfunction and subsequent hypoxia are both a feature in the pathogenesis of several major ocular diseases such as retinopathy of prematurity (ROP), age-related macular degeneration (ARMD), diabetic retinopathy (DR) and glaucoma. The development of methods to measure retinal blood flow and blood oxygen saturation is crucial to improve understanding of the patho-physiology of major ocular diseases. Purpose: The aims of this work were, firstly, to determine the least variable (range ± standard deviation) wavelength combination (610/548, 600/569 and 605/586) and subsequent ODR with the prototype HRC device. Secondly, using the ODR with the lowest measurement variability, we sought to quantify retinal blood SO2 in arterioles and venules and investigate the relationship between retinal blood SO2 and total retinal blood flow (TRBF) in response to stepwise changes in PETO2 in healthy participants. Retinal blood SO2 and TRBF were assessed using the IRIS HRC (Photon etc. Inc. Montreal, Canada) and the RTvue Doppler Fourier Domain OCT (Optovue Inc, Freemont, CA) instruments, respectively. Methods: Ten healthy participants between the ages of 23 and 37, with an average age of 28.3 years were evaluated in two descriptive cross-sectional studies. Two gas provocation protocols; hyperoxia (end-tidal oxygen; PETO2 of 100, 200, 300, 400mmHg) and hypoxia (PETO2 of 100, 80, 60, 50mmHg) were administered in a fixed sequential order. In each phase of gas provocation (via modulation of PETO2), retinal blood SO2 and TRBF measurements were acquired with the HRC and Doppler FD-OCT. The precise and repeated control of the partial end tidal pressures of oxygen (PETO2) and carbon dioxide (PETCO2) over the pre-determined phase duration, irrespective of the individuals’ respiratory rate, was made possible with the RespirAct (Thornhill Research Inc., Toronto, Canada); a sequential re-breathing gas delivery Results: In arterioles, the group range (±SD) of ODR values for baseline measurements (PETO2 of 100mmHg) was 0.169±0.061 for the 605/586 wavelength combination, 0.371±0.099 for the 600/569 wavelength combination and 0.340±0.104 for the 610/548 wavelength combination. In venules, the group range (±SD) of ODR values was 0.600±0.198 for the 605/586 wavelength combination, 0.569±0.169 for the 600/569 wavelength combination and 0.819±0.274 for the 610/548 wavelength combination. With the 605/586 combination at baseline 1 and 2 in arterioles, the group range (±SD) of ODR values was 0.607 ± 0.224 and 0.619 ± 0.158, respectively (p = 0.370), while in venules the group range (±SD) of ODR at baseline 1 and 2 was 0.289±0.750 and 0.284 ± 0.729, respectively (p = 0.714). For the 600/569 combination at baseline 1 and 2 in arterioles, the group range (±SD) of ODR values was 0.747±0.350 and 0.761±0.391, respectively (p = 0.424) while in venules the group range (±SD) of ODR at baseline 1 and 2 was 0.329±0.675 and 0.366±0.659, respectively (p = 0.372). For the 610/548 combination at baseline 1 and 2 in arterioles, the group range (±SD) of ODR values was 0.604±0.263 and 0.685±0.450, respectively (p = 0.056) while in venules, the group range (±SD) of ODR at baseline 1 and 2 was 0.292±0.746 and 0.285±1.009, respectively (p = 0.131). There was no statistical difference found between baseline ODR values (baseline 1 and 2) across all three wavelength combinations in both arterioles and venules. The mean retinal blood SO2 value at baseline in arterioles for 4 participants was 95.19% ± 31.04% and venules was 53.89% ± 17.24% (p = 0.115). There was a negative linear relationship between group retinal blood SO2 and TRBF values in the 10 participants studied, although the results of any of the 10 individuals did not show evidence of such a relationship using the described methodology. The Pearson’s correlation coefficient (r) between TRBF and SaO2 was r = -0.354 and p = 0.001 and between TRBF and SvO2 was r = - 0.295, p = 0.008 Conclusion: Of the three wavelength combinations investigated (605/586, 600/569 and 610/548), the 605/586 combination was shown to have the overall least variability. It would be unwise at this stage to adopt this wavelength combination for clinical usage, however, since it is presupposed that the 605/586 combination is also the most reliable combination to detect change in retinal blood SO2 i.e. lower variability of the 605/586 combination may be irrelevant if this combination proves to be insensitive to change in retinal blood SO2. The absolute mean ± SD retinal blood SO2 in the arterioles (SaO2) was 95.19% ± 31.04% and in the venules (SvO2) was 53.89% ± 17.24%. These values fell within the range expected and described in the literature. The magnitude of the difference between the SaO2 and SvO2 was also consistent with the literature. These findings were all appropriate for a low flow, high oxygen exchange vascular network typical of the inner retinal vascular system. Using group rather than individual data, TRBF was found in this study to relate inversely with SaO2 (r = -0.354 and p = 0.001) and SvO2 (r = – 0.295 and p=0.008), respectively. This relationship between TRBF and SaO2 and SvO2, was as expected based upon data derived primarily from animal models. This study is ground-breaking and unique, in that, it is the first study to concomitantly measure both retinal blood SO2 and TRBF in human participants. Individual data showed extensive variability and noise, thus limiting the strength of the association between TRBF and SaO2 and SvO2..

Master of Science / Department of Kinesiology / Thomas J. Barstow / Dietary nitrate supplementation via beetroot juice (BR) has been shown to have positive effects on mitochondrial and muscle efficiency during large muscle mass exercise in humans, and more recently on locomotory muscle blood flow [Q-dot] in rats. To date, an integrated measure of these effects has not been performed in humans. Therefore, we assessed the influence of BR on [Q-dot] and muscle oxygenation characteristics during moderate and severe intensity handgrip exercise. Seven healthy men (age: 25 ± 3 yrs; height: 179 ± 4 cm; weight: 82 ± 9 kg) completed four constant-power exercise tests randomly assigned to condition (BR or placebo (PL)) and intensity (moderate (40% peak) or severe (85% peak)). Resting mean arterial pressure was significantly lower after BR compared to PL (79.3 ± 5.8 vs 86.8 ± 6.7 mmHg; p < 0.01). All subjects were able to sustain 10 min of exercise at moderate intensity in both conditions. BR had no significant effect on exercise tolerance during severe (342 ± 83 vs 382 ± 138 s, p = 0.382). Brachial artery [Q-dot] was not significantly different after BR at rest or any time during exercise in either intensity. Deoxygenated-[hemoglobin + myoglobin] was elevated at min 2 & 3 for moderate (p < 0.05) and throughout severe exercise (p = 0.03) after BR. The estimated metabolic cost ([V-dot]O₂) was not significantly different during either intensity after BR. These findings support the notion that an acute dose of BR may be valuable to reduce blood pressure in young adults, but revealed that it does not augment [Q-dot] or [V-dot]O₂ during small muscle mass handgrip exercise.

Dissertation submitted in partial compliance with the requirements for the Master’s Degree in Technology: Chiropractic Durban University of Technology, 2009 / Aim: To investigate the effect of therapeutic and sham ultrasound on radial artery blood flow (m.s-1) and radial arterial lumen diameter (mm). Subjects: Fifty healthy asymptomatic volunteers between the ages of 18-38 years. Methodology: The subjects were randomly allocated into one of five intervention groups (A-E). Group A received continuous ultrasound at 0.2 W.cm-² for 5 minutes, Group B received pulse ultrasound at 0.2 W.cm-² for 5 minutes, Group C received continuous ultrasound at 1.5 W.cm-² for 5 minutes, Group D received pulse ultrasound at 1.5 W.cm-² for 5 minutes and Group E received sham ultrasound at 0 W.cm-² for 5 minutes. Baseline radial artery blood flow (m.s-1) and radial artery lumen diameter (mm) readings were taken prior to the commencement of the therapeutic or sham ultrasound application using a Doppler ultrasound. At four minutes of application (during the therapeutic or sham ultrasound application), another set of blood flow and arterial lumen diameter measurements were taken. The final blood flow and arterial lumen diameter measurements were taken one minute after the therapeutic or sham ultrasound application was stopped. Results: The mean (± SD) radial artery blood flow and radial artery lumen diameter at baseline was 0.197 (± 0.060) m.s-1 and 2.4 (± 0.6) mm respectively. In Group A, the mean (± SD) radial artery blood flow during ultrasound application and one-minute after ultrasound application was 0.193 (± 0.070) m.s-1 and 0.179 (± 0.073) m.s-1 respectively. The mean (± SD) radial artery lumen diameter in Group A at the two time intervals was 2.2 (± 0.5) mm and 2.2 (± 0.3) mm respectively. In Group B, the mean (± SD) radial artery blood flow during ultrasound application and one-minute after ultrasound application was 0.187 (± 0.067) m.s-1 and 0.195 (± 0.041) m.s-1 respectively. The mean (± SD) radial artery lumen diameter in Group B at the two time intervals was 2.4 (± 0.4) mm and 2.3 (± 0.5) mm respectively. In Group C, the mean (± SD) radial artery blood flow during ultrasound application and one-minute after ultrasound application was 0.225 (± 0.088) m.s-1 and 0.186 (± 0.071) m.s-1 respectively. The mean (± SD) radial artery lumen diameter in Group C at the two time intervals was 2.4 (± 0.7) mm and 2.7 (± 0.8) mm respectively. In Group D, the mean (± SD) radial artery blood flow during ultrasound application and one-minute after ultrasound application was 0.215 (± 0.080) m.s-1 and 0.200 (± 0.081) m.s-1 respectively. The mean (± SD) radial artery lumen diameter in Group iv D at the two time intervals was 2.4 (± 0.8) mm and 2.4 (± 0.7) mm respectively. In Group E, the mean (± SD) radial artery blood flow during ultrasound application and one-minute after ultrasound application was 0.200 (± 0.067) m.s-1 and 0.182 (± 0.075) m.s-1 respectively. The mean (± SD) radial artery lumen diameter in Group E at the two time intervals was 2.5 (± 0.7) mm and 2.3 (± 0.5) mm respectively. There was no significant change in radial artery blood flow and radial artery lumen diameter over time in any individual group or between groups (p > 0.05; repeated measures ANOVA). There was an overall weak positive correlation between radial artery blood flow and radial artery lumen diameter at baseline (r = 0.508), during (r = 0.541) and after (r = 0.532) the therapeutic or sham ultrasound application. Conclusion: The results of this study showed that continuous, pulse or sham ultrasound had no significant effect on radial artery blood flow and radial artery lumen diameter. Furthermore, active ultrasound (continuous and pulse) was not superior to sham ultrasound in significantly affecting blood flow in a muscular artery.

The structural integrity of the white matter is required for neuronal communication within the brain which is essential for normal cognitive function. Post-mortem and clinical imaging studies of elderly individuals have demonstrated that white matter integrity is weakened with increasing age which is proposed to underlie age-related cognitive decline. Whilst the exact mechanisms are unknown it is thought that modest age-related reductions in cerebral blood flow, termed chronic cerebral hypoperfusion, may contribute to white matter disruption and impaired cognition with ageing. Investigating the effects of white matter integrity in humans is limited as it is difficult to definitively ascertain a cause and effect relationship. Indeed, elderly individuals with cerebral hypoperfusion often have co-existing disease such as hypertension thus the effects of hypoperfusion in isolation cannot be determined. This has led to the development of a mouse model of chronic cerebral hypoperfusion which provides the opportunity to directly assess whether cerebral hypoperfusion results in disruption to white matter and cognitive impairment. This is achieved by applying small wire coils around both common carotid arteries of the mouse resulting in a global reduction in cerebral blood flow. Importantly the extent of blood flow reduction is dependent on the internal diameter of the coils meaning that differing severities of hypoperfusion can be studied. Previous studies using this model have demonstrated diffuse white matter pathology in white matter tracts including the corpus callosum, internal capsule and optic tract following 1 month of hypoperfusion which is accompanied by impaired spatial working memory. This thesis sought to test the hypothesis that chronic cerebral hypoperfusion would influence the structural integrity of nodal and paranodal domains of myelinated axons of the white matter and result in decreased numbers of oligodendroglial cells. It was additionally hypothesised that treatment with the anti-inflammatory and antioxidant drug dimethyl fumarate (DMF) would ameliorate structural and functional alterations to white matter following hypoperfusion. Aim 1 – To determine the impact of chronic cerebral hypoperfusion on the structural integrity of nodal and paranodal domains of myelinated axons The first aim of this thesis was to investigate the effects of chronic cerebral hypoperfusion on the structural integrity of nodal and paranodal domains of myelinated axons. This was addressed by examining key myelin and axonal proteins found at nodal, paranodal and internodal domains. This revealed significant alterations to the distribution of voltage-gated sodium (Nav1.6) channels at nodes of Ranvier which were differentially altered in response to increasing durations of chronic cerebral hypoperfusion. Specifically an increase in the Nav1.6+ domain length was observed in the corpus callosum following 3 days (p < 0.0001) and 1 month (p < 0.001) of chronic cerebral hypoperfusion but was not significantly different from sham controls following 6 weeks of hypoperfusion (p = 0.066). A significant decrease in Nav1.6 domain length was observed following 3 months of hypoperfusion (p = 0.003). Assessment of paranodal integrity was carried out by measuring nodal gap length and by ultrastructural analysis of paranodal domains. This revealed pronounced alterations to nodal gap length, loss of paranodal septate-like junctions and abnormal morphology of paranodal loops. Furthermore this study revealed a significant loss of myelin associated glycoprotein, a key protein involved in the maintenance of axon-glial integrity, as early as 3 days following the onset of hypoperfusion. A further aim of this study was to examine potential mechanisms underlying the observed alterations to nodal and paranodal domains following cerebral hypoperfusion. It was hypothesised that increased inflammation and accumulation of mitochondria at nodes of Ranvier would be observed following hypoperfusion. The extent of inflammation was assessed by counting numbers of microglia which revealed no significant difference between groups following 3 days of hypoperfusion (p = 0.425) but a significant increase in microglial number was observed following 1 month of hypoperfusion (p = 0.001). In addition, assessment of mitochondrial distribution along myelinated axons revealed decreased numbers of nodes containing mitochondria following 6 weeks of hypoperfusion (p = 0.03) with no difference between groups observed following 3 months (p = 0.742). Taken together the results from this study provide evidence that chronic cerebral hypoperfusion results in dynamic alterations in the localisation of Nav1.6 channels which are accompanied by disruption to paranodal domains and impaired axon-glial integrity. Furthermore microglial number does not appear to mediate nodal and paranodal disruption following 3 days but may contribute to ongoing pathology following 1 month of chronic cerebral hypoperfusion. Aim 2 – To determine the effects of chronic cerebral hypoperfusion on oligodendroglial populations. The second aim of this thesis was to determine the effect of chronic cerebral hypoperfusion on numbers of mature oligodendrocytes and oligodendrocyte precursor cells (OPCs). This revealed a significant decrease in numbers of both populations following 3 days of cerebral hypoperfusion however following 1 month numbers of OPCs were restored and a significant increase in mature oligodendrocyte number was observed. Assessment of OPC proliferation demonstrated low numbers of proliferating cells but revealed that a proportion of newly generated cells had differentiated into mature oligodendrocytes. To determine a potential mechanism involved in OPC differentiation following cerebral hypoperfusion the expression of the GPR17 receptor was examined which has recently been reported to mediate OPC differentiation in response to injury. The results demonstrated decreased expression of GPR17 following 3 days of hypoperfusion (p = 0.007) with no difference between groups observed following 1 month (p = 0.362) indicating that this receptor is not involved in differentiation of OPCs following hypoperfusion. Taken together the results from this study show that mature oligodendrocytes and OPCs are lost early in response to hypoperfusion but that these cells recover over time, highlighting the regenerative capacity of the white matter following cerebral hypoperfusion.Aim 3 – To investigate whether modulation of inflammation and oxidative stress could ameliorate alterations to white matter structure and function following severe chronic cerebral hypoperfusion The third and final aim of this thesis was to determine whether treatment with the anti-inflammatory and antioxidant drug DMF could ameliorate structural and functional alterations to white matter following severe chronic cerebral hypoperfusion. This was achieved by examining myelin and axonal integrity in addition to numbers of oligodendrocytes and OPCs following 7 days of severe chronic cerebral hypoperfusion. This revealed that myelin integrity was significantly decreased in vehicle-treated hypoperfused animals as compared to shams (p = 0.005). However no differences in myelin integrity were observed between sham and hypoperfused mice treated with DMF (p = 0.312). In contrast to the previous study, numbers of oligodendrocytes and OPCs were not altered following severe hypoperfusion however DMF treatment led to significantly increased numbers of oligodendrocytes in sham animals (p = 0.003). Assessment of white matter function using electrophysiology revealed that the conduction velocity of myelinated axons was significantly increased in DMF-treated hypoperfused animals as compared to those treated with vehicle (p = 0.04). Taken together the results of this study demonstrate that modulation of inflammation and oxidative stress may improve structural and functional white matter alterations following chronic cerebral hypoperfusion. Conclusions: The results presented in this thesis demonstrate that chronic cerebral hypoperfusion results in structural alterations to myelinated axons and to oligodendroglial populations within the white matter which are accompanied by impaired spatial working memory. Whilst previous studies using the model have reported that cerebral hypoperfusion results in diffuse white matter pathology, this study has highlighted the vulnerability of nodal and paranodal domains of myelinated axons as regions which are altered early in response to hypoperfusion. Furthermore, characterisation of oligodendroglial populations has revealed that these cells are replaced over time despite ongoing hypoperfusion which demonstrates the regenerative capacity of the white matter following cerebral hypoperfusion. Critically the results presented in this thesis demonstrate that treatment with DMF improved the function of myelinated axons in response to severe reductions in cerebral blood flow and thus may represent an appropriate therapeutic strategy for chronic cerebral hypoperfusion.

Time-resolved velocity information of carotid blood flow can be used to estimate haemodynamic conditions associated with carotid artery disease leading to stroke. MRI provides high-resolution measurement of such information but long scan time limits its clinical application in this area. In order to reduce scan time the MRI signal is often undersampled by skipping part of the signal during data acquisition. The aim of this work is to implement and evaluate different undersampling techniques for carotid velocity measurement on a 1.5 T clinical scanner. Most recent undersampling techniques assume spatial and temporal redundancies of real time-resolved MRI signal. In these techniques different undersampling strategies were proposed. Prior information or different assumptions of the nature of true signal were used in signal reconstruction. A brief review of these techniques and details of a representative technique, known as k-t BLAST, are presented. Another undersampling scheme, termed ktVD, is proposed to use predesigned undersampling patterns with variable sampling densities in both temporal and spatial dimensions. It aims to collect enough signal content at the signal acquisition stage and simplify signal reconstruction. Fidelity of the results from undersampled data is affected by many factors, such as signal dynamic content, degree of signal redundancy, noise level, degree of undersampling, undersampling patterns, and parameters of post-processing algorithms. Simulations and in vivo scans were conducted to investigate the effects of these factors in time-resolved 2D scans and time-resolved 3D scans. The results suggested velocity measurement became less reliable when they were obtained from less than 25% of the full signal. In time-resolved 3D scans the signal can be undersampled in either one or two spatial dimensions in addition to the temporal dimension. This allows more options in the design of undersampling patterns, which were tested in vivo. In order to test undersampling in three dimensions in high resolution 3D scans and measure velocity in three dimensions, a flow phantom was also scanned at high degrees of undersampling to test the proposed method.

published_or_final_version / abstract / Medicine / Master / Doctor of Medicine

No description available.

In these experiments, we tested the hypothesis that sympathetic escape in skeletal muscle is mediated through a fall in parenchymal cell oxygen levels following blood flow reduction. This hypothesis predicts that if the fall in parenchymal cell PO₂ during stimulation can be minimized, escape should be reduced. To test this prediction, we studied the behavior of superficial arterioles of the cat sartorius muscle during 3 minutes of sympathetic nerve stimulation. The muscle was covered with silicone oil equilibrated with 0%, 5% and 10% oxygen. During stimulation under 0% oxygen, 90% of visible arterioles showed a significant secondary relaxation (escape). The relaxation averaged 55% of the initial constriction. Under 5% oxygen, resting arteriolar diameter was reduced by an average of 12% and escape was significantly reduced throughout the arteriolar network. Under 10% ambient oxygen, there was an additional 5% reduction in resting diameter and a further reduction of escape. Escape was not attenuated when control diameter was reduced to the same degree with arginine vasopressin, suggesting that the effect of oxygen was specific rather than secondary to an increase in vascular tone. The above observations are also consistent with the hypothesis that escape is mediated through a fall in vascular wall PO₂. To evaluate this possibility, periarteriolar and parenchymal tissue PO₂ were measured with oxygen microelectrodes during sympathetic stimulation under 0% and 10% oxygen suffusion of the muscle. In the proximal arterioles, the periarteriolar PO₂ during control and during stimulation was identical under 0% and 10% oxygen yet escape was reduced by 75% under 10% oxygen. Similarly, escape was reduced 90% in the distal arterioles under 10% oxygen but periarteriolar PO₂ was very nearly the same as that measured under 0% oxygen. In contrast, mean parenchymal tissue PO₂ fell to low levels during stimulation under 0% oxygen but did not fall below normal levels during stimulation under 10% oxygen. These findings argue against the hypothesis that a fall in vascular wall PO₂ is responsible for escape. The findings are consistent with the hypothesis that sympathetic escape in skeletal muscle is mediated through a fall in parenchymal cell PO₂. (Abstract shortened with permission of author.)

Evidence suggests different sections of the arteriolar network supplying muscle can respond independently and this may provide a mechanism for the localized distribution of blood flow. This hypothesis was tested in the microcirculation of the cat sartorius muscle by measurement of arteriolar diameter changes during muscle contraction and sympathetic nerve stimulation in each consecutive section of the network. The diameter changes were referenced to the initial distribution of resistance across the network, as determined from arteriolar pressure measurements and morphometric data. This led to an estimate of the change in network resistance. Unlike previous reports, the most distal arterioles dilated little during muscle contraction and our resistance estimate indicates these vessels play an insignificant role in functional hyperemia. The more proximal, third order arterioles dilated proportionately more than other arteriolar orders and made the largest single contribution to resting resistance. Similarly, these vessels were the largest single site of resistance change during sympathetic stimulation. Together, these findings suggest the third order arterioles play a dominant role in regulating flow to the capillaries that each supplies. Antagonism of sympathetic control during muscle contraction has been attributed to direct inhibition of vascular smooth muscle contraction and to inhibition of sympathetic neurotransmission. Evidence to support the latter mechanism comes from the observation that functional dilation is reduced with exogenous norepinephrine as compared to sympathetic stimulation. However, exogenous norepinephrine may bind to both alpha-1 and alpha-2 adrenergic receptors, whereas that released by sympathetic stimulation may bind primarily to alpha-1 receptors. Since this difference could be significant, functional dilation after systemic injection of norepinephrine or phenylephrine, a selective alpha-1 agonist, was compared to that during sympathetic stimulation. In contrast to the findings with norepinephrine, functional dilation after phenylephrine did not differ from that observed during sympathetic stimulation. This indicates the dilator substance(s) released during exercise may selectively inhibit alpha-1 mediated vasoconstriction but less effectively inhibit vasoconstriction mediated by alpha-2 receptors. Furthermore, these findings suggest that the vasodilator mechanism may act primarily at the level of the vascular smooth muscle, without appreciable pre-junctional inhibition of sympathetic nerves.