Post-junctional alpha-adrenoceptors and basal limb vascular tone in healthy men

Genetic and functional analysis of Raynaud's syndrome implicates loci in vasculature and immunity

Raynaud's syndrome is a dysautonomia where exposure to cold causes vasoconstriction and hypoxia, particularly in the extremities. We performed meta-analysis in four cohorts and discovered eight loci (ADRA2A, IRX1, NOS3, ACVR2A, TMEM51, PCDH10-DT, HLA, and RAB6C) where ADRA2A, ACVR2A, NOS3, TMEM51, and IRX1 co-localized with expression quantitative trait loci (eQTLs), particularly in distal arteries. CRISPR gene editing further showed that ADRA2A and NOS3 loci modified gene expression and in situ RNAscope clarified the specificity of ADRA2A in small vessels and IRX1 around small capillaries in the skin. A functional contraction assay in the cold showed lower contraction in ADRA2A-deficient and higher contraction in ADRA2A-overexpressing smooth muscle cells. Overall, our study highlights the power of genome-wide association testing with functional follow-up as a method to understand complex diseases. The results indicate temperature-dependent adrenergic signaling through ADRA2A, effects at the microvasculature by IRX1, endothelial signaling by NOS3, and immune mechanisms by the HLA locus in Raynaud's syndrome.

Central α2-adrenergic mechanisms regulate human sympathetic neuronal discharge strategies

The present study investigated the impact of central α2-adrenergic mechanisms on sympathetic action potential (AP) discharge, recruitment and latency strategies. We used the microneurographic technique to record muscle sympathetic nerve activity and a continuous wavelet transform to investigate postganglionic sympathetic AP firing during a baseline condition and an infusion of a α2-adrenergic receptor agonist, dexmedetomidine (10 min loading infusion of 0.225 µg kg-1; maintenance infusion of 0.1-0.5 µg kg h-1) in eight healthy individuals (28 ± 7 years, five females). Dexmedetomidine reduced mean pressure (92 ± 7 to 80 ± 8 mmHg, P < 0.001) but did not alter heart rate (61 ± 13 to 60 ± 14 bpm; P = 0.748). Dexmedetomidine reduced sympathetic AP discharge (126 ± 73 to 27 ± 24 AP 100 beats-1, P = 0.003) most strongly for medium-sized APs (normalized cluster 2: 21 ± 10 to 5 ± 5 AP 100 beats-1; P < 0.001). Dexmedetomidine progressively de-recruited sympathetic APs beginning with the largest AP clusters (12 ± 3 to 7 ± 2 clusters, P = 0.002). Despite de-recruiting large AP clusters with shorter latencies, dexmedetomidine reduced AP latency across remaining clusters (1.18 ± 0.12 to 1.13 ± 0.13 s, P = 0.002). A subset of six participants performed a Valsalva manoeuvre (20 s, 40 mmHg) during baseline and the dexmedetomidine infusion. Compared to baseline, AP discharge (Δ 361 ± 292 to Δ 113 ± 155 AP 100 beats-1, P = 0.011) and AP cluster recruitment elicited by the Valsalva manoeuvre were lower during dexmedetomidine (Δ 2 ± 1 to Δ 0 ± 2 AP clusters, P = 0.041). The reduction in sympathetic AP latency elicited by the Valsalva manoeuvre was not affected by dexmedetomidine (Δ -0.09 ± 0.07 to Δ -0.07 ± 0.14 s, P = 0.606). Dexmedetomidine reduced baroreflex gain, most strongly for medium-sized APs (normalized cluster 2: -6.0 ± 5 to -1.6 ± 2 % mmHg-1; P = 0.008). These data suggest that α2-adrenergic mechanisms within the central nervous system modulate sympathetic postganglionic neuronal discharge, recruitment and latency strategies in humans. KEY POINTS: Sympathetic postganglionic neuronal subpopulations innervating the human circulation exhibit complex patterns of discharge, recruitment and latency. However, the central neural mechanisms governing sympathetic postganglionic discharge remain unclear. This microneurographic study investigated the impact of a dexmedetomidine infusion (α2-adrenergic receptor agonist) on muscle sympathetic postganglionic action potential (AP) discharge, recruitment and latency patterns. Dexmedetomidine infusion inhibited the recruitment of large and fast conducting sympathetic APs and attenuated the discharge of medium sized sympathetic APs that fired during resting conditions and the Valsalva manoeuvre. Dexmedetomidine infusion elicited shorter sympathetic AP latencies during resting conditions but did not affect the reductions in latency that occurred during the Valsalva manoeuvre. These data suggest that α2-adrenergic mechanisms within the central nervous system modulate sympathetic postganglionic neuronal discharge, recruitment and latency strategies in humans.

Attenuated peripheral oxygen extraction and greater cardiac output in women with posttraumatic stress disorder during exercise

Posttraumatic stress disorder (PTSD) is associated with an increased risk of developing cardiovascular disease, especially in women. Evidence indicates that men with PTSD exhibit lower maximal oxygen uptake (V̇o2max) relative to controls; however, whether V̇o2max is blunted in women with PTSD remains unknown. Furthermore, it is unclear what determinants (i.e., central and/or peripheral) of V̇o2max are impacted by PTSD. Therefore, we evaluated the central (i.e., cardiac output; Q̇c) and peripheral (i.e., arteriovenous oxygen difference) determinants of V̇o2max in women with PTSD; hypothesizing that V̇o2max would be lower in women with PTSD compared with women without PTSD (controls), primarily due to smaller increases in stroke volume (SV), and therefore Q̇c. Oxygen uptake (V̇o2), heart rate (HR), Q̇c, SV, and arteriovenous oxygen difference were measured in women with PTSD (n = 14; mean [SD]: 43 [11] yr,) and controls (n = 17; 45 [11] yr) at rest, and during an incremental maximal treadmill exercise test, and the Q̇c/V̇o2 slope was calculated. V̇o2max was not different between women with and without PTSD (24.3 [5.6] vs. 26.4 [5.0] mL/kg/min; P = 0.265). However, women with PTSD had higher Q̇c [P = 0.002; primarily due to greater SV (P = 0.069), not HR (P = 0.285)], and lower arteriovenous oxygen difference (P = 0.002) throughout exercise compared with controls. Furthermore, the Q̇c/V̇o2 slope was steeper in women with PTSD relative to controls (6.6 [1.4] vs. 5.7 [1.0] AU; P = 0.033). Following maximal exercise, women with PTSD exhibited slower HR recovery than controls (P = 0.046). Thus, despite attenuated peripheral oxygen extraction, V̇o2max is not reduced in women with PTSD, likely due to larger increases in Q̇c.NEW & NOTEWORTHY The current study indicates that V̇o2max is not different between women with and without PTSD; however, women with PTSD exhibit blunted peripheral extraction of oxygen, thus requiring an increase in Q̇c to meet metabolic demand during exercise. Furthermore, following exercise, women with PTSD demonstrate impaired autonomic cardiovascular control relative to sedentary controls. We interpret these data to indicate that women with PTSD demonstrate aberrant cardiovascular responses during and immediately following fatiguing exercise.

Adrenergic control of skeletal muscle blood flow during chronic hypoxia in healthy males

Sympathetic transduction is reduced following chronic high-altitude (HA) exposure; however, vascular α-adrenergic signaling, the primary mechanism mediating sympathetic vasoconstriction at sea level (SL), has not been examined at HA. In nine male lowlanders, we measured forearm blood flow (Doppler ultrasound) and calculated changes in vascular conductance (ΔFVC) during 1) incremental intra-arterial infusion of phenylephrine to assess α1-adrenergic receptor responsiveness and 2) combined intra-arterial infusion of β-adrenergic and α-adrenergic antagonists propranolol and phentolamine (α-β-blockade) to assess adrenergic vascular restraint at rest and during exercise-induced sympathoexcitation (cycling; 60% peak power). Experiments were performed near SL (344 m) and after 3 wk at HA (4,383 m). HA abolished the vasoconstrictor response to low-dose phenylephrine (ΔFVC: SL: -34 ± 15%, vs. HA; +3 ± 18%; P < 0.0001) and markedly attenuated the response to medium (ΔFVC: SL: -45 ± 18% vs. HA: -28 ± 11%; P = 0.009) and high (ΔFVC: SL: -47 ± 20%, vs. HA: -35 ± 20%; P = 0.041) doses. Blockade of β-adrenergic receptors alone had no effect on resting FVC (P = 0.500) and combined α-β-blockade induced a similar vasodilatory response at SL and HA (P = 0.580). Forearm vasoconstriction during cycling was not different at SL and HA (P = 0.999). Interestingly, cycling-induced forearm vasoconstriction was attenuated by α-β-blockade at SL (ΔFVC: Control: -27 ± 128 vs. α-β-blockade: +19 ± 23%; P = 0.0004), but unaffected at HA (ΔFVC: Control: -20 ± 22 vs. α-β-blockade: -23 ± 11%; P = 0.999). Our results indicate that in healthy males, altitude acclimatization attenuates α1-adrenergic receptor responsiveness; however, resting α-adrenergic restraint remains intact, due to concurrent resting sympathoexcitation. Furthermore, forearm vasoconstrictor responses to cycling are preserved, although the contribution of adrenergic receptors is diminished, indicating a reliance on alternative vasoconstrictor mechanisms.

Global REACH 2018: increased adrenergic restraint of blood flow preserves coupling of oxygen delivery and demand during exercise at high-altitude

Chronic exposure to hypoxia (high-altitude, HA; >4000 m) attenuates the vasodilatory response to exercise and is associated with a persistent increase in basal sympathetic nerve activity (SNA). The mechanism(s) responsible for the reduced vasodilatation and exercise hyperaemia at HA remains unknown. We hypothesized that heightened adrenergic signalling restrains skeletal muscle blood flow during handgrip exercise in lowlanders acclimatizing to HA. We tested nine adult males (n = 9) at sea-level (SL; 344 m) and following 21-28 days at HA (∼4300 m). Forearm blood flow (FBF; duplex ultrasonography), mean arterial pressure (MAP; brachial artery catheter), forearm vascular conductance (FVC; FBF/MAP), and arterial and venous blood sampling (O₂ delivery ( DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ ) and uptake ( V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ )) were measured at rest and during graded rhythmic handgrip exercise (5%, 15% and 25% of maximum voluntary isometric contraction; MVC) before and after local α- and β-adrenergic blockade (intra-arterial phentolamine and propranolol). HA reduced ΔFBF (25% MVC: SL: 138.3 ± 47.6 vs. HA: 113.4 ± 37.1 ml min^-1 ; P = 0.022) and Δ V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ (25% MVC: SL: 20.3 ± 7.5 vs. HA: 14.3 ± 6.2 ml min^-1 ; P = 0.014) during exercise. Local adrenoreceptor blockade at HA restored FBF during exercise (25% MVC: SL_{α-β blockade} : 164.1 ± 71.7 vs. HA_{α-β blockade} : 185.4 ± 66.6 ml min^-1 ; P = 0.947) but resulted in an exaggerated relationship between DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ and V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ ( DO2${D}_{{{\rm{O}}}_{\rm{2}}}$ / V̇O2${\dot{V}}_{{{\rm{O}}}_{\rm{2}}}$ slope: SL: 1.32; HA: slope: 1.86; P = 0.037). These results indicate that tonic adrenergic signalling restrains exercise hyperaemia in lowlanders acclimatizing to HA. The increase in adrenergic restraint is necessary to match oxygen delivery to demand and prevent over perfusion of contracting muscle at HA. KEY POINTS: In exercising skeletal muscle, local vasodilatory signalling and sympathetic vasoconstriction integrate to match oxygen delivery to demand and maintain arterial blood pressure. Exposure to chronic hypoxia (altitude, >4000 m) causes a persistent increase in sympathetic nervous system activity that is associated with impaired functional capacity and diminished vasodilatation during exercise. In healthy male lowlanders exposed to chronic hypoxia (21-28 days; ∼4300 m), local adrenoreceptor blockade (combined α- and β-adrenergic blockade) restored skeletal muscle blood flow during handgrip exercise. However, removal of tonic adrenergic restraint at high altitude caused an excessive rise in blood flow and subsequently oxygen delivery for any given metabolic demand. This investigation is the first to identify greater adrenergic restraint of blood flow during acclimatization to high altitude and provides evidence of a functional role for this adaptive response in regulating oxygen delivery and demand.

Global REACH 2018: the adaptive phenotype to life with chronic mountain sickness and polycythaemia

KEY POINTS

Humans suffering from polycythaemia undergo multiple circulatory adaptations including changes in blood rheology and structural and functional vascular adaptations to maintain normal blood pressure and vascular shear stresses, despite high blood viscosity. During exercise, several circulatory adaptations are observed, especially involving adrenergic and non-adrenergic mechanisms within non-active and active skeletal muscle to maintain exercise capacity, which is not observed in animal models. Despite profound circulatory stress, i.e. polycythaemia, several adaptations can occur to maintain exercise capacity, therefore making early identification of the disease difficult without overt symptomology. Pharmacological treatment of the background heightened sympathetic activity may impair the adaptive sympathetic response needed to match local oxygen delivery to active skeletal muscle oxygen demand and therefore inadvertently impair exercise capacity.

ABSTRACT

Excessive haematocrit and blood viscosity can increase blood pressure, cardiac work and reduce aerobic capacity. However, past clinical investigations have demonstrated that certain human high-altitude populations suffering from excessive erythrocytosis, Andeans with chronic mountain sickness, appear to have phenotypically adapted to life with polycythaemia, as their exercise capacity is comparable to healthy Andeans and even with sea-level inhabitants residing at high altitude. By studying this unique population, which has adapted through natural selection, this study aimed to describe how humans can adapt to life with polycythaemia. Experimental studies included Andeans with (n = 19) and without (n = 17) chronic mountain sickness, documenting exercise capacity and characterizing the transport of oxygen through blood rheology, including haemoglobin mass, blood and plasma volume and blood viscosity, cardiac output, blood pressure and changes in total and local vascular resistances through pharmacological dissection of α-adrenergic signalling pathways within non-active and active skeletal muscle. At rest, Andeans with chronic mountain sickness had a substantial plasma volume contraction, which alongside a higher red blood cell volume, caused an increase in blood viscosity yet similar total blood volume. Moreover, both morphological and functional alterations in the periphery normalized vascular shear stress and blood pressure despite high sympathetic nerve activity. During exercise, blood pressure, cardiac work and global oxygen delivery increased similar to healthy Andeans but were sustained by modifications in both non-active and active skeletal muscle vascular function. These findings highlight widespread physiological adaptations that can occur in response to polycythaemia, which allow the maintenance of exercise capacity.

Effects of sex and exercise training on β-adrenoreceptor-mediated opposition of evoked sympathetic vasoconstriction in resting and contracting muscle of rats

This study investigated the hypothesis that β-adrenoreceptor-mediated inhibition of sympathetic vasoconstriction would be enhanced in female compared with male rats, and that endurance exercise training would augment β-adrenoreceptor-mediated inhibition of sympathetic vasoconstriction in male and female rats. Sprague-Dawley rats were randomized into sedentary (male: n = 7; female: n = 8) and exercise-trained (male: n = 9; female: n = 9) groups. Following 4 wk of exercise training or being sedentary, rats were anesthetized and surgically instrumented for stimulation of the lumbar sympathetic chain, muscle contraction and measurement of arterial blood pressure and femoral artery blood flow (FBF). Femoral vascular conductance (FVC) was calculated as FBF/mean arterial pressure. The percentage change of FVC in response to sympathetic stimulation delivered at 2 and 5 Hz was measured at rest and during contraction of the triceps surae muscles before and after β-adrenoreceptor blockade (propranolol: 0.075 mg·kg^-1 iv). We found that, at rest, β-adrenoreceptor blockade decreased (main effect of drug, 2 Hz: P < 0.001; 5 Hz: P < 0.001) sympathetic vasoconstriction. During contraction, sympathetic vasoconstrictor responsiveness was lower (main effect of sex, 2 Hz: P = 0.001; 5 Hz: P = 0.023) in female compared with male rats, and sympatholysis was enhanced (main effect of sex, 2 Hz: P = 0.001; 5 Hz: P < 0.001) in female rats. β-adrenoreceptor blockade decreased (main effect of drug, 2 Hz: P = 0.049; 5 Hz: P < 0.001) evoked sympathetic vasoconstriction in contracting muscle. The present study demonstrated that β-adrenoreceptors do not blunt sympathetic vasoconstriction in resting or contracting skeletal muscle of male or female rats. Sympatholysis was enhanced in female rats; however, this was not attributable to β-adrenoreceptor-mediated blunting of sympathetic vasoconstriction.NEW & NOTEWORTHY β-adrenoreceptors do not inhibit sympathetic vasoconstriction in resting or contracting muscle of male or female rats, regardless of training status. Sympatholysis was enhanced in female, compared to male rats; however, β-adrenoreceptors were not responsible for the enhanced sympatholysis. These findings indicate that β-adrenoreceptors do not contribute to the regulation of sympathetic vasoconstriction in resting and contracting skeletal muscle and suggest that β-adrenoreceptors do not underlie sex differences in the neural control of the circulation.

Mechanisms of sympathetic restraint in human skeletal muscle during exercise: role of α-adrenergic and nonadrenergic mechanisms

Sympathetic vasoconstriction is mediated by α-adrenergic receptors under resting conditions. During exercise, increased sympathetic nerve activity (SNA) is directed to inactive and active skeletal muscle; however, it is unclear what mechanism(s) are responsible for vasoconstriction during large muscle mass exercise in humans. The aim of this study was to determine the contribution of α-adrenergic receptors to sympathetic restraint of inactive skeletal muscle and active skeletal muscle during cycle exercise in healthy humans. In ten male participants (18-35 yr), mean arterial pressure (intra-arterial catheter) and forearm vascular resistance (FVR) and conductance (FVC) were assessed during cycle exercise (60% total peak workload) alone and during combined cycle exercise + handgrip exercise (HGE) before and after intra-arterial blockade of α- and β-adrenoreceptors via phentolamine and propranolol, respectively. Cycle exercise caused vasoconstriction in the inactive forearm that was attenuated ~80% with adrenoreceptor blockade (%ΔFVR, +81.7 ± 84.6 vs. +9.7 ± 30.7%; P = 0.05). When HGE was performed during cycle exercise, the vasodilatory response to HGE was restrained by ~40% (ΔFVC HGE, +139.3 ± 67.0 vs. cycle exercise: +81.9 ± 66.3 ml·min^-1·100 mmHg^-1; P = 0.03); however, the restraint of active skeletal muscle blood flow was not due to α-adrenergic signaling. These findings highlight that α-adrenergic receptors are the primary, but not the exclusive mechanism by which sympathetic vasoconstriction occurs in inactive and active skeletal muscle during exercise. Metabolic activity or higher sympathetic firing frequencies may alter the contribution of α-adrenergic receptors to sympathetic vasoconstriction. Finally, nonadrenergic vasoconstrictor mechanisms may be important for understanding the regulation of blood flow during exercise.NEW & NOTEWORTHY Sympathetic restraint of vascular conductance to inactive skeletal muscle is critical to maintain blood pressure during moderate- to high-intensity whole body exercise. This investigation shows that cycle exercise-induced restraint of inactive skeletal muscle vascular conductance occurs primarily because of activation of α-adrenergic receptors. Furthermore, exercise-induced vasoconstriction restrains the subsequent vasodilatory response to hand-grip exercise; however, the restraint of active skeletal muscle vasodilation was in part due to nonadrenergic mechanisms. We conclude that α-adrenergic receptors are the primary but not exclusive mechanism by which sympathetic vasoconstriction restrains blood flow in humans during whole body exercise and that metabolic activity modulates the contribution of α-adrenergic receptors.

Assessment of resistance vessel function in human skeletal muscle: guidelines for experimental design, Doppler ultrasound, and pharmacology

The introduction of duplex Doppler ultrasound almost half a century ago signified a revolutionary advance in the ability to assess limb blood flow in humans. It is now widely used to assess blood flow under a variety of experimental conditions to study skeletal muscle resistance vessel function. Despite its pervasive adoption, there is substantial variability between studies in relation to experimental protocols, procedures for data analysis, and interpretation of findings. This guideline results from a collegial discussion among physiologists and pharmacologists, with the goal of providing general as well as specific recommendations regarding the conduct of human studies involving Doppler ultrasound-based measures of resistance vessel function in skeletal muscle. Indeed, the focus is on methods used to assess resistance vessel function and not upstream conduit artery function (i.e., macrovasculature), which has been expertly reviewed elsewhere. In particular, we address topics related to experimental design, data collection, and signal processing as well as review common procedures used to assess resistance vessel function, including postocclusive reactive hyperemia, passive limb movement, acute single limb exercise, and pharmacological interventions.

β-Adrenoreceptors do not oppose sympathetic vasoconstriction in resting and contracting skeletal muscle of male rats

Sympathetic nervous system (SNS) vasoconstriction is primarily achieved through the binding of norepinephrine (NE) to α-adrenoreceptors. However, NE may also bind to β-adrenoreceptors and cause vasodilation that may oppose/blunt SNS-mediated vasoconstriction. Therefore, this study investigated the hypothesis that β-adrenoreceptor–mediated vasodilation opposes evoked vasoconstriction in resting and contracting skeletal muscle. Male (n = 9) Sprague–Dawley rats were anesthetized and surgically instrumented for stimulation of the lumbar sympathetic chain and measurement of arterial blood pressure and femoral artery blood flow. The percentage change of femoral vascular conductance in response to sympathetic chain stimulation delivered at 2 and 5 Hz was determined at rest and during triceps surae skeletal muscle contraction before (control) and after β-adrenoreceptor blockade (propranolol; 0.075 mg·kg−1, intravenously). β-Adrenoreceptor blockade did not alter (P > 0.05) baseline hemodynamics or the hyperemic response to exercise. At the 2 Hz stimulation frequency, sympathetic vasoconstriction was similar (P > 0.05) in control and β-blockade conditions in resting (control, −34% ± 6%; β-blockade, −33% ± 8%) and contracting (control, −16% ± 6%; β-blockade, −14% ± 7%) muscle. At the 5 Hz stimulation frequency, sympathetic vasoconstrictor responsiveness was reduced (main effect of drug, P < 0.05) following β-blockade (rest: control, −52% ± 7%; β-blockade, −51% ± 9%; contraction: control, −32% ± 11%; β-blockade, −29% ± 13%).

Novelty These data indicate that β-adrenoreceptor blockade did not augment sympathetic vasoconstriction at rest or during exercise. The study demonstrates that β-adrenoreceptors do not oppose evoked sympathetic vasoconstriction in resting or contracting skeletal muscle or contribute to functional sympatholysis.

The role of α2-adrenergic receptors in hypertensive preeclampsia: A hypothesis

Preeclampsia, a major disorder of human pregnancy, manifests as persistent hypertension and proteinuria presenting after 20 weeks of pregnancy. Multiple systemic symptoms might be associated with preeclampsia including thrombocytopenia, liver impairment, pulmonary edema, and cerebral disturbances. However, vascular dysfunction remains the core pathological driver of preeclampsia. Defective placental implantation followed by dysfunctional placental spiral artery development promotes a hypoxic environment. Massive endothelial dysfunction characterized by reduced vasodilation, augmented vasoconstriction, and increased vascular permeability and inflammation ensues. Interestingly, the same signaling and inflammatory pathways implicated in preeclampsia appear to be shared with other vascular disorders involving alteration of α₂ -AR function. The role of α₂ -ARs in the regulation of microcirculatory function has long been recognized, thus raising the question of whether they are involved in the pathogenesis of vascular dysfunction in preeclampsia. Here, we review possible interplay between signaling and inflammatory pathways common to preeclampsia and α₂ -AR function/regulation. We speculate on the potential contribution of these receptors to the observed phenotype and the potential role for their pharmacological modulators as therapeutic interventions with preeclampsia.

Alpha2A-adrenergic receptor and eNOS genetic polymorphisms are associated with exercise muscle vasodilatation in apparently healthy individuals

PURPOSE

Muscle vasodilatation during exercise has been associated with cardiovascular health and may be influenced by genetic variability. The purpose of this study was to evaluate functional genetic polymorphisms of physiologic pathways related to the regulation of the cardiovascular function (alpha-adrenergic receptors, endothelial nitric oxide synthase and bradykinin B2 receptor) and exercise muscle vasodilatation in apparently healthy men and women.

METHODS

We enrolled 689 individuals without established cardiovascular disease that had attended a check-up program. The vasodilatation was studied with venous occlusion plethysmography and determined by the increase of vascular conductance during handgrip exercise. Genotypes for ADRA1A Arg347Cys (rs1048101), ADRA2A 1780 C > T (rs553668), ADRA2B Del 301-303 (rs28365031), eNOS 786 T > C (rs2070744), eNOS Glu298Asp (rs1799983) and BDKRB2 (rs5810761) polymorphisms were assessed by polymerase chain reaction followed by high resolution melting analysis.

RESULTS

The eNOS rs2070744 polymorphism was significantly associated with forearm vascular conductance during exercise in women. Women with CC genotype showed higher vasodilatation than carriers of TC and TT genotypes (p = 0.043). The ADRA2A rs553668 polymorphism was significantly associated with forearm vascular conductance during exercise in men. Men with TT genotype had higher vasodilatation than carriers of CT and CC genotypes (p = 0.025).

CONCLUSIONS

eNOS rs207074 polymorphism in women and ADRA2A rs553668 polymorphism in men were associated with the increase of forearm vascular conductance during handgrip exercise. These findings suggest that eNOS and ADRA2A genetic polymorphisms may be potential markers of exercise muscle vasodilatation.

Regulation of increased blood flow (hyperemia) to muscles during exercise: a hierarchy of competing physiological needs

This review focuses on how blood flow to contracting skeletal muscles is regulated during exercise in humans. The idea is that blood flow to the contracting muscles links oxygen in the atmosphere with the contracting muscles where it is consumed. In this context, we take a top down approach and review the basics of oxygen consumption at rest and during exercise in humans, how these values change with training, and the systemic hemodynamic adaptations that support them. We highlight the very high muscle blood flow responses to exercise discovered in the 1980s. We also discuss the vasodilating factors in the contracting muscles responsible for these very high flows. Finally, the competition between demand for blood flow by contracting muscles and maximum systemic cardiac output is discussed as a potential challenge to blood pressure regulation during heavy large muscle mass or whole body exercise in humans. At this time, no one dominant dilator mechanism accounts for exercise hyperemia. Additionally, complex interactions between the sympathetic nervous system and the microcirculation facilitate high levels of systemic oxygen extraction and permit just enough sympathetic control of blood flow to contracting muscles to regulate blood pressure during large muscle mass exercise in humans.

Contracting human skeletal muscle maintains the ability to blunt α1 -adrenergic vasoconstriction during KIR channel and Na(+) /K(+) -ATPase inhibition

KEY POINTS

During exercise there is a balance between vasoactive factors that facilitate increases in blood flow and oxygen delivery to the active tissue and the sympathetic nervous system, which acts to limit muscle blood flow for the purpose of blood pressure regulation. Functional sympatholysis describes the ability of contracting skeletal muscle to blunt the stimulus for vasoconstriction, yet the underlying signalling of this response in humans is not well understood. We tested the hypothesis that activation of inwardly rectifying potassium channels and the sodium-potassium ATPase pump, two potential vasodilator pathways within blood vessels, contributes to the ability to blunt α1 -adrenergic vasoconstriction. Our results show preserved blunting of α1 -adrenergic vasconstriction despite blockade of these vasoactive factors. Understanding this complex phenomenon is important as it is impaired in a variety of clinical populations.

ABSTRACT

Sympathetic vasoconstriction in contracting skeletal muscle is blunted relative to that which occurs in resting tissue; however, the mechanisms underlying this 'functional sympatholysis' remain unclear in humans. We tested the hypothesis that α1 -adrenergic vasoconstriction is augmented during exercise following inhibition of inwardly rectifying potassium (KIR ) channels and Na(+) /K(+) -ATPase (BaCl2  + ouabain). In young healthy humans, we measured forearm blood flow (Doppler ultrasound) and calculated forearm vascular conductance (FVC) at rest, during steady-state stimulus conditions (pre-phenylephrine), and after 2 min of phenylephrine (PE; an α1 -adrenoceptor agonist) infusion via brachial artery catheter in response to two different stimuli: moderate (15% maximal voluntary contraction) rhythmic handgrip exercise or adenosine infusion. In Protocol 1 (n = 11 subjects) a total of six trials were performed in three conditions: control (saline), combined enzymatic inhibition of nitric oxide (NO) and prostaglandin (PG) synthesis (l-NMMA + ketorolac) and combined inhibition of NO, PGs, KIR channels and Na(+) /K(+) -ATPase (l-NMMA + ketorolac + BaCl2  + ouabain). In Protocol 2 (n = 6) a total of four trials were performed in two conditions: control (saline), and combined KIR channel and Na(+) /K(+) -ATPase inhibition. All trials occurred after local β-adrenoceptor blockade (propranolol). PE-mediated vasoconstriction was calculated (%ΔFVC) in each condition. Contrary to our hypothesis, despite attenuated exercise hyperaemia of ∼30%, inhibition of KIR channels and Na(+) /K(+) -ATPase, combined with inhibition of NO and PGs (Protocol 1) or alone (Protocol 2) did not enhance α1 -mediated vasoconstriction during exercise (Protocol 1: -27 ± 3%; P = 0.2 vs. control, P = 0.4 vs. l-NMMA + ketorolac; Protocol 2: -21 ± 7%; P = 0.9 vs. control). Thus, contracting human skeletal muscle maintains the ability to blunt α1 -adrenergic vasoconstriction during combined KIR channel and Na(+) /K(+) -ATPase inhibition.

A molecular signature in the pannexin1 intracellular loop confers channel activation by the α1 adrenoreceptor in smooth muscle cells

Both purinergic signaling through nucleotides such as ATP (adenosine 5'-triphosphate) and noradrenergic signaling through molecules such as norepinephrine regulate vascular tone and blood pressure. Pannexin1 (Panx1), which forms large-pore, ATP-releasing channels, is present in vascular smooth muscle cells in peripheral blood vessels and participates in noradrenergic responses. Using pharmacological approaches and mice conditionally lacking Panx1 in smooth muscle cells, we found that Panx1 contributed to vasoconstriction mediated by the α1 adrenoreceptor (α1AR), whereas vasoconstriction in response to serotonin or endothelin-1 was independent of Panx1. Analysis of the Panx1-deficient mice showed that Panx1 contributed to blood pressure regulation especially during the night cycle when sympathetic nervous activity is highest. Using mimetic peptides and site-directed mutagenesis, we identified a specific amino acid sequence in the Panx1 intracellular loop that is essential for activation by α1AR signaling. Collectively, these data describe a specific link between noradrenergic and purinergic signaling in blood pressure homeostasis.

Neural control of the circulation: how sex and age differences interact in humans

The autonomic nervous system is a key regulator of the cardiovascular system. In this review, we focus on how sex and aging influence autonomic regulation of blood pressure in humans in an effort to understand general issues related to the cardiovascular system as a whole. Younger women generally have lower blood pressure and sympathetic activity than younger men. However, both sexes show marked interindividual variability across age groups with significant overlap seen. Additionally, while men across the lifespan show a clear relationship between markers of whole body sympathetic activity and vascular resistance, such a relationship is not seen in young women. In this context, the ability of the sympathetic nerves to evoke vasoconstriction is lower in young women likely as a result of concurrent β2-mediated vasodilation that offsets α-adrenergic vasoconstriction. These differences reflect both central sympatho-inhibitory effects of estrogen and also its influence on peripheral vasodilation at the level of the vascular smooth muscle and endothelium. By contrast postmenopausal women show a clear relationship between markers of whole body sympathetic traffic and vascular resistance, and sympathetic activity rises progressively in both sexes with aging. These major findings in humans are discussed in the context of differences in population-based trends in blood pressure and orthostatic intolerance. The many areas where there is little sex-specific data on how the autonomic nervous system participates in the regulation of the human cardiovascular system are highlighted.

Role of α-adrenergic vasoconstriction in regulating skeletal muscle blood flow and vascular conductance during forearm exercise in ageing humans

In healthy humans, ageing is typically associated with reduced skeletal muscle blood flow and vascular conductance during exercise. Further, there is a marked increase in resting sympathetic nervous system (SNS) activity with age, yet whether augmented SNS-mediated α-adrenergic vasoconstriction contributes to the age-associated impairment in exercising muscle blood flow and vascular tone in humans is unknown. We tested the hypothesis that SNS-mediated vasoconstriction is greater in older than young adults and limits muscle (forearm) blood flow (FBF) during graded handgrip exercise (5, 15, 25% maximal voluntary contraction (MVC)). FBF was measured (Doppler ultrasound) and forearm vascular conductance (FVC) was calculated in 11 young (21 ± 1 years) and 12 older (62 ± 2 years) adults in control conditions and during combined local α- and β-adrenoreceptor blockade via intra-arterial infusions of phentolamine and propranolol, respectively. Under control conditions, older adults exhibited significantly lower FBF and FVC at 15% MVC exercise (22.6 ± 1.3 vs. 29 ± 3.3 ml min(-1) 100 g forearm fat-free mass (FFM)(-1) and 21.7 ± 1.2 vs. 33.6 ± 4.0 ml min(-1) 100 g FFM(-1) 100 mmHg(-1); P < 0.05) and 25% MVC exercise (37.4 ± 1.4 vs. 46.0 ± 4.9 ml min(-1) 100 g FFM(-1) and 33.7 ± 1.4 vs. 49.0 ± 5.7 ml min(-1) 100 g FFM(-1) 100 mmHg(-1); P < 0.05), whereas there was no age group difference at 5% MVC exercise. Local adrenoreceptor blockade increased FBF and FVC at rest and during exercise in both groups, although the increase in FBF and FVC from rest to steady-state exercise was similar in young and older adults across exercise intensities, and thus the age-associated impairment in FBF and FVC persisted. Our data indicate that during graded intensity handgrip exercise, the reduced FVC and subsequently lower skeletal muscle blood flow in older healthy adults is not due to augmented sympathetic vasoconstriction, but rather due to impairments in local signalling or structural limitations in the peripheral vasculature with advancing age.

Perivascular innervation: a multiplicity of roles in vasomotor control and myoendothelial signaling

The control of vascular resistance and tissue perfusion reflect coordinated changes in the diameter of feed arteries and the arteriolar networks they supply. Against a background of myogenic tone and metabolic demand, vasoactive signals originating from perivascular sympathetic and sensory nerves are integrated with endothelium-derived signals to produce vasodilation or vasoconstriction. PVNs release adrenergic, cholinergic, peptidergic, purinergic, and nitrergic neurotransmitters that lead to SMC contraction or relaxation via their actions on SMCs, ECs, or other PVNs. ECs release autacoids that can have opposing actions on SMCs. Respective cell layers are connected directly to each other through GJs at discrete sites via MEJs projecting through holes in the IEL. Whereas studies of intercellular communication in the vascular wall have centered on endothelium-derived signals that govern SMC relaxation, attention has increasingly focused on signaling from SMCs to ECs. Thus, via MEJs, neurotransmission from PVNs can evoke distinct responses from ECs subsequent to acting on SMCs. To integrate this emerging area of investigation in light of vasomotor control, the present review synthesizes current understanding of signaling events that originate within SMCs in response to perivascular neurotransmission in light of EC feedback. Although often ignored in studies of the resistance vasculature, PVNs are integral to blood flow control and can provide a physiological stimulus for myoendothelial communication. Greater understanding of these underlying signaling events and how they may be affected by aging and disease will provide new approaches for selective therapeutic interventions.

Short-term exercise training augments 2-adrenoreceptor-mediated sympathetic vasoconstriction in resting and contracting skeletal muscle

We hypothesized that exercise training (ET) would alter α2-adrenoreceptor-mediated sympathetic vasoconstriction. Sprague-Dawley rats (n = 30) were randomized to sedentary (S), mild- (M) or heavy-intensity (H) treadmill ET groups (5 days per week for 4 weeks). Following the ET component of the study, rats were anaesthetized, and instrumented for lumbar sympathetic chain stimulation, triceps surae muscle contraction and measurement of femoral vascular conductance (FVC). The percentage change of FVC in response to sympathetic stimulation was determined at rest and during contraction in control, α2 blockade (yohimbine) and combined α2 + nitric oxide (NO) synthase (NOS) blockade (N-nitro-L-arginine methyl ester hydrochloride, L-NAME) conditions. ET augmented (P < 0.05) sympathetic vasoconstrictor responses at rest and during contraction. Yohimbine reduced (P < 0.05) the vasoconstrictor response in ET rats at rest (M: 2 Hz: 8 ± 2%, 5 Hz: 9 ± 4%; H: 2 Hz: 14 ± 5%, 5 Hz: 11 ± 6%) and during contraction (M: 2 Hz: 9 ± 2%, 5 Hz: 9 ± 5%; H: 2 Hz: 8 ± 3%, 5 Hz: 6 ± 6%) but did not change the response in S rats. The addition of L-NAME caused a larger increase (P < 0.05) in the vasoconstrictor response in ET than in S rats at rest (2 Hz: S: 8 ± 2%, M: 15 ± 3%, H: 23 ± 7%; 5 Hz: S: 8 ± 5%, M: 15 ± 3%, H: 17 ± 5%) and during contraction (2 Hz: S: 9 ± 3%, M: 18 ± 3%, H: 22 ± 6%; 5 Hz: S: 9 ± 5%, M: 22 ± 4%, H:26 ± 9%). Sympatholysis was greater (P < 0.05) in ET than in S rats. Blockade of α2-adrenoreceptors and NOS reduced (P < 0.05) sympatholysis in ET rats, but had no effect on sympatholysis in S rats. In conclusion, ET increased α2-mediated vasoconstriction at rest and during contraction.

The role of α-adrenergic receptors in mediating beat-by-beat sympathetic vascular transduction in the forearm of resting man

Sympathetic vascular transduction is commonly understood to act as a basic relay mechanism, but under basal conditions, competing dilatory signals may interact with and alter the ability of sympathetic activity to decrease vascular conductance. Thus, we determined the extent to which spontaneous bursts of muscle sympathetic nerve activity (MSNA) mediate decreases in forearm vascular conductance (FVC) and the contribution of local α-adrenergic receptor-mediated pathways to the observed FVC responses. In 19 young men, MSNA (microneurography), arterial blood pressure and brachial artery blood flow (duplex Doppler ultrasound) were continuously measured during supine rest. These measures were also recorded in seven men during intra-arterial infusions of normal saline, phentolamine (PHEN) and PHEN with angiotensin II (PHEN+ANG). The latter was used to control for increases in resting blood flow with α-adrenergic blockade. Spike-triggered averaging was used to characterize beat-by-beat changes in FVC for 15 cardiac cycles following each MSNA burst and a peak response was calculated. Following MSNA bursts, FVC initially increased by +3.3 ± 0.3% (P = 0.016) and then robustly decreased to a nadir of -5.8 ± 1.6% (P < 0.001). The magnitude of vasoconstriction appeared graded with the number of consecutive MSNA bursts; while individual burst size only had a mild influence. Neither PHEN nor PHEN+ANG infusions affected the initial rise in FVC, but both infusions significantly attenuated the subsequent decrease in FVC (-2.1 ± 0.7% and -0.7 ± 0.8%, respectively; P < 0.001 vs. normal saline). These findings indicate that spontaneous MSNA bursts evoke robust beat-by-beat decreases in FVC that are exclusively mediated via α-adrenergic receptors.

Vasoconstrictor responsiveness during hyperbaric hyperoxia in contracting human muscle

Large increases in systemic oxygen content cause substantial reductions in exercising forearm blood flow (FBF) due to increased vascular resistance. We hypothesized that 1) functional sympatholysis (blunting of sympathetic α-adrenergic vasoconstriction) would be attenuated during hyperoxic exercise and 2) α-adrenergic blockade would limit vasoconstriction during hyperoxia and increase FBF to levels observed under normoxic conditions. Nine male subjects (age 28 ± 1 yr) performed forearm exercise (20% of maximum) under normoxic and hyperoxic conditions. Studies were performed in a hyperbaric chamber at 1 atmosphere absolute (ATA; sea level) while breathing 21% O(2) and at 2.82 ATA while breathing 100% O(2) (estimated change in arterial O(2) content ∼6 ml O(2)/100 ml). FBF (ml/min) was measured using Doppler ultrasound. Forearm vascular conductance (FVC) was calculated from FBF and blood pressure (arterial catheter). Vasoconstrictor responsiveness was determined using intra-arterial tyramine. FBF and FVC were substantially lower during hyperoxic exercise than normoxic exercise (∼20-25%; P < 0.01). At rest, vasoconstriction to tyramine (% decrease from pretyramine values) did not differ between normoxia and hyperoxia (P > 0.05). During exercise, vasoconstrictor responsiveness was slightly greater during hyperoxia than normoxia (-22 ± 3 vs. -17 ± 2%; P < 0.05). However, during α-adrenergic blockade, hyperoxic exercise FBF and FVC remained lower than during normoxia (P < 0.01). Therefore, our data suggest that although the vasoconstrictor responsiveness during hyperoxic exercise was slightly greater, it likely does not explain the majority of the large reductions in FBF and FVC (∼20-25%) during hyperbaric hyperoxic exercise.

α-adrenergic vasoconstriction contributes to the age-related increase in conduit artery retrograde and oscillatory shear

Aging is associated with increased retrograde and oscillatory shear in peripheral conduit arteries of humans. Although the mechanisms responsible for these age-related changes are not completely understood, augmented downstream α-adrenergic tone likely plays a significant role in this phenomenon. Therefore, in protocol 1, brachial artery diameter and blood velocity were measured via Doppler ultrasound during (1) rest (control), (2) endogenous norepinephrine release via intra-arterial infusions of tyramine, and (3) α-adrenergic blockade via infusions of phentolamine in young healthy humans (n=12). Tyramine increased brachial artery retrograde (-4.0±1.4 to -9.5±1.4 s(-1)) and oscillatory shear (0.05±0.02 to 0.18±0.05 arbitrary units), whereas phentolamine abolished retrograde and oscillatory shear (P<0.05). Additionally, in protocol 2, we examined brachial artery shear patterns in young (n=12; 29±2 years) and older (n=13; 69±2 years) healthy adults during (1) rest (control), (2) sympathetic activation via lower body negative pressure, and (3) infusion of phentolamine. At rest, older adults exhibited greater brachial artery retrograde and oscillatory shear (-9.9±2.7 s(-1) and 0.11±0.03 arbitrary units, respectively) compared with younger adults (-3.1±1.0 s(-1) and 0.05±0.02 arbitrary units, respectively; P<0.05 for both). Lower body negative pressure increased retrograde and oscillatory shear in young (P<0.05), but not older adults (P=0.85-0.97), such that differences between young and older were eliminated (P>0.05). During infusion of phentolamine, retrograde and oscillatory shear were abolished in young adults (P<0.05) and markedly reduced, yet still persistent, in older adults (P<0.01). Our data indicate that α-adrenergic vasoconstriction is a major contributor to age-related discrepancies in conduit artery shear-rate patterns at rest.

α-Adrenergic Blockade Unmasks a Greater Compensatory Vasodilation in Hypoperfused Contracting Muscle

We previously demonstrated that acute hypoperfusion in exercising human muscle causes an immediate increase in vascular resistance that is followed by a partial restoration (less than 100% recovery) of flow. In the current study we examined the contribution of α-adrenergic vasoconstriction in the initial changes in vascular resistance at the onset of hypoperfusion as well as in the recovery of flow over time. Nine healthy male subjects (29 ± 2) performed rhythmic forearm exercise (20% of maximum) during hypoperfusion evoked by intra-arterial balloon inflation. Each trial included; baseline, exercise prior to inflation, exercise with inflation, and exercise after deflation (3 min each). Forearm blood flow (FBF; ultrasound), local (brachial artery), and systemic arterial pressure (MAP; Finometer) were measured. The trial was repeated during phentolamine infusion (α-adrenergic receptor blockade). Forearm vascular conductance (FVC; ml min(-1) 100 mmHg(-1)) and resistance (mmHg ml min(-1)) was calculated from BF (ml min(-1)) and local MAP (mmHg). Recovery of FBF and FVC (steady state inflation plus exercise value - nadir)/[steady state exercise (control) value - nadir] with phentolamine was enhanced compared with the respective control (no drug) trial (FBF = 97 ± 5% vs. 81 ± 6%, P < 0.05; FVC = 126 ± 9% vs. 91 ± 5%, P < 0.01). However, the absolute (0.05 ± 0.01 vs. 0.06 ± 0.01 mmHg ml min(-1); P = 0.17) and relative (35 ± 5% vs. 31 ± 2%; P = 0.41) increase in vascular resistance at the onset of balloon inflation was not different between the α-adrenergic receptor inhibition and control (no drug) trials. Therefore, our data indicate that α-adrenergic mediated vasoconstriction restricts compensatory vasodilation during forearm exercise with hypoperfusion, but is not responsible for the initial increase in vascular resistance at the onset of hypoperfusion.

Relation between digital peripheral arterial tonometry and brachial artery ultrasound measures of vascular function in patients with coronary artery disease and in healthy volunteers

Digital peripheral arterial tonometry (PAT) is an emerging, noninvasive method to assess vascular function. The physiology underlying this phenotype, however, remains unclear. Therefore, we evaluated the relation between digital PAT and established brachial artery ultrasound measures of vascular function under basal conditions and after reactive hyperemia. Using a cross-sectional study design, digital PAT and brachial artery ultrasonography with pulsed wave Doppler were simultaneously completed at baseline and after reactive hyperemia in both those with established coronary artery disease (n = 99) and healthy volunteers with low cardiovascular disease risk (n = 40). Under basal conditions, the digital pulse volume amplitude demonstrated a significant positive correlation with the brachial artery velocity-time integral that was independent of the arterial diameter, in both the healthy volunteer (r(s) = 0.64, p <0.001) and coronary artery disease (r(s) = 0.63, p <0.001) cohorts. Similar positive relations were observed with the baseline brachial artery blood flow velocity and blood flow. In contrast, no relation between the reactive hyperemia-evoked digital PAT ratio and either brachial artery flow-mediated dilation or shear stress was observed in either cohort (p = NS). In conclusion, these findings demonstrate that the digital PAT measures of vascular function more closely reflect basal blood flow in the brachial artery than reactive hyperemia-induced changes in the arterial diameter or flow velocity, and the presence of vascular disease does not modify the physiology underlying the digital PAT phenotype.

α1-Adrenoreceptor activity does not explain lower morning endothelial-dependent, flow-mediated dilation in humans

Early morning reduction in endothelium-dependent, flow-mediated dilation (FMD) may contribute to the high incidence of sudden cardiac death at this time of day. The mechanisms underpinning diurnal variation in FMD are unclear, but potentially relate to a circadian rhythm in sympathetic nerve activity. We hypothesized that blockade of α1-mediated sympathetic nerve activity would act to attenuate the diurnal variation in FMD. In a randomized and placebo-controlled design, we measured brachial artery FMD in 12 participants (mean age = 26 yr, SD = 3) at 0600 and 1600 after ingestion of an α1-blocker (prazosin, 1 mg/20 kg body mass) or placebo. Arterial diameter and shear rate were assessed using edge-detection software. Heart rate and blood pressure were also measured. Data were analyzed using linear mixed modeling. Following placebo, FMD was 8 ± 2% in the morning compared with 10 ± 3% in the afternoon ( P = 0.04). Blockade with prazosin led to a slight but nonsignificant increase in morning FMD ( P = 0.24) and a significant ( P = 0.04) decrease in afternoon FMD, resulting in no diurnal variation ( P = 0.20). Shear rate did not differ in the morning or afternoon under either condition ( P > 0.23). Blood pressure was lower following prazosin compared with placebo ( P < 0.02), an effect that was similar at both times of day ( P > 0.34). Heart rate and norepinephrine levels were higher in the afternoon following prazosin. These data indicate that α1-adrenoreceptor activity does not explain lower morning endothelium-dependent FMD.

Independent regulation of α1 and α2 adrenergic receptor-mediated vasoconstriction in vivo

BACKGROUND

Vascular α1 and α2 adrenergic receptors mediate vasoconstriction and are major determinants of peripheral vascular tone. There is a wide variability in vasoconstrictor sensitivity to α1 and α2 adrenergic receptor agonists among individuals. In previous studies, this variability was not explained by identified α1 and α2 adrenergic receptor genetic variants. Thus, we hypothesized that adrenergic vasoconstrictor sensitivity is determined by shared constrictor mechanisms downstream of the individual receptors and that α1 and α2 adrenergic receptor-mediated vasoconstrictor sensitivity would therefore be correlated.

METHODS

Dorsal hand vein responses to increasing doses of the α1 adrenergic receptor agonist phenylephrine (12-12 000 ng/min) and the α2 adrenergic receptor agonist dexmedetomidine (0.01-100 ng/min) were measured in healthy individuals using a linear variable differential transformer. From individual dose-response curves, we calculated the dose of phenylephrine and dexmedetomidine that produced 50% (ED50) of maximum venoconstriction (Emax) for each patient. We examined the correlation between phenylephrine and dexmedetomidine ED50 and Emax before and after adjustment for covariates (age, sex, ethnicity, BMI, blood pressure, heart rate, and baseline plasma norepinephrine concentrations).

RESULTS

In 62 patients (36 men, 34 African-American, 28 whites), the median ED50 for dexmedetomidine was 1.32 ng/min [interquartile range (IQR) 0.45-5.37 ng/min] and for phenylephrine 177.8 ng/min (IQR 40.7-436.5 ng/min). The Emax for phenylephrine was 90.8% (82.2-99.6%) and for dexmedetomidine 80.0% (64.7-95.2%). There was no correlation between individual sensitivities (ED50) to phenylephrine and dexmedetomidine, before and after adjustment for covariates (P > 0.30).

CONCLUSION

Both phenylephrine and dexmedetomidine produce strong venoconstriction in the dorsal hand vein; however, there is no significant correlation between vascular sensitivity to an α1 and α2 adrenergic receptor agonist. These findings suggest the independent regulation of vascular α1 and α2 adrenergic receptor-mediated responses.

Chronic lowering of blood pressure by carotid baroreflex activation: mechanisms and potential for hypertension therapy

Recent technical advances have renewed interest in device-based therapy for the treatment of drug-resistant hypertension. Findings from recent clinical trials regarding the efficacy of electric stimulation of the carotid sinus for the treatment of resistant hypertension are reviewed here. The main goal of this article, however, is to summarize the preclinical studies that have provided insight into the mechanisms that account for the chronic blood pressure-lowering effects of carotid baroreflex activation. Some of the mechanisms identified were predictable and confirmed by experimentation. Others have been surprising and controversial, and resolution will require further investigation. Although feasibility studies have been promising, firm conclusions regarding the value of this device-based therapy for the treatment of resistant hypertension awaits the results of current multicenter trials.

Sustained suppression of sympathetic activity and arterial pressure during chronic activation of the carotid baroreflex

Following sinoaortic denervation, which eliminates arterial baroreceptor input into the brain, there are slowly developing adaptations that abolish initial sympathetic activation and hypertension. In comparison, electrical stimulation of the carotid sinus for 1 wk produces sustained reductions in sympathetic activity and arterial pressure. However, whether compensations occur subsequently to diminish these responses is unclear. Therefore, we determined whether there are important central and/or peripheral adaptations that diminish the sympathoinhibitory and blood pressure-lowering effects of more sustained carotid sinus stimulation. To this end, we measured whole body plasma norepinephrine spillover and alpha(1)-adrenergic vascular reactivity in six dogs over a 3-wk period of baroreflex activation. During the first week of baroreflex activation, there was an approximately 45% decrease in plasma norepinephrine spillover, along with reductions in mean arterial pressure and heart rate of approximately 20 mmHg and 15 beats/min, respectively; additionally, plasma renin activity did not increase. Most importantly, these responses during week 1 were largely sustained throughout the 3 wk of baroreflex activation. Acute pressor responses to alpha-adrenergic stimulation during ganglionic blockade were similar throughout the study, indicating no compensatory increases in adrenergic vascular reactivity. These findings indicate that the sympathoinhibition and lowering of blood pressure and heart rate induced by chronic activation of the carotid baroreflex are not diminished by adaptations in the brain and peripheral circulation. Furthermore, by providing evidence that baroreflexes have long-term effects on sympathetic activity and arterial pressure, they present a perspective that is opposite from studies of sinoaortic denervation.

Desensitization of vascular response in vivo: contribution of genetic variation in the [alpha]2B-adrenergic receptor subtype

OBJECTIVES

Vascular alpha2B-adrenergic receptors (alpha2B-ARs) mediate vasoconstriction and contribute to peripheral regulation of vascular tone. In vitro, a common 301-303 deletion in the alpha2B-AR gene, ADRA2B, results in loss of alpha2B-AR desensitization. We examined the hypothesis that ADRA2B del301-303 or other common ADRA2B variants alter vascular desensitization in vivo.

METHODS

We measured sensitivity to a highly selective alpha2-AR agonist, dexmedetomidine, (0.01-1000 ng/min) in the dorsal hand vein in 41 healthy individuals. To induce desensitization a dose of dexmedetomidine that resulted in submaximal constriction was infused for 180 min and dorsal hand vein responses measured. Desensitization was defined as the ratio between the area-under-the-effect curve for each individual's response and the hypothetical area-under-the-effect curve assuming that the initial response had been maintained for 180 min (ratio below 1 reflecting desensitization). The relationship between six ADRA2B variants (one promoter, three coding, and two in the 3' untranslated region ) with an allele frequency of more than 5% and desensitization was determined.

RESULTS

Forty-one individuals (22 men, 21 whites, age 18-45 years) were studied. The ADRA2B 301-303 deletion allele (ins/del and del/del, n = 18) was associated with resistance to desensitization [1.01 (interquartile range 0.90-1.06)] as compared with ins/ins homozygous individuals (n = 23) [0.91 (interquartile range 0.73-0.99)], P = 0.026. In addition, the -98 GG, 1182 CC, and 1776 CC genotypes were associated with significantly less desensitization than GC or CC, and CA or AA genotypes, respectively.

CONCLUSION

Common ADRA2B variants contribute to the interindividual variability in vascular desensitization to an alpha2-AR agonist in vivo.

Sympathetic Nonadrenergic Transmission Contributes to Autonomic Dysreflexia in Spinal Cord–Injured Individuals

Autonomic dysreflexia is a hypertensive episode in spinal cord–injured individuals induced by exaggerated sympathetic activity and thought to be α-adrenergic mediated. α-Adrenoceptor antagonists have been a rational first choice; nevertheless, calcium channel blockers are primarily used in autonomic dysreflexia management. However, α-adrenoceptor blockade may leave a residual vasoconstrictor response to sympathetic nonadrenergic transmission unaffected. The aim was to assess the α-adrenergic contribution and, in addition, the role of supraspinal control to leg vasoconstriction during exaggerated sympathetic activity provoked by autonomic dysreflexia in spinal cord–injured individuals and by a cold pressure test in control individuals. Upper leg blood flow was measured using venous occlusion plethysmography during supine rest and during exaggerated sympathetic activity in 6 spinal cord–injured individuals and 7 able-bodied control individuals, without and with phentolamine (α-adrenoceptor antagonist) and nicardipine (calcium channel blocker) infusion into the right femoral artery. Leg vascular resistance was calculated. In spinal cord–injured individuals, phentolamine significantly reduced the leg vascular resistance increase during autonomic dysreflexia (8±5 versus 24±13 arbitrary units; P =0.04) in contrast to nicardipine (15±10 versus 24±13 arbitrary units; P =0.12). In controls, phentolamine completely abolished the leg vascular resistance increase during a cold pressure test (1±2 versus 18±14 arbitrary units; P =0.02). The norepinephrine increase during phentolamine infusion was larger ( P =0.04) in control than in spinal cord–injured individuals. These results indicate that the leg vascular resistance increase during autonomic dysreflexia in spinal cord–injured individuals is not entirely α-adrenergic mediated and is partly explained by nonadrenergic transmission, which may, in healthy subjects, be suppressed by supraspinal control.

Phenylephrine but not ephedrine reduces frontal lobe oxygenation following anesthesia-induced hypotension

BACKGROUND

Vasopressor agents are used to correct anesthesia-induced hypotension. We describe the effect of phenylephrine and ephedrine on frontal lobe oxygenation (S(c)O(2)) following anesthesia-induced hypotension.

METHODS

Following induction of anesthesia by fentanyl (0.15 mg kg(-1)) and propofol (2.0 mg kg(-1)), 13 patients received phenylephrine (0.1 mg iv) and 12 patients received ephedrine (10 mg iv) to restore mean arterial pressure (MAP). Heart rate (HR), MAP, stroke volume (SV), cardiac output (CO), and frontal lobe oxygenation (S(c)O(2)) were registered.

RESULTS

Induction of anesthesia was followed by a decrease in MAP, HR, SV, and CO concomitant with an elevation in S(c)O(2). After administration of phenylephrine, MAP increased (51 +/- 12 to 81 +/- 13 mmHg; P < 0.001; mean +/- SD). However, a 14% (from 70 +/- 8% to 60 +/- 7%) reduction in S(c)O(2) (P < 0.05) followed with no change in CO (3.7 +/- 1.1 to 3.4 +/- 0.9 l min(-1)). The administration of ephedrine led to a similar increase in MAP (53 +/- 9 to 79 +/- 8 mmHg; P < 0.001), restored CO (3.2 +/- 1.2 to 5.0 +/- 1.3 l min(-1)), and preserved S(c)O(2).

CONCLUSIONS

The utilization of phenylephrine to correct hypotension induced by anesthesia has a negative impact on S(c)O(2) while ephedrine maintains frontal lobe oxygenation potentially related to an increase in CO.

Prolonged activation of the baroreflex decreases arterial pressure even during chronic adrenergic blockade

Previous studies suggest that prolonged electric activation of the baroreflex may reduce arterial pressure more than chronic blockade of alpha(1)- and beta(1,2)-adrenergic receptors. To determine whether central inhibition of sympathetic outflow has appreciable effects to chronically reduce arterial pressure by actions distinct from well-established mechanisms, we hypothesized that chronic baroreflex activation would lower arterial pressure substantially even during complete alpha(1)- and beta(1,2)-adrenergic receptor blockade. This hypothesis was tested in 6 dogs during adrenergic blockade (AB; 18 days) with and without electric activation of the carotid baroreflex (7 days). During chronic AB alone, there was a sustained decrease in the mean arterial pressure of 21+/-2 mm Hg (control: 95+/-4 mm Hg) and an approximately 3-fold increase in plasma norepinephrine concentration (control: 138+/-6 pg/mL), likely attributed to baroreceptor unloading. In comparison, during AB plus prolonged baroreflex activation, plasma norepinephrine concentration decreased to control levels, and mean arterial pressure fell an additional 10+/-1 mm Hg. Because of differences in plasma norepinephrine concentration, we also tested the acute blood pressure-lowering effects of MK-467, a peripherally acting alpha(2)-antagonist. After administration of MK-467, there was a significantly greater fall in arterial pressure during AB (15+/-3 mm Hg) than during AB plus prolonged baroreflex activation (7+/-3 mm Hg). These findings suggest that reflex-induced increases in sympathetic activity attenuate reductions in arterial pressure during chronic AB and that inhibition of central sympathetic outflow by prolonged baroreflex activation lowers arterial pressure further by previously undefined mechanisms, possibly by diminishing attendant activation of postjunctional alpha(2)-adrenergic receptors.

Angiotensin II in the elderly: impact of angiotensin II type 1 receptor sensitivity on peripheral hemodynamics

Exercise hyperemia is attenuated in the elderly, which may be attributed to local vasoregulatory pathways within the skeletal muscle vasculature. Therefore, we sought to determine whether healthy aging is associated with changes in angiotensin II (Ang II) receptor sensitivity through measurements of leg blood flow in resting and exercising skeletal muscle. In 12 (n=6 young, 24+/-1 years; n=6 older, 68+/-3 years) healthy volunteers, we determined changes in leg blood flow (ultrasound Doppler) before and during intra-arterial infusion of Ang II (0.8 ng/mL of leg blood flow per minute). Heart rate, arterial blood pressure, common femoral artery diameter, and mean blood velocity were measured at rest and during knee-extensor exercise at 20% and 40% of the maximal work rate (WR(max)). At rest, Ang II infusion decreased leg blood flow to a greater extent in older (-61+/-8%) subjects compared with younger subjects (-31+/-5%). Compared with rest, Ang II-mediated vasoconstriction (leg blood flow) during exercise was diminished in both older and younger subjects at 20% (older: -7+/-5%; younger: -21+/-2%) and 40% WR(max) (older: -5+/-4%; younger: -9+/-3%). These data identify a clear age-related hypersensitivity to Ang II in the resting leg, which may contribute to the recognized decrement in leg blood flow in this cohort. However, the diminished vasoconstriction to Ang II during exercise suggests that the elevation in Ang II type 1 receptor sensitivity documented at rest does not contribute significantly to the blunted exercise hyperemia experienced with advancing age.

Autonomic cardiovascular control during a novel pharmacologic alternative to ganglionic blockade

The purpose of this study was to compare ganglionic blockade with trimethaphan (TMP) and an alternative drug strategy using combined muscarinic antagonist (glycopyrrolate, GLY) and alpha-2 agonist (dexmedetomidine, DEX). Protocol 1: incremental phenylephrine was administered during control and combined GLY-DEX, or control and TMP on two control combined GLY and DEX or TMP infusion on two randomized days. Protocol 2: muscle sympathetic nerve activity (MSNA) and the baroreflex MSNA relationship was determined before and after GLY-DEX. Blood pressure was higher with GLY-DEX (99+/-3 mm Hg) and lower with TMP (78+/-3 mm Hg) relative to control (GLY-DEX: 90+/-2 mm Hg; TMP: 91+/-2 mm Hg; P<0.05). Incremental phenylephrine increased pressure during GLY-DEX (P<0.01 vs control) and TMP (P<0.01 vs control) to a similar degree. Both GLY-DEX and TMP infusion inhibited norepinephrine release (P<0.01 vs control). GLY-DEX inhibited baseline MSNA (P<0.05) and baroreflex changes in MSNA (P<0.01). We conclude that the GLY-DEX alternative drug strategy can be used as a reasonable alternative to pharmacologic ganglionic blockade to examine autonomic cardiovascular control.

Contribution of alpha2-adrenoceptors and Y1 neuropeptide Y receptors to the blunting of sympathetic vasoconstriction induced by systemic hypoxia in the rat

There is evidence that sympathetically evoked vasoconstriction in skeletal muscle is blunted in systemic hypoxia, but the mechanisms underlying this phenomenon are not clear. In Saffan-anaesthetized Wistar rats, we have studied the role of α₂-adrenoceptors and neuropeptide Y (NPY) Y₁ receptors in mediating vasoconstriction evoked by direct stimulation of the lumbar sympathetic chain by different patterns of impulses in normoxia (N) and systemic hypoxia (H: breathing 8% O₂). Patterns comprised 120 impulses delivered in bursts over a 1 min period at 40 or 20 Hz, or continuously at 2 Hz. Hypoxia attenuated the evoked increases in femoral vascular resistance (FVR) by all patterns, the response to 2 Hz being most affected (40 Hz bursts: N = 3.25 ± 0.75 arbitrary resistance units (RU); H = 1.14 ± 0.29 RU). Yohimbine (Yoh, α₂-adrenoceptor antagonist) or BIBP 3226 (Y₁-receptor antagonist) did not affect baseline FVR. In normoxia, Yoh attenuated the responses evoked by high frequency bursts and 2 Hz, whereas BIBP 3226 only attenuated the response to 40 Hz (40 Hz bursts: N + Yoh = 2.1 ± 0.59 RU; N + BIBP 3226 = 1.9 ± 0.4 RU). In hypoxia, Yoh did not further attenuate the evoked responses, but BIBP 3226 further attenuated the response to 40 Hz bursts. These results indicate that neither α₂-adrenoceptors nor Y₁ receptors contribute to basal vascular tone in skeletal muscle, but both contribute to constrictor responses evoked by high frequency bursts of sympathetic activity. We propose that in systemic hypoxia, the α₂-mediated component represents about 50% of the sympathetically evoked constriction that is blunted, whereas the contribution made by Y₁ receptors is resistant. Thus we suggest the importance of NPY in the regulation of FVR and blood pressure increases during challenges such as systemic hypoxia.

Alternative to ganglionic blockade with anticholinergic and alpha-2 receptor agents

The ganglionic blocking agent trimethaphan (TMP) is no longer produced. Therefore, a need exists for alternative pharmacological approaches to investigate baroreflex control of the circulation. The aim of the present study was to examine baroreflex-mediated cardiovascular responses during the administration of a muscarinic receptor antagonist (glycopyrrolate; GLY: ) and a selective alpha-2 receptor agonist (dexmedetomidine; DEX: ) and to compare responses to ganglionic blockade with TMP. We hypothesized that combined GLY-: DEX: would inhibit the baroreflex similar to TMP. Ten volunteers participated in two study days and were instrumented with pulse oximeter, nasal cannula, ECG, continuous blood pressure monitoring (Finapres), and I.V. catheter for drug infusions. Each study day consisted of a control condition followed by either combined GLY: -DEX: or TMP on alternating days. A Valsalva maneuver was performed under each condition with every subject and six subjects received bolus phenylephrine (25 mug) during GLY: -DEX: and TMP. Combined GLY: -DEX: increased (P < 0.05) blood pressure (99 +/- 4 mmHg) and heart rate (99 +/- 3 bpm) relative to control condition (BP: 90 +/- 2 mmHg; HR: 64 +/- 3 bpm) and TMP infusion decreased (P < 0.05) blood pressure (79 +/- 3 mmHg) while increasing heart rate (88 +/- 3 bpm). Valsalva maneuver elicited a persistent drop in arterial pressure (no phase IIb recovery) with the absence of a phase IV overshoot during both GLY: -DEX: and TMP conditions. Phenylephrine increased systolic pressure 34 +/- 4 mmHg under GLY: -DEX: and 23 +/- 3 mmHg with TMP (P < 0.05). Heart rate only decreased 1 +/- 2 bpm during GLY: -DEX: and 1 +/- 1 bpm with TMP. Taken together, our results suggest that GLY: -DEX: is a reasonable alternative to TMP for baroreflex inhibition.

Exercise-induced inhibition of angiotensin II vasoconstriction in human thigh muscle

It is well established that metabolic inhibition of adrenergic vasoconstriction contributes to the maintenance of adequate perfusion to exercising skeletal muscle. However, little is known regarding nonadrenergic vasoconstriction during exercise. We tested the hypothesis that a non-adrenergic vasoconstrictor, angiotensin II (AngII), would be less sensitive to metabolic inhibition than an alpha1-agonist, phenylephrine (PE), in the exercising human thigh. In 11 healthy men, femoral blood flow (FBF, ultrasound Doppler and thermodilution) and blood pressure were evaluated during wide-ranging doses of intra-arterial (femoral) infusions of PE and AngII at rest and during two workloads of steady-state knee-extensor exercise (7 W and 27 W). At rest, the maximal decrease in femoral artery diameter (FAD) during AngII (9.0+/-0.2 to 8.4+/-0.4 mm) was markedly less than during PE (9.0+/-0.3 to 5.7+/-0.5 mm), whereas maximal reductions in FBF and femoral vascular conductance (FVC) were similar during AngII (FBF: -65+/-6 and FVC: -66+/-6%) and PE (-57+/-5 and -59+/-4%). During exercise, FAD was not changed by AngII, but moderately decreased by PE. The maximal reductions in FBF and FVC were blunted during exercise compared to rest for both AngII (7 W: -28+/-5 and -40+/-5%; 27 W: -15+/-4% and -29+/-5%) and PE (7 W: -30+/-4 and -37+/-6%; 27 W: -15+/-2 and -24+/-6%), with no significant differences between drugs. The major new findings are (1) an exercise-induced intensity-dependent metabolic attenuation of non-adrenergic vasoconstriction in the human leg; and (2) functional evidence that AngII-vasoconstriction is predominantly distal, whereas alpha1-vasoconstriction is proximal and distal within the muscle vascular bed of the human thigh.

Systemic hypoxia and vasoconstrictor responsiveness in exercising human muscle

Exercise blunts sympathetic alpha-adrenergic vasoconstriction (functional sympatholysis). We hypothesized that sympatholysis would be augmented during hypoxic exercise compared with exercise alone. Fourteen subjects were monitored with ECG and pulse oximetry. Brachial artery and antecubital vein catheters were placed in the nondominant (exercising) arm. Subjects breathed hypoxic gas to titrate arterial O2 saturation to 80% while remaining normocapnic via a rebreath system. Baseline and two 8-min bouts of rhythmic forearm exercise (10 and 20% of maximum) were performed during normoxia and hypoxia. Forearm blood flow, blood pressure, heart rate, minute ventilation, and end-tidal CO2 were measured at rest and during exercise. Vasoconstrictor responsiveness was determined by responses to intra-arterial tyramine during the final 3 min of rest and each exercise bout. Heart rate was higher during hypoxia (P < 0.01), whereas blood pressure was similar (P = 0.84). Hypoxic exercise potentiated minute ventilation compared with normoxic exercise (P < 0.01). Forearm blood flow was higher during hypoxia compared with normoxia at rest (85 +/- 9 vs. 66 +/- 7 ml/min), at 10% exercise (276 +/- 33 vs. 217 +/- 27 ml/min), and at 20% exercise (464 +/- 32 vs. 386 +/- 28 ml/min; P < 0.01). Arterial epinephrine was higher during hypoxia (P < 0.01); however, venoarterial norepinephrine difference was similar between hypoxia and normoxia before (P = 0.47) and during tyramine administration (P = 0.14). Vasoconstriction to tyramine (%decrease from pretyramine values) was blunted in a dose-dependent manner with increasing exercise intensity (P < 0.01). Interestingly, vasoconstrictor responsiveness tended to be greater (P = 0.06) at rest (-37 +/- 6% vs. -33 +/- 6%), at 10% exercise (-27 +/- 5 vs. -22 +/- 4%), and at 20% exercise (-22 +/- 5 vs. -14 +/- 4%) between hypoxia and normoxia, respectively. Thus sympatholysis is not augmented by moderate hypoxia nor does it contribute to the increased blood flow during hypoxic exercise.

Variation in the alpha2B-adrenergic receptor gene (ADRA2B) and its relationship to vascular response in vivo

The alpha2B-adrenergic receptor (ADRA2B) plays an important role in vasoconstriction and blood pressure regulation. One common variant in the ADRA2B gene (del 301--303) has been identified, and results in markedly decreased receptor desensitization in vitro but does not alter vascular sensitivity in vivo. Therefore, we fully characterized genetic variations in ADRA2B and related them to phenotype in vivo. We examined 5812 bp of contiguous sequence of ADRA2B (promoter, exonic, and 3'-untranslated region; 3'-UTR) using the polymerase chain reaction to amplify the genomic target followed by bidirectional sequencing (n=68). Haplotypes were inferred using an expectation maximization algorithm. Vasoconstriction in response to increasing doses of the highly selective alpha2-adrenergic receptor agonist, dexmedetomidine (0.01--1000 ng/min) was measured in the dorsal hand vein using a linear variable differential transformer. The dose that produced 50% (ED50) of maximum venoconstriction (Emax) was determined for each subject from the individual dose--response curves. ED50 and Emax were compared in subjects with and without variant alleles and haplotypes of interest. We identified 24 variable sites, 12 in the promoter region, five in the coding region (including two previously described as non-synonymous variants) and seven in the 3'-UTR region. Four haplotypes were inferred, representing approximately 95% of the cohort. One haplotype, characterized by two single nucleotide polymorphisms in the promoter region, and one in the 3'-UTR, occurred in seven of 38 African-Americans, and was associated with a lower Emax, 61.3% [95% confidence interval (CI) 39.5--83.0, n=7] compared to 78.1% (CI 73.8--82.5) in wild-types (n=61) (P=0.02). There was no association between the nine common variants and dexmedetomidine ED50. We have described novel variants and haplotypes of the ADRA2B gene. These do not alter sensitivity to a selective alpha2-adrenergic receptor agonist but some may decrease maximal venoconstriction in vivo.

Reduced forearm alpha1-adrenergic vasoconstriction is associated with enhanced heart rate fluctuations in humans

In the present study, we assessed whether heart rate (HR) or arterial pressure fluctuations are enhanced in healthy young humans with reduced alpha-adrenergic vasoconstrictor responses and, if so, whether this occurs for both alpha1- and alpha2-adrenergic receptor-mediated vasoconstriction. Arterial pressure (brachial artery catheter) and HR (ECG) were monitored continuously, and alpha1- and alpha2-adrenergic responsiveness was determined by assessing the effects of brachial artery infusions of phenylephrine (alpha1-adrenergic agonist) and dexmedetomidine (alpha2-adrenergic agonist), respectively, on forearm blood flow (strain gauge plethysmography). alpha1-Adrenergic responsiveness varied markedly among the subjects (n=20) and was inversely correlated with coefficient of variation for HR (R2=0.37, P<0.01), whereas the responsiveness was not correlated with the coefficient of variation for either systolic or diastolic arterial pressure. alpha1-Adrenergic responsiveness was inversely and more strongly correlated with baroreflex sensitivity (R2=0.62, P<0.0001), determined from beat-to-beat changes in HR and systolic arterial pressure, than the coefficient of variation for HR. On the other hand, alpha2-adrenergic responsiveness was not correlated with any of the parameters determined above. These results suggest that, in healthy young subjects, the enhanced HR response to changes in systolic pressure helps maintain the stability of arterial blood pressure when alpha1-adrenergic responsiveness is reduced.

Blood Flow to Exercising Limbs Varies With Age, Gender, and Training Status

Understanding the effects of physiological aging on blood flow to active skeletal muscle and its regulation during exercise has important functional, hemodynamic, and metabolic implications for our rapidly expanding elderly population. During peak exercise involving a large muscle mass, blood flow to the legs is lower in healthy older compared to younger persons; this results from central (reduced cardiac output) and peripheral (reduced leg vascular conductance) limitations. There is considerable variability in the literature concerning age-related changes in leg blood flow during submaximal exercise, with reports of similar or reduced leg blood flaw and vascular conductance in older vs. younger subjects depending on the exercise intensity and the gender and training status of the subjects. However, all the studies involving non-endurance-trained subjects are consistent in that older subjects achieve the requisite leg blood flow at higher arterial perfusion pressures than young subjects, suggesting altered local vasoregulatory mechanisms with aging. Although the nature of these age- related alterations is poorly understood, we have preliminary evidence for augmented sympathetic vasoconstrictor responsiveness in the legs of older men during exercise, and blunted leg vasodilator responsiveness in older women. Systematic research will be needed in order to define the central and local mechanisms underlying these age- and gender-specific differences in muscle vascular responsiveness. Such information will be important for designing future interventions aimed at improving muscle blood supply and functional capacity in older persons. Key words: exercise, vascular responsiveness, human

Selective alpha2-adrenergic properties of dexmedetomidine over clonidine in the human forearm

We tested the hypothesis that dexmedetomidine (Dex) has greater alpha(2)- vs. alpha(1) selectivity than clonidine and causes more alpha(2)-selective vasoconstriction in the human forearm. After local beta-adrenergic blockade with propranolol, forearm blood flow (plethysmography) responses to brachial artery administration of Dex, clonidine, and phenylephrine (alpha(1)-agonist) were determined in healthy young adults before and after alpha(2)-blockade with yohimbine (n = 10) or alpha(1)-blockade with prazosin (n = 9). Yohimbine had no effect on phenylephrine-mediated vasoconstriction but blunted Dex-mediated vasoconstriction (mean +/- SE: -41 +/- 5 vs. -11 +/- 2%; before vs. after yohimbine) more than clonidine-mediated vasoconstriction (-39 +/- 5 vs. -28 +/- 4%; before vs. after yohimbine) (P < 0.02). Prazosin blunted phenylephrine-mediated vasoconstriction (-39 +/- 4 vs. -8 +/- 2%; before vs. after prazosin) but had similar effects on both Dex- (-30 +/- 4 vs. -39 +/- 6%; before vs. after prazosin) and clonidine-mediated vasoconstriction (-29 +/- 3 vs. -41 +/- 7%; before vs. after prazosin) (P > 0.7). Both Dex and clonidine reduced deep forearm venous norepinephrine concentrations to a similar extent (-59 +/- 12 vs. -55 +/- 10 pg/ml; Dex vs. clonidine, P > 0.6); this effect was abolished by yohimbine and blunted by prazosin. These results suggest that Dex causes more alpha(2)-selective vasoconstriction in the forearm than clonidine. The similar vasoconstrictor responses to both drugs after prazosin might be explained by the presynaptic effects on norepinephrine release.

??2B Adrenergic receptor 301???303 deletion polymorphism and vascular ??2 adrenergic receptor response

Postsynaptic alpha2B adrenergic receptors (ARs) mediate vasoconstriction. There is more than 1000-fold variability in vascular sensitivity to an alpha2-AR agonist. Genetic variability may contribute to such interindividual differences in sensitivity. A 301-303 deletion (del) polymorphism has been identified in the coding region of the alpha2B-AR gene and has functional effects in vitro. Thus, we examined the hypothesis that the del301-303 polymorphism contributes to variability in vascular alpha2-AR responses in vivo. Healthy subjects were recruited based on their alpha2B-AR genotype. Their vascular sensitivity was determined using a linear variable differential transformer following the infusion of increasing doses (range 0.01-1000 ng/min) of the alpha2-AR agonist, dexmedetomidine, into a dorsal hand vein. The dose that produced 50% (ED50) of maximum venoconstriction (Emax) was determined for each subject. Vascular response was compared among the three genotypes. Forty-nine subjects were studied [28 wild-type wt/wt, 13 wt/del, 8 del/del]. There was no difference in dexmedetomidine ED50 and Emax among the alpha2B-AR del301-303 genotypes. The ED50 was 1.39 ng/min [95% confidence interval (CI) 0.03-63.0 ng/min] in wt/wt subjects, 1.63 ng/min (95% CI 0.01-177.8 ng/min) in wt/del and 2.37 ng/min (95% CI 0.17-33.7 ng/min) in del/del (P=0.80). The average Emax was 75.4+/-14.9% in wt/wt, 75.7+/-21.3% in wt/del and 82.2+/-12.9% in del/del subjects (P=0.26). These findings suggest that the del301-303 polymorphism does not contribute significantly to interindividual in vivo variability in response to alpha2-AR activation in the hand vein.

Annexin A5 scintigraphy of forearm as a novel in vivo model of skeletal muscle preconditioning in humans

BACKGROUND

Nonlethal ischemia and reperfusion reduce ischemia-reperfusion-induced cell death, a phenomenon called ischemic preconditioning. In animal models, this potent endogenous protection is mimicked in vivo by administration of adenosine. In humans, exploitation of ischemic preconditioning is hindered by the lack of an appropriate in vivo model to study this phenomenon. To solve this problem, we aimed to set up an easy-to-use human in vivo model to study ischemic or pharmacological preconditioning.

METHODS AND RESULTS

Healthy male volunteers performed unilateral ischemic handgrip. At reperfusion, we intravenously injected technetium-99m-labeled Annexin A5, a presumed marker of ischemic injury, and we imaged both forearms and hands simultaneously with a gamma camera. Region of interest analysis (counts per pixel) and subsequent calculation of the percentage difference in radioactivity between experimental and control hands (thenar muscle; mean+/-SE) revealed significant uptake to the ischemically exercised tissue (26+/-3% at 4 hours after reperfusion; P<0.05). This selective localization of Annexin A5 was reduced by ischemic preconditioning (10 minutes of ischemia plus reperfusion before ischemic exercise) or by infusion of adenosine into the brachial artery to 6+/-1% and 10+/-3%, respectively (P<0.05 versus ischemic exercise alone), resembling observations in animal models with infarct size as an end point. Appropriate control experiments supported our conclusion.

CONCLUSIONS

Annexin A5 scintigraphy can be applied to test pharmacological or physiological interventions for their ability to prevent ischemia-reperfusion injury.

{alpha}-Adrenoceptor constrictor responses and their modulation in slow-twitch and fast-twitch mouse skeletal muscle

Vasoconstrictor responses to sympathetic nerve stimulation and their sensitivity to metabolic modulation reportedly differ in fast-twitch and slow-twitch muscles, but the underlying mechanisms are not known. Both alpha(1)- and alpha(2)-adrenoceptors mediate these vascular responses in fast-twitch muscle, while their roles in slow-twitch muscle are less well defined. In this study, the phosphorylation of smooth muscle myosin regulatory light chain (smRLC) was measured as an index of vasoconstriction in slow-twitch soleus muscles and fast-twitch extensor digitorum longus (EDL) muscles isolated from C57BL/6J mice. In soleus muscles, incubation with phenylephrine (PE) or UK 14,304 to selectively activate alpha(1)- or alpha(2)-adrenoceptors resulted in concentration-dependent increases in smRLC phosphorylation. To evaluate metabolic modulation of these responses, vasodilator pathways previously implicated in such modulation in fast-twitch muscle were activated in soleus muscles by treatment with the nitric oxide (NO) donor nitroprusside or the ATP-sensitive potassium (K(ATP)) channel opener cromakalim. Both drugs inhibited responses to UK 14,304, but not to PE. The effect of nitroprusside to antagonize UK 14,304 responses was prevented by inhibition of guanylyl cyclase or by blockade of K(ATP) channels, but not by blockade of other potassium channels. Results were similar in EDL muscles. These data provide the first evidence for alpha(2)-adrenoceptor-mediated constriction in slow-twitch muscle, and show that it is sensitive to modulation by NO via a cGMP-dependent mechanism that requires K(ATP) channel activation. Based on the similar findings in soleus and EDL muscles, fibre type does not appear to determine the innate vascular response to alpha(1)- or alpha(2)-adrenoceptor activation.

Neural control of muscle blood flow during exercise

Activation of skeletal muscle fibers by somatic nerves results in vasodilation and functional hyperemia. Sympathetic nerve activity is integral to vasoconstriction and the maintenance of arterial blood pressure. Thus the interaction between somatic and sympathetic neuroeffector pathways underlies blood flow control to skeletal muscle during exercise. Muscle blood flow increases in proportion to the intensity of activity despite concomitant increases in sympathetic neural discharge to the active muscles, indicating a reduced responsiveness to sympathetic activation. However, increased sympathetic nerve activity can restrict blood flow to active muscles to maintain arterial blood pressure. In this brief review, we highlight recent advances in our understanding of the neural control of the circulation in exercising muscle by focusing on two main topics: 1) the role of motor unit recruitment and muscle fiber activation in generating vasodilator signals and 2) the nature of interaction between sympathetic vasoconstriction and functional vasodilation that occurs throughout the resistance network. Understanding how these control systems interact to govern muscle blood flow during exercise leads to a clear set of specific aims for future research.

Combined NO and PG inhibition augments alpha-adrenergic vasoconstriction in contracting human skeletal muscle

Sympathetic alpha-adrenergic vasoconstrictor responses are blunted in the vascular beds of contracting muscle (functional sympatholysis). We tested the hypothesis that combined inhibition of nitric oxide (NO) and prostaglandins (PGs) restores sympathetic vasoconstriction in contracting human muscle. We measured forearm blood flow via Doppler ultrasound and calculated the reduction in forearm vascular conductance in response to alpha-adrenergic receptor stimulation during rhythmic handgrip exercise (6.4 kg) and during a control nonexercise vasodilator condition (using intra-arterial adenosine) before and after combined local inhibition of NO synthase (NOS; via N(G)-nitro-L-arginine methyl ester) and cyclooxygenase (via ketorolac) in healthy men. Before combined inhibition of NO and PGs, the forearm vasoconstrictor responses to intra-arterial tyramine (which evoked endogenous noradrenaline release), phenylephrine (a selective alpha1-agonist), and clonidine (an alpha2-agonist) were significantly blunted during exercise compared with adenosine treatment. After combined inhibition of NO and PGs, the vasoconstrictor responses to all alpha-adrenergic receptor stimuli were augmented by approximately 10% in contracting muscle (P <0.05), whereas the responses to phenylephrine and clonidine were also augmented by approximately 10% during passive vasodilation in resting muscle (P <0.05). In six additional subjects, PG inhibition alone did not alter the vasoconstrictor responses in resting or contracting muscles. Thus in light of our previous findings, it appears that inhibition of either NO or PGs alone does not affect functional sympatholysis in healthy humans. However, the results from the present study indicate that combined inhibition of NO and PGs augments alpha-adrenergic vasoconstriction in contracting muscle but does not completely restore the vasoconstrictor responses compared with those observed during passive vasodilation in resting muscle.