Impact of combined NO and PG blockade on rapid vasodilation in a forearm mild-to-moderate exercise transition in humans

Rapid onset vasodilation during baroreceptor loading and unloading

The purpose of these experiments was to determine if the increase in vascular conductance following a single muscle contraction (50% of maximal voluntary contraction) (6 male and 6 female subjects) was altered during baroceptor loading and unloading. Rapid onset vasodilation (ROV) was determined by measuring brachial artery blood flow (Doppler ultrasound) and blood pressure (Finapress monitor). Brachial artery vascular conductance was calculated by dividing blood flow by mean arterial pressure. ROV was described by the area under the Δvascular conductance (VC)-time curve during the 30 s following muscle contraction. ROV was determined using chamber pressures of +20, +10, 0, -10, -20, and -40 mmHg (lower body positive and negative pressure, LBPP, and LBNP). We tested the hypothesis that the impact of baroreceptor loading and unloading produces a proportion change in ROV. The level of ROV following each contraction was proportional to the peak force (r2 = 0.393, P = 0.0001). Peak force was therefore used as a covariate in further analysis. ROV during application of -40 mmHg LBNP (0.345 ± 0.229 mL·mmHg-1) was lower than that observed at Control (0.532 ± 0.284 mL·mmHg-1, P = 0.034) and +20 mmHg LBPP (0.658 ± 0.364 mL·mmHg-1, P = 0.0008). ROV was linearly related to chamber pressure from -40 to +20 mmHg chamber pressure (r2 = 0.512, P = 0.022, n = 69) and from -20 to +10 mmHg chamber pressure (r2= 0.973, P < 0.0425, n = 45), Overall, vasoconstrictor tone altered with physiologically relevant baroreceptor loading and unloading resulted in a proportion change in ROV.NEW & NOTEWORTHY Rapid onset vasodilation (ROV) was linearly related to the peak force of each single 1-s muscle contraction. In addition, ROV is reduced by baroreceptor unloading (LBNP: -10, -120, and -40 mmHg) and increased by baroreceptor loading (LBPP: +10 and +20 mmHg). Without accounting for peak force and the level of baroreceptor engagement makes comparison of ROV in subjects of differing muscle size or strength untenable.

Nitric oxide mediated hypoxia dynamics in COVID-19

Several COVID-19 patients frequently experience with happy hypoxia. Sometimes, the level of nitric oxide (NO) in COVID-19 patients was found to be greater than in non-COVID-19 hypoxemics and most of the cases lower. Induced or inhaled NO has a long history of usage as a therapy for hypoxemia. Excessive production of ROS and oxidative stress lower the NO level and stimulates mitochondrial malfunction is the primary cause of hypoxia-mediated mortality in COVID-19. Higher level of NO in mitochondria also the cause of dysfunction, because, excess NO can also diffuse quickly into mitochondria or through mitochondrial nitric oxide synthase (NOS). A precise dose of NO may increase oxygenation while also acting as an effective inhibitor of cytokine storm. NOS inhibitors may be used in conjunction with iNO therapy to compensate for the patient's optimal NO level. NO play a key role in COVID-19 happy hypoxia and a crucial component in the COVID-19 pathogenesis that demands a reliable and easily accessible biomarker to monitor.

KIR channel activation links local vasodilatation with muscle fibre recruitment during exercise in humans

KEY POINTS

During exercise, blood flow to working skeletal muscle increases in parallel with contractile activity such that oxygen delivery is sufficient to meet metabolic demand. K⁺ released from active skeletal muscle fibres could facilitate vasodilatation in proportion to the degree of muscle fibre recruitment. Once released, K⁺ stimulates inwardly rectifying K⁺ (K_IR ) channels on the vasculature to elicit an increase in blood flow. In the present study, we demonstrate that K_IR channels mediate the rapid vasodilatory response to an increase in exercise intensity. We also show that K_IR channels augment vasodilatation during exercise which demands greater muscle fibre recruitment independent of the total amount of work performed. These results suggest that K⁺ plays a key role in coupling the magnitude of vasodilatation to the degree of contractile activity. Ultimately, the findings from this study help us understand the signalling mechanisms that regulate muscle blood flow in humans.

ABSTRACT

Blood flow to active skeletal muscle is augmented with greater muscle fibre recruitment. We tested whether activation of inwardly rectifying potassium (K_IR ) channels underlies vasodilatation with elevated muscle fibre recruitment when work rate is increased (Protocol 1) or held constant (Protocol 2). We assessed forearm vascular conductance (FVC) during rhythmic handgrip exercise under control conditions and during local inhibition of K_IR channels (intra-arterial BaCl₂ ). In Protocol 1, healthy volunteers performed mild handgrip exercise for 3 min, then transitioned to moderate intensity for 30 s. BaCl₂ eliminated vasodilatation during the first contraction at the moderate workload (ΔFVC, BaCl₂ : -1 ± 17 vs. control: 30 ± 28 ml min^-1  100 mmHg^-1 ; n = 9; P = 0.004) and attenuated the 30 s area under the curve by 56 ± 14% (n = 9; P < 0.0001). In Protocol 2, participants performed two exercise bouts in which muscle fibre recruitment was manipulated while total contractile work was held constant via reciprocal changes in contraction frequency: (1) low fibre recruitment, with contractions at 12.5% maximal voluntary contraction once every 4 s and (2) high fibre recruitment, with contractions at 25% maximal voluntary contraction once every 8 s. Under control conditions, steady-state FVC was augmented in high vs. low fibre recruitment (211 ± 90 vs. 166 ± 73 ml min^-1 ⋅100 mmHg^-1 ; n = 10; P = 0.0006), whereas BaCl₂ abolished the difference between high and low fibre recruitment (134 ± 59 vs. 134 ± 63 ml min^-1  100 mmHg^-1 ; n = 10; P = 0.85). These findings demonstrate that K_IR channel activation is a key mechanism linking local vasodilatation with muscle fibre recruitment during exercise.

The role of vascular endothelium in nitroglycerin-mediated vasodilation

AIMS

Nitroglycerin (or glyceryl trinitrate, GTN) has been long considered an endothelium-independent vasodilator because GTN vasodilation is intact in the absence of the endothelium and in the presence of endothelial dysfunction. However, in animal and in vitro models, GTN has been shown to stimulate the release of certain endothelium-derived vasodilators such as nitric oxide (NO) and prostacyclin (PGI₂ ). In addition, chronic GTN therapy leads to endothelial dysfunction. In this series of experiments, we explored how GTN might interact with the vascular endothelium in normal humans, without cardiovascular disease or risk factors associated with abnormalities in vascular function.

METHODS

We examined the effect of inhibition of NO, PGI₂ , and epoxyeicosatrienoic acids (EETs, a class of endothelium-derived hyperpolarizing factor) on GTN-mediated vasodilation. We measured arterial blood flow responses to brachial artery infusions of GTN in the absence and presence of L-NMMA (n = 13), ketorolac (n = 14) and fluconazole (n = 16), which are inhibitors of endothelium-derived NO, PGI₂ and EETs, respectively, in healthy volunteers.

RESULTS

Our results demonstrate that inhibition of endothelium-dependent vasodilator mechanisms does not alter forearm resistance vessel responses to GTN.

CONCLUSION

We conclude that GTN-mediated dilation of forearm resistance vessels is largely independent of vascular endothelium.

Single passive leg movement assessment of vascular function: contribution of nitric oxide

Broxterman RM, Trinity JD, Gifford JR, Kwon OS, Kithas AC, Hydren JR, Nelson AD, Morgan DE, Jessop JE, Bledsoe AD, Richardson RS. Single passive leg movement assessment of vascular function: contribution of nitric oxide. J Appl Physiol 123: 1468-1476, 2017. First published August 31, 2017; doi:10.1152/japplphysiol.00533.2017.-The assessment of passive leg movement (PLM)-induced leg blood flow (LBF) and vascular conductance (LVC) is a novel approach to assess vascular function that has recently been simplified to only a single PLM (sPLM), thereby increasing the clinical utility of this technique. As the physiological mechanisms mediating the robust increase in LBF and LVC with sPLM are unknown, we tested the hypothesis that nitric oxide (NO) is a major contributor to the sPLM-induced LBF and LVC response. In nine healthy men, sPLM was performed with and without NO synthase inhibition by intra-arterial infusion of N^G-monomethyl-l-arginine (l-NMMA). Doppler ultrasound and femoral arterial pressure were used to determine LBF and LVC, which were characterized by the peak change (ΔLBF_peak and ΔLVC_peak) and area under the curve (LBF_AUC and LVC_AUC). l-NMMA significantly attenuated ΔLBF_peak [492 ± 153 (l-NMMA) vs. 719 ± 238 (control) ml/min], LBF_AUC [57 ± 34 (l NMMA) vs. 147 ± 63 (control) ml], ΔLVC_peak [4.7 ± 1.1 (l-NMMA) vs. 8.0 ± 3.0 (control) ml·min^-1·mmHg^-1], and LVC_AUC [0.5 ± 0.3 (l-NMMA) vs. 1.6 ± 0.9 (control) ml/mmHg]. The magnitude of the NO contribution to LBF and LVC was significantly correlated with the magnitude of the control responses ( r = 0.94 for ΔLBF_peak, r = 0.85 for LBF_AUC, r = 0.94 for ΔLVC_peak, and r = 0.95 for LVC_AUC). These data establish that the sPLM-induced hyperemic and vasodilatory response is predominantly (~65%) NO-mediated. As such, sPLM appears to be a promising, simple, in vivo assessment of NO-mediated vascular function and NO bioavailability. NEW & NOTEWORTHY Passive leg movement (PLM), a novel assessment of vascular function, has been simplified to a single PLM (sPLM), thereby increasing the clinical utility of this technique. However, the role of nitric oxide (NO) in mediating the robust sPLM hemodynamic responses is unknown. This study revealed that sPLM induces a hyperemic and vasodilatory response that is predominantly NO-mediated and, as such, appears to be a promising simple, in vivo, clinical assessment of NO-mediated vascular function and, therefore, NO bioavailability.

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.

Passive leg movement and nitric oxide-mediated vascular function: the impact of age

In young healthy men, passive leg movement (PLM) elicits a robust nitric oxide (NO)-dependent increase in leg blood flow (LBF), thus providing a novel approach to assess NO-mediated vascular function. While the magnitude of the LBF response to PLM is markedly reduced with age, the role of NO in this attenuated response in the elderly is unknown. Therefore, this study sought to determine the contribution of NO in the PLM-induced LBF with age. Fourteen male subjects (7 young, 24 ± 1 yr; and 7 old, 75 ± 3 yr) underwent PLM with and without NO synthase (NOS) inhibition achieved by intra-arterial infusion of N(G)-monomethyl-L-arginine (L-NMMA). LBF was determined second-by-second by Doppler ultrasound, and central hemodynamics were measured by finger photoplethysmography. NOS inhibition blunted the PLM-induced peak increase in LBF in the young (control: 668 ± 106;

L-NMMA

431 ± 95 Δml/min; P = 0.03) but had no effect in the old (control: 266 ± 98;

L-NMMA

251 ± 92 Δml/min; P = 0.59). Likewise, the magnitude of the reduction in the overall (i.e., area under the curve) PLM-induced LBF response to NOS inhibition was less in the old (LBF: -31 ± 18 ml) than the young (LBF: -129 ± 21 ml; P < 0.01). These findings suggest that the age-associated reduction in PLM-induced LBF in the elderly is primarily due to a reduced contribution to vasodilation from NO and therefore support the use of PLM as a novel approach to assess NO-mediated vascular function across the lifespan.

Regulation of the skeletal muscle blood flow in humans

In humans, skeletal muscle blood flow is regulated by an interaction between several locally formed vasodilators, including NO and prostaglandins. In plasma, ATP is a potent vasodilator that stimulates the formation of NO and prostaglandins and, very importantly, can offset local sympathetic vasoconstriction. Adenosine triphosphate is released into plasma from erythrocytes and endothelial cells, and the plasma concentration increases in both the feed artery and the vein draining the contracting skeletal muscle. Adenosine also stimulates the formation of NO and prostaglandins, but the plasma adenosine concentration does not increase during exercise. In the skeletal muscle interstitium, there is a marked increase in the concentration of ATP and adenosine, and this increase is tightly coupled to the increase in blood flow. The sources of interstitial ATP and adenosine are thought to be skeletal muscle cells and endothelial cells. In the interstitium, both ATP and adenosine stimulate the formation of NO and prostaglandins, but ATP has also been suggested to induce vasoconstriction and stimulate afferent nerves that signal to increase sympathetic nerve activity. Adenosine has been shown to contribute to exercise hyperaemia, whereas the role of ATP remains uncertain due to lack of specific purinergic receptor blockers for human use. The purpose of this review is to address the interaction between vasodilator systems and to discuss the multiple proposed roles of ATP in human skeletal muscle blood flow regulation.

Redundant vasodilator pathways underlying radial artery flow-mediated dilation are preserved in healthy aging

Background. Blocking nitric oxide (NO) and vasodilator prostanoids (PN) does not consistently reduce flow-mediated dilation (FMD) in young adults. The impact of aging on the contribution of NO and PG to FMD is unknown. Methods. FMD was measured in older adults (n = 10, 65 ± 3 y) after arterial infusion of saline, N(G)-monomethyl-L-arginine (L-NMMA), and ketorolac + L-NMMA. Data were compared to published data in young adults. Results. L-NMMA reduced FMD in older adults (8.9 ± 3.6 to 5.9 ± 3.7%) although this was not statistically significant (P = 0.08) and did not differ (P = 0.74) from the reduction observed in young adults (10.0 ± 3.8 to 7.6 ± 4.7%; P = 0.03). Blocking PN did not affect FMD in young or older adults. In older adults, L-NMMA reduced (n = 6; range = 36-123% decrease), augmented (n = 3; 10-122% increase), or did not change FMD (n = 1; 0.4% increase). After PN blockade, FMD responses were reduced (n = 2), augmented (n = 6), or unaffected (n = 1). Conclusions. NO or PN blockade did not consistently reduce FMD in healthy older adults, suggesting the existence of redundant vasodilator phenotypes as observed previously in young adults.

Prostaglandins induce vasodilatation of the microvasculature during muscle contraction and induce vasodilatation independent of adenosine

Blood flow data from contracting muscle in humans indicates that adenosine (ADO) stimulates the production of nitric oxide (NO) and vasodilating prostaglandins (PG) to produce arteriolar vasodilatation in a redundant fashion such that when one is inhibited the other can compensate. We sought to determine whether these redundant mechanisms are employed at the microvascular level. First, we determined whether PGs were involved in active hyperaemia at the microvascular level. We stimulated four to five skeletal muscle fibres in the anaesthetized hamster cremaster preparation in situ and measured the change in diameter of 2A arterioles (maximum diameter 40 μm, third arteriolar level up from the capillaries) at a site of overlap with the stimulated muscle fibres before and after 2 min of contraction [stimulus frequencies: 4, 20 and 60 Hz at 15 contractions per minute (CPM) or contraction frequencies of 6, 15 or 60 CPM at 20 Hz; 250 ms train duration]. Muscle fibres were stimulated in the absence and presence of the phospholipase A2 inhibitor quinacrine. Further, we applied a range of concentrations of ADO (10(-7)-10(-5) M) extraluminally, (to mimic muscle contraction) in the absence and presence of L-NAME (NO synthase inhibitor), indomethacin (INDO, cyclooxygenase inhibitor) and L-NAME + INDO and observed the response of 2A arterioles. We repeated the latter experiment on a different level of the cremaster microvasculature (1A arterioles) and on the microvasculature of a different skeletal muscle (gluteus maximus, 2A arterioles). We observed that quinacrine inhibited vasodilatation during muscle contraction at intermediate and high contraction frequencies (15 and 60 CPM). L-NAME, INDO and L-NAME + INDO were not effective at inhibiting vasodilatation induced by any concentration of ADO tested in 2A and 1A arterioles in the cremaster muscle or 2A arterioles in the gluteus maximus muscle. Our data show that PGs are involved in the vasodilatation of the microvasculature in response to muscle contraction but did not obtain evidence that extraluminal ADO causes vasodilatation through NO or PG or both. Thus, we propose that PG-induced microvascular vasodilation during exercise is independent of ADO.

Nitric oxide and passive limb movement: a new approach to assess vascular function

Passive limb movement elicits a robust increase in limb blood flow (LBF) and limb vascular conductance (LVC), but the peripheral vascular mechanisms associated with this increase in LBF and LVC are unknown. This study sought to determine the contribution of nitric oxide (NO) to movement-induced LBF and LVC and document the potential for passive-limb movement to assess NO-mediated vasodilatation and therefore NO bioavailability. Six subjects underwent passive knee extension with and without nitric oxide synthase (NOS) inhibition via intra-arterial infusion of N(G)-monomethyl-L-arginine (L-NMMA). LBF was determined second-by-second by Doppler ultrasound, and central haemodynamics were measured by finger photoplethysmography. Although L-NMMA did not alter the immediate increase (initial ∼9 s) in LBF and LVC, NOS blockade attenuated the peak increase in LBF (control: 653 ± 81; L-NMMA: 399 ± 112 ml(−1) min(−1), P = 0.03) and LVC (control: 7.5 ± 0.8; L-NMMA: 4.1 ± 1.1 ml min(−1) mmHg(−1), P = 0.02) and dramatically reduced the overall vasodilatory and hyperaemic response (area under the curve) by nearly 80% (LBF: control: 270 ± 51; L-NMMA: 75 ± 32 ml, P = 0.001; LVC: control: 2.9 ± 0.5; L-NMMA: 0.8 ± 0.3 ml mmHg(−1), P < 0.001). Passive movement in control and L-NMMA trials evoked similar increases in heart rate, stroke volume, cardiac output and a reduction in mean arterial pressure. As movement-induced increases in LBF and LVC are predominantly NO dependent, passive limb movement appears to have significant promise as a new approach to assess NO-mediated vascular function, an important predictor of cardiovascular disease risk.

Role of nitric oxide and prostanoids in the regulation of leg blood flow and blood pressure in humans with essential hypertension: effect of high-intensity aerobic training

We examined the role of nitric oxide (NO) and prostanoids in the regulation of leg blood flow and systemic blood pressure before and after 8 weeks of aerobic high-intensity training in individuals with essential hypertension (n = 10) and matched healthy control subjects (n = 11). Hypertensive subjects were found to have a lower (P < 0.05) blood flow to the exercising leg than normotensive subjects (30 W: 2.92 ± 0.16 vs. 3.39 ± 0.37 l min(−1)). Despite the lower exercise hyperaemia, pharmacological inhibition of the NO and prostanoid systems reduced leg blood flow to a similar extent during exercise in the two groups and vascular relaxation to the NO-dependent vasodilator acetylcholine was also similar between groups. High-intensity aerobic training lowered (P < 0.05) resting systolic (∼9 mmHg) and diastolic (∼12 mmHg) blood pressure in subjects with essential hypertension, but this effect of training was abolished when the NO and prostanoid systems were inhibited. Skeletal muscle vascular endothelial NO synthase uncoupling, expression and phosphorylation status were similar in the two groups before and after training. These data demonstrate that a reduction in exercise hyperaemia in hypertensive subjects is not associated with a reduced capacity of the NO and prostanoid systems to induce vasodilatation or with altered acetylcholine-induced response. However, our data suggest that the observed reduction in blood pressure is related to a training-induced change in the tonic effect of NO and/or prostanoids on vascular tone.

Local control of skeletal muscle blood flow during exercise: influence of available oxygen

Reductions in oxygen availability (O(2)) by either reduced arterial O(2) content or reduced perfusion pressure can have profound influences on the circulation, including vasodilation in skeletal muscle vascular beds. The purpose of this review is to put into context the present evidence regarding mechanisms responsible for the local control of blood flow during acute systemic hypoxia and/or local hypoperfusion in contracting muscle. The combination of submaximal exercise and hypoxia produces a "compensatory" vasodilation and augmented blood flow in contracting muscles relative to the same level of exercise under normoxic conditions. A similar compensatory vasodilation is observed in response to local reductions in oxygen availability (i.e., hypoperfusion) during normoxic exercise. Available evidence suggests that nitric oxide (NO) contributes to the compensatory dilator response under each of these conditions, whereas adenosine appears to only play a role during hypoperfusion. During systemic hypoxia the NO-mediated component of the compensatory vasodilation is regulated through a β-adrenergic receptor mechanism at low-intensity exercise, while an additional (not yet identified) source of NO is likely to be engaged as exercise intensity increases during hypoxia. Potential candidates for stimulating and/or interacting with NO at higher exercise intensities include prostaglandins and/or ATP. Conversely, prostaglandins do not appear to play a role in the compensatory vasodilation during exercise with hypoperfusion. Taken together, the data for both hypoxia and hypoperfusion suggest NO is important in the compensatory vasodilation seen when oxygen availability is limited. This is important from a basic biological perspective and also has pathophysiological implications for diseases associated with either hypoxia or hypoperfusion.

Heterogenous vasodilator pathways underlie flow-mediated dilation in men and women

This study investigated the sex differences in the contribution of nitric oxide (NO) and prostaglandins (PGs) to flow-mediated dilation (FMD). Radial artery (RA) FMD, assessed as the dilatory response to 5-min distal cuff occlusion, was repeated after three separate brachial artery infusions of saline (SAL), N(G)-monomethyl-L-arginine (L-NMMA), and ketorolac (KETO) + L-NMMA in healthy younger men (M; n = 8) and women (W; n = 8). In eight subjects (4 M, 4W) RA FMD was reassessed on a separate day with drug order reversed (SAL, KETO, and L-NMMA + KETO). RA FMD was calculated as the peak dilatory response observed relative to baseline (%FMD) and expressed relative to the corresponding area under the curve shear stress (%FMD/AUC SS). L-NMMA reduced %FMD similarly and modestly (P = 0.68 for sex * trial interaction) in M and W (all subjects: 10.0 ± 3.8 to 7.6 ± 4.7%; P = 0.03) with no further effect of KETO (P = 0.68). However, all sex * trial and trial effects on %FMD/AUC SS for l-NMMA and KETO + l-NMMA were insignificant (all P > 0.20). There was also substantial heterogeneity of the magnitude and direction of dilator responses to blockade. After l-NMMA infusion, subjects exhibited both reduced (n = 14; range: 11 to 78% decrease) and augmented (n = 2; range: 1 to 96% increase) %FMD. Following KETO + l-NMMA, seven subjects exhibited reduced dilation (range: 10 to 115% decrease) and nine subjects exhibited augmented dilation (range: 1 to 212% increase). Reversing drug order did not change the nature of the findings. These findings suggest that RA FMD is not fully or uniformly NO dependent in either men or women, and that there is heterogeneity in the pathways underlying the conduit dilatory response to ischemia.

Prostaglandins do not contribute to the nitric oxide-mediated compensatory vasodilation in hypoperfused exercising muscle

We tested the hypothesis that 1) prostaglandins (PGs) contribute to compensatory vasodilation in contracting human forearm subjected to acute hypoperfusion, and 2) the combined inhibition of PGs and nitric oxide would attenuate the compensatory vasodilation more than PG inhibition alone. In separate protocols, subjects performed forearm exercise (20% of maximum) during hypoperfusion evoked by intra-arterial balloon inflation. Each trial included baseline, exercise before inflation, exercise with inflation, and exercise after deflation. Forearm blood flow (FBF; ultrasound) and local (brachial artery) and systemic arterial pressure [mean arterial pressure (MAP); Finometer] were measured. In protocol 1 (n = 8), exercise was repeated during cyclooxygenase (COX) inhibition (Ketorolac) alone and during Ketorolac-NOS inhibition [N(G)-monomethyl-l-arginine (l-NMMA)]. In protocol 2 (n = 8), exercise was repeated during l-NMMA alone and during l-NMMA-Ketorolac. Forearm vascular conductance (FVC; ml·min(-1)·100 mmHg(-1)) was calculated from FBF (ml/min) and local MAP (mmHg). The percent recovery in FVC during inflation was calculated as (steady-state inflation + exercise value - nadir)/[steady-state exercise (control) value - nadir] × 100. In protocol 1, COX inhibition alone did not reduce the %FVC recovery compared with the control (no drug) trial (92 ± 11 vs. 100 ± 10%, P = 0.83). However, combined COX-nitric oxide synthase (NOS) inhibition caused a substantial reduction in %FVC recovery (54 ± 8%, P < 0.05 vs. Ketorolac alone). In protocol 2, the percent recovery in FVC was attenuated with NOS inhibition alone (69 ± 9 vs. 107 ± 10%, P < 0.01) but not attenuated further during combined NOS-COX inhibition (62 ± 10%, P = 0.74 vs. l-NMMA alone). Our data indicate that PGs are not obligatory to the compensatory dilation observed during forearm exercise with hypoperfusion.

Nitric oxide-mediated vasodilation becomes independent of beta-adrenergic receptor activation with increased intensity of hypoxic exercise

Hypoxic vasodilation in skeletal muscle at rest is known to include β-adrenergic receptor-stimulated nitric oxide (NO) release. We previously reported that the augmented skeletal muscle vasodilation during mild hypoxic forearm exercise includes β-adrenergic mechanisms. However, it is unclear whether a β-adrenergic receptor-stimulated NO component exists during hypoxic exercise. We hypothesized that NO-mediated vasodilation becomes independent of β-adrenergic receptor activation with increased exercise intensity during hypoxic exercise. Ten subjects (7 men, 3 women; 23 ± 1 yr) breathed hypoxic gas to titrate arterial O(2) saturation to 80% while remaining normocapnic. Subjects performed two consecutive bouts of incremental rhythmic forearm exercise (10% and 20% of maximum) with local administration (via a brachial artery catheter) of propranolol (β-adrenergic receptor inhibition) alone and with the combination of propranolol and nitric oxide synthase inhibition [N(G)-monomethyl-l-arginine (l-NMMA)] under normoxic and hypoxic conditions. Forearm blood flow (FBF, ml/min; Doppler ultrasound) and blood pressure [mean arterial pressure (MAP), mmHg; brachial artery catheter] were assessed, and forearm vascular conductance (FVC, ml·min(-1)·100 mmHg(-1)) was calculated (FBF/MAP). During propranolol alone, the rise in FVC (Δ from normoxic baseline) due to hypoxic exercise was 217 ± 29 and 415 ± 41 ml·min(-1)·100 mmHg(-1) (10% and 20% of maximum, respectively). Combined propranolol-l-NMMA infusion during hypoxic exercise attenuated ΔFVC at 20% (352 ± 44 ml·min(-1)·100 mmHg(-1); P < 0.001) but not at 10% (202 ± 28 ml·min(-1)·100 mmHg(-1); P = 0.08) of maximum compared with propranolol alone. These data, when integrated with earlier findings, demonstrate that NO contributes to the compensatory vasodilation during mild and moderate hypoxic exercise; a β-adrenergic receptor-stimulated NO component exists during low-intensity hypoxic exercise. However, the source of the NO becomes less dependent on β-adrenergic mechanisms as exercise intensity increases.

Crucial importance of the endothelial K+ channel SK3 and connexin40 in arteriolar dilations during skeletal muscle contraction

Skeletal muscle activity requires substantial increases in blood flow, and the underlying vasodilation involves endothelial activity, but the contribution of the endothelium-dependent hyperpolarizing factor (EDHF) is only poorly defined. In EDHF signaling, endothelial hyperpolarization mediated by the Ca(2+)-activated K(+) channels SK3 and IK1 is a key step and also initiates gap junction-dependent conducted dilations. We assessed the role of SK3, IK1, and connexin40 (Cx40) in muscular contraction-induced dilations in the microcirculation in vivo. Hitherto, arterioles were observed in the electrically stimulated cremaster skeletal muscle of anesthetized mice lacking SK3, IK1, or Cx40 using intravital microscopy. Genetic deficiency of SK3, but not of IK1, strongly attenuated dilations to muscular contraction. Similarly, pharmacologic blockade of SK3 by the specific blocker UCL1684 impaired such dilations in wild-type and IK1-deficient mice. In contrast, IK1 was required for acetylcholine-induced dilations. Genetic deficiency of Cx40 also attenuated dilations induced by muscular contraction but not by acetylcholine. These data support the concept that endothelial hyperpolarization through activation of SK3 contributes to exercise hyperemia and the hyperpolarization ascends the vascular tree through gap junctions formed by Cx40 to orchestrate dilation. The differential impact of SK3- and IK1-deficiency on dilations to distinct stimuli suggests stimulus-dependent activation of these endothelial channels.

Nitric oxide contributes to the augmented vasodilatation during hypoxic exercise

We tested the hypotheses that (1) nitric oxide (NO) contributes to augmented skeletal muscle vasodilatation during hypoxic exercise and (2) the combined inhibition of NO production and adenosine receptor activation would attenuate the augmented vasodilatation during hypoxic exercise more than NO inhibition alone. In separate protocols subjects performed forearm exercise (10% and 20% of maximum) during normoxia and normocapnic hypoxia (80% arterial O(2) saturation). In protocol 1 (n = 12), subjects received intra-arterial administration of saline (control) and the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA). In protocol 2 (n = 10), subjects received intra-arterial saline (control) and combined L-NMMA-aminophylline (adenosine receptor antagonist) administration. Forearm vascular conductance (FVC; ml min(-1) (100 mmHg)(-1)) was calculated from forearm blood flow (ml min(-1)) and blood pressure (mmHg). In protocol 1, the change in FVC (Delta from normoxic baseline) due to hypoxia under resting conditions and during hypoxic exercise was substantially lower with L-NMMA administration compared to saline (control; P < 0.01). In protocol 2, administration of combined L-NMMA-aminophylline reduced the DeltaFVC due to hypoxic exercise compared to saline (control; P < 0.01). However, the relative reduction in DeltaFVC compared to the respective control (saline) conditions was similar between L-NMMA only (protocol 1) and combined L-NMMA-aminophylline (protocol 2) at 10% (-17.5 +/- 3.7 vs. -21.4 +/- 5.2%; P = 0.28) and 20% (-13.4 +/- 3.5 vs. -18.8 +/- 4.5%; P = 0.18) hypoxic exercise. These findings suggest that NO contributes to the augmented vasodilatation observed during hypoxic exercise independent of adenosine.

Adenosine contributes to blood flow regulation in the exercising human leg by increasing prostaglandin and nitric oxide formation

Adenosine can induce vasodilation in skeletal muscle, but to what extent adenosine exerts its effect via formation of other vasodilators and whether there is redundancy between adenosine and other vasodilators remain unclear. We tested the hypothesis that adenosine, prostaglandins, and NO act in synergy to regulate skeletal muscle hyperemia by determining the following: (1) the effect of adenosine receptor blockade on skeletal muscle exercise hyperemia with and without simultaneous inhibition of prostaglandins (indomethacin; 0.8 to 1.8 mg/min) and NO (N(G)-mono-methyl-l-arginine; 29 to 52 mg/min); (2) whether adenosine-induced vasodilation is mediated via formation of prostaglandins and/or NO; and (3) the femoral arterial and venous plasma adenosine concentrations during leg exercise with the microdialysis technique in a total of 24 healthy, male subjects. Inhibition of adenosine receptors (theophylline; 399+/-9 mg, mean +/- SEM) or combined inhibition of prostaglandins and NO formation inhibited the exercise-induced increase in leg blood flow by 14+/-1% and 29+/-2% (P<0.05), respectively, but combined inhibition of prostaglandins, NO, and adenosine receptors did not result in an additive reduction of leg blood flow (31+/-5%). Femoral arterial infusion of adenosine increased leg blood flow from approximately 0.3 to approximately 2.5 L/min. Inhibition of prostaglandins or NO, or prostaglandins and NO combined, inhibited the adenosine-induced increase in leg blood flow by 51+/-3%, 39+/-8%, and 66+/-8%, respectively (P<0.05). Arterial and venous plasma adenosine concentrations were similar at rest and during exercise. These results suggest that adenosine contributes to the regulation of skeletal muscle blood flow by stimulating prostaglandin and NO synthesis.

Epigenetic regulation of vascular endothelial gene expression

Epigenetics refers to chromatin-based pathways important in the regulation of gene expression and includes 3 distinct, but highly interrelated, mechanisms: DNA methylation, histone density and posttranslational modifications, and RNA-based mechanisms. Together, they offer a newer perspective on transcriptional control paradigms in vascular endothelial cells and provide a molecular basis for how the environment impacts the genome to modify disease susceptibility. This review provides an introduction to epigenetic concepts for vascular biologists. Using endothelial nitric oxide synthase (NOS3) as an example, we examine the growing body of evidence implicating epigenetic pathways in the control of vascular endothelial gene expression in health and disease.

Enhanced exocytotic-like insertion of Orai1 into the plasma membrane upon intracellular Ca2+ store depletion

Ca+ release-activated Ca2+ (CRAC) channels are activated when free Ca2+ concentration in the intracellular stores is substantially reduced and mediate sustained Ca2+ entry. Recent studies have identified Orai1 as a CRAC channel subunit. Here we demonstrate that passive Ca2+ store depletion using the inhibitor of the sarcoendoplasmic reticulum Ca2+-ATPase, thapsigargin (TG), enhances the surface expression of Orai1, a process that depends on rises in cytosolic free Ca2+ concentration, as demonstrated in cells loaded with dimethyl BAPTA, an intracellular Ca2+ chelator that prevented TG-evoked cytosolic free Ca2+ concentration elevation. Similar results were observed with a low concentration of carbachol. Cleavage of the soluble N-ethylmaleimide-sensitive-factor attachment protein receptor, synaptosomal-assiciated protein-25 (SNAP-25), with botulinum neurotoxin A impaired TG-induced increase in the surface expression of Orai1. In addition, SNAP-25 cleaving by botulinum neurotoxin A reduces the maintenance but not the initial stages of store-operated Ca2+ entry. In aggregate, these findings demonstrate that store depletion enhances Orai1 plasma membrane expression in an exocytotic manner that involves SNAP-25, a process that contributes to store-dependent Ca2+ entry.

Nitric oxide and muscle blood flow in exercise

Despite being the subject of investigation for well over 100 years, the nature of exercising muscle blood flow control remains, in many respects, poorly understood. In this review we focus on the potential role of nitric oxide in vasodilation of muscle resistance vessels during a bout of exercise. Its contribution is explored in the context of whether it contributes to steady-state exercise hyperemia, the dynamic adjustment of muscle blood flow to exercise, or the modulation of sympathetic vasoconstriction in exercising muscle. It appears that the obligatory role of nitric oxide in all three of these categories is modest at best. The elucidation of the integrated nature of exercise hyperemia control in terms of synergy and redundancy of mechanism interaction remains in its infancy, and much more remains to be learned about the role of nitric oxide in this type of integrated control.

Energy metabolism in intensively exercising calf muscle under a simulated orthostasis

We conducted non-invasive methods to investigate the mechanisms how an orthostasis improves fatigue resistance in human calf muscle during intense exercise. Eleven healthy volunteers performed two series of ten intervals of maximum dynamic exercise (15 s) and recovery (45 s) at almost horizontal body position under both, control conditions (CON) and lower body negative pressure (LBNP, -40 mbar). As from the second work interval, LBNP significantly improved fatigue resistance shown as a lower reduction in work and in contraction velocity (P < 0.01). During each work interval, EMG showed a small increase in amplitude (P < 0.01) and a steep drop by 20% in median frequency (P < 0.01). Under LBNP, both EMG parameters completely recovered during subsequent rest, whereas under CON recovery was incomplete (P < 0.01). During the first work interval, consumption of phosphocreatine (PCr) was almost the same for both conditions. In periods of recovery under LBNP, resynthesis of PCr and inorganic phosphate were significantly faster. PCr reached 10 to 20% higher levels (P < 0.01). LBNP caused an initial increase in intracellular pH (0.08 U (P < 0.01)). The subsequent time courses of pH were similar for CON and LBNP. During work, pH steeply increased by about 0.3 U. During subsequent recovery, pH dropped to values between 6.3 and 6.5. LBNP caused significantly higher levels of total haemoglobin and oxy-haemoglobin (P < 0.05). A simulated orthostasis increased fatigue resistance during high intense interval exercise because of a faster PCr resynthesis and may be because of improvements in the maintenance of motoneuronal activity.

Mechanical influences on skeletal muscle vascular tone in humans: insight into contraction-induced rapid vasodilatation

We tested the hypothesis that mechanical deformation of forearm blood vessels via acute increases in extravascular pressure elicits rapid vasodilatation in humans. In healthy adults, we measured forearm blood flow (Doppler ultrasound) and calculated forearm vascular conductance (FVC) responses to whole forearm compressions and isometric muscle contractions with the arm above heart level. We used several experimental protocols to gain insight into how mechanical factors contribute to contraction-induced rapid vasodilatation. The findings from the present study clearly indicate that acute increases in extravascular pressure (200 mmHg for 2 s) elicit a significant rapid vasodilatation in the human forearm (peak DeltaFVC approximately 155%). Brief, 6 s sustained compressions evoked the greatest vasodilatation (DeltaFVC approximately 260%), whereas the responses to single (2 s) and repeated compressions (five repeated 2 s compressions) were not significantly different (DeltaFVC approximately 155% versus approximately 115%, respectively). This mechanically induced vasodilatation peaks within 1-2 cardiac cycles, and thus is dissociated from the temporal pattern normally observed in response to brief muscle contractions ( approximately 4-7 cardiac cycles). A non-linear relation was found between graded increases in extravascular pressure and both the immediate and peak rapid vasodilatory response, such that the responses increased sharply from 25 to 100 mmHg, with no significant further dilatation until 300 mmHg (maximal DeltaFVC approximately 185%). This was in contrast to the linear intensity-dependent relation observed with muscle contractions. Our collective findings indicate that mechanical influences contribute largely to the immediate vasodilatation (first cardiac cycle) observed in response to a brief, single contraction. However, it is clear that there are additional mechanisms related to muscle activation that continue to cause and sustain vasodilatation for several more cardiac cycles after contraction. Additionally, the potential contribution of mechanical influences to the total contraction-induced hyperaemia appears greatest for low to moderate intensity single muscle contractions, and this contribution becomes less significant for sustained and repeated contractions. Nevertheless, this mechanically induced vasodilatation could serve as a feedforward mechanism to increase muscle blood flow at the onset of exercise, as well as in response to changes in contraction intensity, prior to alterations in local vasodilating substances that influence vascular tone.

Inhibition of nitric oxide and prostaglandins, but not endothelial-derived hyperpolarizing factors, reduces blood flow and aerobic energy turnover in the exercising human leg

Prostaglandins, nitric oxide (NO) and endothelial-derived hyperpolarizing factors (EDHFs) are substances that have been proposed to be involved in the regulation of skeletal muscle blood flow during physical activity. We measured haemodynamics, plasma ATP at rest and during one-legged knee-extensor exercise (19 +/- 1 W) in nine healthy subjects with and without intra-arterial infusion of indomethacin (Indo; 621 +/- 17 microg min(-1)), Indo + N(G)-monomethyl-L-arginine (L-NMMA; 12.4 +/- 0.3 mg min(-1)) (double blockade) and Indo + L-NMMA + tetraethylammonium chloride (TEA; 12.4 +/- 0.3 mg min(-1)) (triple blockade). Double and triple blockade lowered leg blood flow (LBF) at rest (P<0.05), while it remained unchanged with Indo. During exercise, LBF and vascular conductance were 2.54 +/- 0.10 l min(-1) and 25 +/- 1 mmHg, respectively, in control and they were lower with double (33 +/- 3 and 36 +/- 4%, respectively) and triple (26 +/- 4 and 28 +/- 3%, respectively) blockade (P<0.05), while there was no difference with Indo. The lower LBF and vascular conductance with double and triple blockade occurred in parallel with a lower O(2) delivery, cardiac output, heart rate and plasma [noradrenaline] (P<0.05), while blood pressure remained unchanged and O(2) extraction and femoral venous plasma [ATP] increased. Despite the increased O(2) extraction, leg was 13 and 17% (triple and double blockade, respectively) lower than control in parallel to a lower femoral venous temperature and lactate release (P<0.05). These results suggest that NO and prostaglandins play important roles in skeletal muscle blood flow regulation during moderate intensity exercise and that EDHFs do not compensate for the impaired formation of NO and prostaglandins. Moreover, inhibition of NO and prostaglandin formation is associated with a lower aerobic energy turnover and increased concentration of vasoactive ATP in plasma.

Muscle blood-flow dynamics at exercise onset: do the limbs differ?

Common approaches to understanding control of muscle blood flow in exercise focus on the contributions of various putative vasoregulatory mechanisms to the magnitude of the steady-state response. The application of systems-control principles offers a unique approach to characterizing and quantifying the non-steady-state adaptation of muscle blood flow with exercise onset. Information gained from this approach provides novel insight into the nature of control mechanisms governing physiological responses to exercise. This review is intended to provide the reader with an understanding of 1) exercise models, methodology for measuring muscle blood flow, and analysis approaches for quantifying muscle blood-flow dynamics; 2) what is currently known about the dynamic response of muscle blood-flow control mechanisms in humans; and 3) the similarities and differences in exercising muscle blood-flow control in the upper versus the lower limbs in humans.

Aberrant cytoplasmic sequestration of eNOS in endothelial cells after monocrotaline, hypoxia, and senescence: live-cell caveolar and cytoplasmic NO imaging

We previously reported the disruption of caveolae/rafts, dysfunction of Golgi tethers, N-ethylmaleimide-sensitive factor-attachment protein (SNAP) receptor proteins (SNAREs), and SNAPs, and inhibition of anterograde trafficking in endothelial cells in culture and rat lung exposed to monocrotaline pyrrole (MCTP) as a prelude to the development of pulmonary hypertension. We have now investigated 1) whether this trafficking block affects subcellular localization and function of endothelial nitric oxide (NO) synthase (eNOS) and 2) whether Golgi blockade and eNOS sequestration are observed after hypoxia and senescence. Immunofluorescence data revealed that MCTP-induced "megalocytosis" of pulmonary arterial endothelial cells (PAEC) was accompanied by a loss of eNOS from the plasma membrane, with increased accumulation in the cytoplasm. This cytoplasmic eNOS was sequestered in heterogeneous compartments and partially colocalized with Golgi and endoplasmic reticulum (ER) markers, caveolin-1, NOSTRIN, and ER Tracker, but not Lyso Tracker. Hypoxia and senescence also produced enlarged PAEC, with dysfunctional Golgi and loss of eNOS from the plasma membrane, with sequestration in the cytoplasm. Live-cell imaging of caveolar and cytoplasmic NO with 4,5-diaminofluorescein diacetate (DAF-2DA) as probe showed a marked loss of caveolar NO after MCTP, hypoxia, and senescence. Although ionomycin stimulated DAF-2DA fluorescence in control PAEC, this ionophore decreased DAF-2DA fluorescence in MCTP-treated and senescent PAEC, suggesting localization of eNOS in an aberrant cytoplasmic compartment that was readily discharged by Ca(2+)-induced exocytosis. Thus monocrotaline, hypoxia, and senescence produce a Golgi blockade in PAEC, leading to sequestration of eNOS away from its functional caveolar location and providing a mechanism for the often-reported reduction in pulmonary arterial NO levels in experimental pulmonary hypertension, despite sustained eNOS protein levels.

Metabolic and Vascular Limb Differences Affected by Exercise, Gender, Age, and Disease

PURPOSE

We studied previously resistance-trained men and compared the effects of concentric and eccentric training on performance and structural muscle parameters.

METHODS

Seventeen trained individuals (age 26.9 +/- 3.4 yr) participated in 12 wk of either maximum concentric (N = 8) or eccentric (N = 9) resistance training of the elbow flexors. The functional performance was measured as the maximum concentric and eccentric strength and angular velocity at standard loads. Muscle cross-sectional area and cross-sectional area of single cells were used as measures of muscular hypertrophy. Fiber-type proportions were assessed by staining cells for myofibrillar ATPase.

RESULTS

Both eccentric and concentric training increased concentric strength to a similar extent (14 vs 18%), whereas eccentric training led to greater increases in eccentric strength than concentric training did (26 vs 9%). The maximum angular velocity at all loads was enhanced equally in both training groups. The cross-sectional area of both the elbow flexors (+11%) and of the type I and type IIA fibers increased only after the eccentric training. In addition, the relative cross-sectional area occupied by the type II fibers increased from 64 to 73% after the eccentric training. There were only minor changes in the fiber-type proportions.

CONCLUSION

The present data suggest that for resistance-trained men, increases in concentric strength and velocity performance after eccentric training are largely mediated by changes in fiber and muscle cross-sectional area. However, hypertrophy alone could not explain the increase in eccentric strength. Because the increases in strength and velocity performance after concentric training could not be ascribed to muscular adaptations alone, we suggest that they may be attributable to additional neural factors.

Rapid vasoregulatory mechanisms in exercising human skeletal muscle: dynamic response to repeated changes in contraction intensity

We tested the hypothesis that vasoregulatory mechanisms exist in humans that can rapidly adjust muscle blood flow to repeated increases and decreases in exercise intensity. Six men and seven women (age, 24.4 ± 1.3 yr) performed continuous dynamic forearm handgrip contractions (1- to 2-s contraction-to-relaxation duty cycle) during repeated step increases and decreases in contraction intensity. Three step change oscillation protocols were examined: Slow (7 contractions per contraction intensity × 10 steps); Fast (2 contractions per contraction intensity × 15 steps); and Very Fast (1 contraction per contraction intensity × 15 steps). Forearm blood flow (FBF; Doppler and echo ultrasonography), heart rate (ECG), and mean arterial pressure (arterial tonometry) were examined for the equivalent of a cardiac cycle during each relaxation phase (FBFrelax). Mean arterial pressure and heart rate did not change during repeated step changes ( P = 0.352 and P = 0.190). For both Slow and Fast conditions, relaxation phase FBFrelaxadjusted immediately and repeatedly to both increases and decreases in contraction intensity, and the magnitude and time course of FBFrelaxchanges were virtually identical. For the Very Fast condition, FBFrelaxincreased with the first contraction and thereafter slowly increased over the course of repeated contraction intensity oscillations. We conclude that vasoregulatory mechanisms exist in human skeletal muscle that are capable of rapidly and repeatedly adjusting muscle blood flow with ongoing step changes in contraction intensity. Importantly, they demonstrate symmetry in response magnitude and time course with increasing versus decreasing contraction intensity but cannot adjust to very fast exercise intensity oscillations.

Effects of aging on microvascular oxygen pressures in rat skeletal muscle

Aging alters skeletal muscle vascular geometry and control such that the dynamics of muscular blood flow (Q) and O2 delivery (Q(O2)) may be impaired across the rest-exercise transition. If, at the onset of muscle contractions, Q dynamics are slowed disproportionately to those of muscle O2 uptake (V(O2), microvascular PO2 (PO2m) would be reduced and blood-tissue O2 transfer compromised. This investigation determined the effects of aging on PO2m (a direct reflection of the Q(O2)-to-V(O2) ratio), at rest and across the rest-contractions transition in the spinotrapezius of young (approximately 6 months, n = 9) and old (>24 months, n = 10) male Fisher 344/Brown Norway hybrid rats. Phosphorescence quenching techniques were used to quantify PO2m, and test the hypothesis that, across the rest-contractions (twitch, 1 Hz; 4-6 V, 240 s) transition, aging would transiently reduce the Q(O2)-to-V(O2) ratio causing a biphasic profile in which PO2m fell below steady-state contracting values. Old rats had a lower pre-contraction baseline PO2m than young (27.1+/-1.9 versus 33.8+/-1.6 mmHg, P<0.05, respectively). In addition, in old rats PO2m demonstrated a pronounced difference between the absolute nadir and end-contracting values (2.5+/-0.9 mmHg), which was absent in young rats. In conclusion, unlike their young counterparts, old rats exhibited a transiently reduced PO2m across the rest-contractions transition that may impair blood-tissue O2 exchange and elevate the O2 deficit, thereby contributing to premature fatigue.

Effect of physical activity on MRI-measured blood oxygen level-dependent transients in skeletal muscle after brief contractions

The signal intensity (SI) in gradient-echo, echo-planar magnetic resonance images (repetition time/echo time = 1,000/40) of anterior tibialis muscle in active [estimated energy expenditure 42.4 +/- 3.7 (SD), n = 8] vs. sedentary (32.3 +/- 0.6 kcal.kg(-1).day(-1), n = 8) young adult (18-34 yr old) human subjects was measured after single, 1-s-duration maximum voluntary ankle dorsiflexion contractions. There was no difference between groups in anterior tibial muscle cross-sectional area or peak force. In both groups there was a transient increase in anterior tibialis muscle SI, which peaked 5-7 s after the end of each contraction. The magnitude of the SI transient was over threefold greater [5.5 +/- 1.0 (SE) vs. 1.5 +/- 0.4%] and persisted twice as long (half-recovery time 5.4 +/- 0.4 vs. 2.7 +/- 0.3 s) in the active subjects. In the same subjects, blood flow in popliteal, anterior tibial, and posterior tibial arteries was measured by cardiac-gated CINE magnetic resonance angiography before and after 2 min of dynamic, repetitive ankle dorsiflexion exercise. There was no difference between groups in resting or postexercise flow in anterior tibial artery, although popliteal and posterior tibial artery flow after exercise tended to be greater in the active group. The results indicate that transient hyperemia and oxygenation in muscle after single contractions are enhanced by chronic physical activity to a greater extent than peak muscle blood flow.

Dynamic response characteristics of local muscle blood flow regulatory mechanisms in human forearm exercise

We sought to understand the nature of control mechanisms involved in the adaptation of exercising muscle hyperemia. Seven subjects performed rhythmic dynamic forearm exercise under two exercise conditions: small step 1 [step increase from rest to 40% peak forearm vascular conductance (FVC), in ml.min(-1).100 mmHg-1] for 5 min followed by small step 2 (further increase to 80% peak FVC for 5 min), and large step (step increase from rest to 80% peak FVC for 5 min). FVC data were fit with a two- (small step 1) and three-component (small step 2, large step) exponential as appropriate. For the rapid phase I response, FVC dynamic response characteristics (time delay, time constant) were not affected by the magnitude of the work intensity increase when the transition began from rest, but were slower in the 40-80% transition. Rest-80% gain was greater than either rest-40% or 40-80% transitions but represented the same proportion of the phase I + phase II gain across all transitions (57 vs. 56 vs. 57%, respectively, P = 0.975). For the slower phase II response, dynamic response characteristics were not affected by the magnitude of the work intensity increase when initiated from rest. The time constant was not altered when the transition began from exercise vs. rest. We conclude that 1) dynamic response characteristics of exercise hyperemia control mechanisms are not affected by the magnitude of work rate increase when forearm exercise is initiated from rest, 2) phase I but not phase II dynamic response characteristics are sensitive to baseline exercise intensity, and 3) the mechanisms contributing to phase I result in the same relative response magnitude, regardless of the size of the step increase in exercise intensity or the baseline from which it is initiated.