Blood flow control during exercise: role for the venular endothelium?

Oxygen uptake kinetics

Muscular exercise requires transitions to and from metabolic rates often exceeding an order of magnitude above resting and places prodigious demands on the oxidative machinery and O2-transport pathway. The science of kinetics seeks to characterize the dynamic profiles of the respiratory, cardiovascular, and muscular systems and their integration to resolve the essential control mechanisms of muscle energetics and oxidative function: a goal not feasible using the steady-state response. Essential features of the O2 uptake (VO2) kinetics response are highly conserved across the animal kingdom. For a given metabolic demand, fast VO2 kinetics mandates a smaller O2 deficit, less substrate-level phosphorylation and high exercise tolerance. By the same token, slow VO2 kinetics incurs a high O2 deficit, presents a greater challenge to homeostasis and presages poor exercise tolerance. Compelling evidence supports that, in healthy individuals walking, running, or cycling upright, VO2 kinetics control resides within the exercising muscle(s) and is therefore not dependent upon, or limited by, upstream O2-transport systems. However, disease, aging, and other imposed constraints may redistribute VO2 kinetics control more proximally within the O2-transport system. Greater understanding of VO2 kinetics control and, in particular, its relation to the plasticity of the O2-transport/utilization system is considered important for improving the human condition, not just in athletic populations, but crucially for patients suffering from pathologically slowed VO2 kinetics as well as the burgeoning elderly population.

Exercise increases the angiotensin II effects in isolated portal vein of trained rats

Training in rats adapts the portal vein to respond vigorously to sympathetic stimuli even when the animal is re-exposed to exercise. Moreover, changes in the exercise-induced effects of angiotensin II, a potent venoconstrictor agonist, in venous beds remain to be investigated. Therefore, the present study aimed to assess the effects of angiotensin II in the portal vein and vena cava from sedentary and trained rats at rest or submitted to an exercise session immediately before organ bath experiments. We found that training or exposure of sedentary animals to a single bout of running exercise does not significantly change the responses of the rat portal vein to angiotensin II. However, the exposure of trained animals to a single bout of running exercise enhanced the response of the rat portal vein to angiotensin II. This enhancement appeared to be territory-specific because it was not observed in the vena cava. Moreover, it was not observed in endothelium-disrupted preparations and in preparations treated with N(omega)-nitro-l-arginine methyl ester hydrochloride, indomethacin, BQ-123 or BQ-788. These data indicate that training causes adaptations in the rat portal vein that respond vigorously to angiotensin II even upon re-exposure to exercise. This increased response to angiotensin II requires an enhancement of the vasocontractile influence of endothelin beyond the influence of nitric oxide and vasodilator prostanoids.

Losartan and ozagrel reverse retinal arteriolar constriction in non-obese diabetic mice

OBJECTIVE

Reductions in retinal blood flow are observed early in diabetes. Venules may influence arteriolar constriction and flow; therefore, we hypothesized that diabetes would induce the constriction of arterioles that are in close proximity to venules, with the constriction mediated by thromboxane and angiotensin II.

METHODS

Using nonobese diabetic (NOD) mice, retinal measurements were performed three weeks following the age at which glucose levels exceeded 200 mg/dL, with accompanying experiments on age-matched normoglycemic NOD mice. The measurements included retinal arteriolar diameters and red blood cell velocities and were repeated following an injection of the thromboxane synthase inhibitor, ozagrel. Mice were subdivided into equal groups and given drinking water with or without the angiotensin II receptor antagonist, losartan.

RESULTS

Retinal arterioles were constricted in hyperglycemic mice, with a significant reduction in flow. However, not all arterioles were equally affected; the vasoconstriction was limited to arterioles that were in closer proximity to venules. The arteriolar vasoconstriction (mean arteriolar diameters = 51 +/- 1 vs. 61 +/- 1 microm in controls; p < 0.01) was eliminated by both ozagrel (61 +/- 2 microm) and losartan (63 +/- 2 microm).

CONCLUSIONS

Venule-dependent arteriolar vasoconstriction in NOD mice is mediated by thromboxane and/or angiotensin II.

Insulin resistance and impaired functional vasodilation in obese Zucker rats

Individuals with metabolic syndrome exhibit insulin resistance and an attenuated functional vasodilatory response to exercise. We have shown that impaired functional vasodilation in obese Zucker rats (OZRs) is associated with enhanced thromboxane receptor (TP)-mediated vasoconstriction. We hypothesized that insulin resistance, hyperglycemia/hyperlipidemia, and the resultant ROS are responsible for the increased TP-mediated vasoconstriction in OZRs, resulting in impaired functional vasodilation. Eleven-week-old male lean Zucker rats (LZRs) and OZRs were fed normal rat chow or chow containing rosiglitazone (5 mg.kg(-1).day(-1)) for 2 wk. In another set of experiment, LZRs and OZRs were treated with 2 mM tempol (drinking water) for 7-10 days. After the treatments, spinotrapezius muscles were prepared, and arcade arteriolar diameters were measured following muscle stimulation and arachidonic acid (AA) application (10 muM) in the absence and presence of the TP antagonist SQ-29548 (1 muM). OZRs exhibited higher insulin, glucose, triglyceride, and superoxide levels and increased NADPH oxidase activity compared with LZRs. Functional and AA-induced vasodilations were impaired in OZRs. Rosiglitazone treatment improved insulin, glucose, triglyceride, and superoxide levels as well as NADHP oxidase activity in OZRs. Both rosiglitazone and tempol treatment improved vasodilatory responses in OZRs with no effect in LZRs. SQ-29548 treatment improved vasodilatory responses in nontreated OZRs with no effect in LZRs or treated OZRs. These results suggest that insulin resistance and the resultant increased ROS impair functional dilation in OZRs by increasing TP-mediated vasoconstriction.

Functional vasodilation in the rat spinotrapezius muscle: role of nitric oxide, prostanoids and epoxyeicosatrienoic acids

1. The present study was designed to determine the mechanisms responsible for functional vasodilation of arterioles paired and unpaired with venules in the rat spinotrapezius muscle. 2. The spinotrapezius muscle (from Sprague-Dawley rats) was treated with combinations of the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME; 100 micromol/L), the cyclo-oxygenase inhibitor indomethacin (10 micromol/L) and the epoxygenase inhibitor 6-(2-propargyloxyphenyl) hexanoic acid (PPOH; 30 micromol/L) to determine vascular responses to muscle stimulation. Both paired and unpaired arcade arterioles were chosen for microcirculatory observation. Arteriolar diameter was measured following 2 min muscle stimulation before and 30 min after subsequent application of each inhibitor. 3. In all cases, L-NAME treatment resulted in decreased basal diameter that was restored to control levels by the addition of sodium nitroprusside (0.01-0.1 micromol/L) to the superfusion solution. N(G)-Nitro-L-arginine methyl ester significantly inhibited the functional dilation in both paired (-20 +/- 3%) and unpaired (-29 +/- 3%) arterioles, whereas these inhibitory effects of L-NAME were diminished after pretreatment with indomethacin and PPOH. Indomethacin treatment attenuated the dilation in paired (-33 +/- 5%) but not unpaired (-6 +/- 4%) arterioles. Treatment with PPOH had no effect on the functional dilation in either set of arterioles. Approximately 50% of the vasodilatory responses remained in the presence of L-NAME, indomethacin and PPOH. 4. These results suggest that both nitric oxide and vasodilator prostanoid(s) are involved in mediating functional vasodilation in the rat spinotrapezius. The vasodilator prostanoid(s) released from venules is responsible for a portion of the vasodilation of the paired arteriole. The results also suggest possible interactions between the synthesis of nitric oxide and prostaglandin or epoxyeicosatrienoic acids during muscle contraction.

Influence of venous emptying on the reactive hyperemic blood flow response

BACKGROUND

Previous research indicates that venous emptying serves as a stimulus for vasodilation in the human forearm. This suggests the importance of recognizing the potential influence of venous volume on reactive hyperemic blood flow (RHBF) following occlusion. The purpose of this study was to examine the influence of venous emptying on forearm vascular function.

METHODS

Forearm RHBF, venous capacitance and venous outflow were examined in 35 individuals (age = 22 +/- 2 years), using mercury in-Silastic strain gauge plethysmography, at rest and following five minutes of upper arm occlusion using standard procedures (CONTROL). In addition, the same measures were obtained following five minutes of upper arm occlusion preceded by two minutes of passive arm elevation (Pre-elevation).

RESULTS

Average resting arterial inflow was 2.42 +/- 1.11 ml x 100 ml(-1) x min(-1). RHBF and venous capacitance were significantly greater during Pre-elevation compared to CONTROL (RHBF; Pre-elevation: 23.76 +/- 5.95 ml x 100 ml(-1) x min(-1) vs.

CONTROL

19.33 +/- 4.50; p = 0.001), (venous capacitance; Pre-elevation: 2.74 +/- 0.89 % vs.

CONTROL

2.19 +/- 0.97, p = 0.001). Venous outflow did not differ between the two conditions.

CONCLUSION

Venous emptying prior to upper arm occlusion results in a significant greater RHBF response and venous capacitance. Recognition of the influence of venous volume on RHBF is particularly important in studies focusing on arterial inflow, and also provides further evidence for the interplay between the venous and arterial system.

Effect of prior multiple-sprint exercise on pulmonary O2 uptake kinetics following the onset of perimaximal exercise

We hypothesized that the metabolic acidosis resulting from the performance of multiple-sprint exercise would enhance muscle perfusion and result in a speeding of pulmonary oxygen uptake (V̇o2) kinetics during subsequent perimaximal-intensity constant work rate exercise, if O2 availability represented a limitation to V̇o2 kinetics in the control (i.e., no prior exercise) condition. On two occasions, seven healthy subjects completed two bouts of exhaustive cycle exercise at a work rate corresponding to ∼105% of the predetermined V̇o2 peak, separated by 3 × 30-s maximal sprint cycling and 15-min recovery (MAX1 and MAX2). Blood lactate concentration (means ± SD: MAX1: 1.3 ± 0.4 mM vs. MAX2: 7.7 ± 0.9 mM; P < 0.01) was significantly greater immediately before, and heart rate was significantly greater both before and during, perimaximal exercise when it was preceded by multiple-sprint exercise. Near-infrared spectroscopy also indicated that muscle blood volume and oxygenation were enhanced when perimaximal exercise was preceded by multiple-sprint exercise. However, the time constant describing the primary component (i.e., phase II) increase in V̇o2 was not significantly different between the two conditions (MAX1: 33.8 ± 5.5 s vs. MAX2: 33.2 ± 7.7 s). Rather, the asymptotic “gain” of the primary V̇o2 response was significantly increased by the performance of prior sprint exercise (MAX1: 8.1 ± 0.9 ml·min−1·W−1 vs. MAX2: 9.0 ± 0.7 ml·min−1·W−1; P < 0.05), such that V̇o2 was projecting to a higher “steady-state” amplitude with the same time constant. These data suggest that priming exercise, which apparently increases muscle O2 availability, does not influence the time constant of the primary-component V̇o2 response but does increase the amplitude to which V̇o2 may rise following the onset of perimaximal-intensity cycle exercise.

Metabolic control of muscle blood flow during exercise in humans

During muscle contraction, several mechanisms regulate blood flow to ensure a close coupling between muscle oxygen delivery and metabolic demand. No single factor has been identified to constitute the primary metabolic regulator, yet there are signal transduction pathways between skeletal muscle and the vasculature that induce vasodilation. A link between muscle metabolic events and microvascular control of blood flow is illustrated by local dilation of terminal arterioles during contraction of muscle fibers and conduction of vasodilation upstream. Endothelial-derived vasodilator mechanisms are known to exert control of muscle vasodilation. Adenosine, nitric oxide (NO), prostacyclin (PGI2), and endothelial-derived hyperpolarization factor (EDHF) are possible mediators of muscle vasodilation during exercise. In humans, adenosine has been shown to contribute to functional hyperemia as blood flow is reduced under nonselective adenosine-receptor blockade. No clear role has been demonstrated for either NO or PGI2(2), based on studies employing selective inhibition of these substances individually, suggesting a redundancy of vasodilator mechanisms. This is supported by recent work demonstrating that combined blockade of NOS and PGI2, and NOS and cytochrome P450, both attenuate exercise-induced hyperemia in humans. Combined vasodilator blockade studies offer the potential to uncover important interactions and compensatory vasodilator responses. The signaling pathways that link metabolic events evoked by muscle contraction to vasodilatory signals in the local vascular bed remains an important area of study.