Relative contribution of vasodilator prostanoids, NO, and KATP channels to human forearm metabolic vasodilation

Regulation of capillary hemodynamics by KATP channels in resting skeletal muscle

ATP-sensitive K⁺ channels (K_ATP ) have been implicated in the regulation of resting vascular smooth muscle membrane potential and tone. However, whether K_ATP channels modulate skeletal muscle microvascular hemodynamics at the capillary level (the primary site for blood-myocyte O₂ exchange) remains unknown. We tested the hypothesis that K_ATP channel inhibition would reduce the proportion of capillaries supporting continuous red blood cell (RBC) flow and impair RBC hemodynamics and distribution in perfused capillaries within resting skeletal muscle. RBC flux (f_RBC ), velocity (V_RBC ), and capillary tube hematocrit (Hct_cap ) were assessed via intravital microscopy of the rat spinotrapezius muscle (n = 6) under control (CON) and glibenclamide (GLI; K_ATP channel antagonist; 10 µM) superfusion conditions. There were no differences in mean arterial pressure (CON:120 ± 5, GLI:124 ± 5 mmHg; p > 0.05) or heart rate (CON:322 ± 32, GLI:337 ± 33 beats/min; p > 0.05) between conditions. The %RBC-flowing capillaries were not altered between conditions (CON:87 ± 2, GLI:85 ± 1%; p > 0.05). In RBC-perfused capillaries, GLI reduced f_RBC (CON:20.1 ± 1.8, GLI:14.6 ± 1.3 cells/s; p < 0.05) and V_RBC (CON:240 ± 17, GLI:182 ± 17 µm/s; p < 0.05) but not Hct_cap (CON:0.26 ± 0.01, GLI:0.26 ± 0.01; p > 0.05). The absence of GLI effects on the %RBC-flowing capillaries and Hct_cap indicates preserved muscle O₂ diffusing capacity (DO₂ m). In contrast, GLI lowered both f_RBC and V_RBC thus impairing perfusive microvascular O₂ transport (Q̇m) and lengthening RBC capillary transit times, respectively. Given the interdependence between diffusive and perfusive O₂ conductances (i.e., %O₂ extraction∝DO₂ m/Q̇m), such GLI alterations are expected to elevate muscle %O₂ extraction to sustain a given metabolic rate. These results support that K_ATP channels regulate capillary hemodynamics and, therefore, microvascular gas exchange in resting skeletal muscle.

Exercise-Induced Modulation of Angiotensin II Responses in Femoral Veins From 2-Kidney-1-Clip Hypertensive Rats

The present study investigated the angiotensin II (Ang II) responses in rat femoral veins taken from 2-kidney-1clip (2K1C) hypertensive rats at 4 weeks after clipping, as well as the effects of exercise on these responses. In this manner, femoral veins taken from 2K1C rats kept at rest or exposed to acute exercise or to exercise training were challenged with Ang II or endothelin-1 (ET-1) in organ bath. Simultaneously, the presence of cyclooxygenase-1 (COX-1) and cyclooxygenase-2 (COX-2) were determined in these preparations by western blotting. In these experiments, femoral veins exhibited subdued Ang II responses. However, after nitric oxide (NO) synthesis blockade, the responses were higher in the femoral veins taken from animals kept at rest [0.137(0.049–0.245); n = 10] than those obtained in trained animals kept at rest [0.008(0.001–0.041); n = 10] or studied after a single bout of exercise [0.001(0.001–0.054); n = 11]. In preparations in which, in addition to NO synthesis, both the local production of prostanoids and the action of ET-1 on type A (ET_A) or B (ET_B) receptors were inhibited, the differences induced by exercise were no longer observed. In addition, neither ET-1 responses nor the presence of COX-1 and COX-2 in these preparations were modified by the employed exercise protocols. In conclusion, NO maintains Ang II responses reduced in femoral veins of 2K1C animals at rest. However, vasodilator prostanoids as well as other relaxing mechanisms, activated by ET_B stimulation, are mobilized by exercise to cooperate with NO in order to maintain controlled Ang II responses in femoral veins.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

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.

Exercise reduces angiotensin II responses in rat femoral veins

The control of blood flow during exercise involves different mechanisms, one of which is the activation of the renin-angiotensin system, which contributes to exercise-induced blood flow redistribution. Moreover, although angiotensin II (Ang II) is considered a potent venoconstrictor agonist, little is known about its effects on the venous bed during exercise. Therefore, the present study aimed to assess the Ang II responses in the femoral vein taken from sedentary and trained rats at rest or subjected to a single bout of exercise immediately before organ bath experiments. Isolated preparations of femoral veins taken from resting-sedentary, exercised-sedentary, resting-trained and exercised-trained animals were studied in an organ bath. In parallel, the mRNA expression of prepro-endothelin-1 (ppET-1), as well as the ETA and ETB receptors, was quantified by real-time PCR in this tissue. The results show that, in the presence of L-NAME, Ang II responses in resting-sedentary animals were higher compared to the other groups. However, this difference disappeared after co-treatment with indomethacin, BQ-123 or BQ-788. Moreover, exercise reduced ppET-1 mRNA expression. These reductions in mRNA expression were more evident in resting-trained animals. In conclusion, either acute or repeated exercise adapts the rat femoral veins, thereby reducing the Ang II responses. This adaptation is masked by the action of locally produced nitric oxide and involves, at least partially, the ETB- mediated release of vasodilator prostanoids. Reductions in endothelin-1 production may also be involved in these exercise-induced modifications of Ang II responses in the femoral vein.

The effect of glibenclamide on cutaneous laser-Doppler flux

The K(ATP) channels play a crucial role in regulation of vascular tone in conditions of hypoxia. Whether they contribute to peripheral blood flow regulation in human cutaneous microcirculation during a non-hypoxic state is the matter of conflicting in vivo studies that have used plethysmographic method. Our aim was therefore to elucidate the role of K(ATP) channels in human skin microcirculation in three different conditions that evoke different interplays of vascular mechanisms; during resting conditions, during the postocclusive vasodilatation and in the vasoconstriction response to local cold exposure. The laser-Doppler (LD) skin response was monitored in 12 healthy volunteers on the skin of the fingertips of both hands at rest, after the release of an 8-min digital arteries occlusion, and during local cooling of one hand at 15 degrees C. We compared the direct (at the measuring site) and the indirect (at the contralateral non-cooled hand) LD flux response after intradermal microinjection of saline solution (1 mul) and after a microinjection of the K(ATP) channel blocker glibenclamide (8 muM saturated solution) at the measuring site after obtaining the dose-dependent effect of glibenclamide. The effect of the saline solution was used as a reference value. There was a statistically significant lower resting LD flux after the microinjection of glibenclamide 273.6+/-36 PU when compared to the values obtained after the application of the saline solution 375.8+/-31 PU (paired t-test, p=0.016). Glibenclamide also significantly reduced the relative area under the LD flux curve during the PRH response 14551+/-2508 PU*s vs. 6402+/-1476 PU*s (paired t-test, p=0.01) and increased the principal frequency of postocclusive PRH oscillations 0.0931+/-0.01 Hz vs. 0.1309+/-0.02 Hz (p=0.01). In addition, glibenclamide significantly decreased the LD flux during both the direct and indirect response to local cold exposure when compared to the application of saline solution (paired t-test, p<0.01). Our results support the conjecture that ATP sensitive K(+) channels are importantly involved in blood flow regulation of human skin microcirculation in PRH response, in resting conditions as well as in microvascular local cold response.

Near-infrared spectroscopy to monitor peripheral blood flow perfusion

BACKGROUND

Non-invasive evaluation of peripheral perfusion may be useful in many contexts including peri-operative monitoring. We validated a novel non-invasive spectroscopy technique to assess peripheral perfusion. This method, which is based on the measurement of tissue saturation variations after an ischemic period, was compared to strain gauge plethysmography and radionuclide plethysmography. The technique uses near-infrared spectroscopy (NIRS) to determine the rate of change of forearm tissue saturation during reactive hyperemia.

METHODS

In a prospective crossover study, 25 subjects were simultaneously evaluated with NIRS and strain gauge plethysmography. Six baseline flow measurements were performed to assess the reproducibility of each method. Twenty-seven serial measurements were then made to evaluate flow variation during forearm reactive hyperemia.

RESULTS

Strain gauge and NIRS methods showed excellent reproducibility with intra-class correlation coefficients of 0.96 and 0.93, respectively.

CONCLUSION

The NIRS technique appears well suited for the non-invasive evaluation of limb perfusion.

Mobile detection system to evaluate reactive hyperemia using radionuclide plethysmography

We validated a novel mobile detection system to evaluate reactive hyperemia using the radionuclide plethysmography technique. Twenty-six subjects underwent simultaneously radionuclide plethysmography with strain gauge plethysmography. Strain gauge and radionuclide methods showed excellent reproducibility with intraclass correlation coefficients of 0.96 and 0.89 respectively. There was also a good correlation of flows between the two methods during reactive hyperemia (r = 0.87). We conclude that radionuclide plethysmography using this mobile detection system is a non-invasive alternative to assess forearm blood flow and its dynamic variations during reactive hyperemia.

Flavonoid-rich grapeseed extracts: a new approach in high cardiovascular risk patients?

The management of traditional risk factors such as hypertension and dyslipidaemia has been successful in reducing the development of cardiovascular disease. However, this has not resulted in the amelioration of complications; prompting attention to be focused on novel markers of vascular risk such as endothelial dysfunction (a determinant of vascular tone), vascular inflammation, oxidative stress and insulin resistance. With an ever-growing interest in plant-derived products, agents that could have a beneficial effect on this complex web of pathophysiology have thus been a major area of research and interest. Flavonoids have been a major focus of attention since the days of the French paradox and the presence of high quantity of flavonoids in grapeseed extracts has prompted research looking at its effects on novel markers of vascular risk. This review briefly summarises mechanisms implicated in the development of vascular disease and then focuses upon the potential role of the antioxidant properties of flavonoid-rich grapeseed extracts in the reversal of these processes.

Change in cutaneous perfusion following 10 weeks of aerobic training in Type 2 diabetes

A small, but positive, association between aerobic training status or prior maximal exercise and enhanced dorsal foot skin perfusion in active individuals with Type 2 diabetes has been shown. This study, therefore, was designed to examine whether an aerobic training intervention would positively affect cutaneous perfusion in sedentary Type 2 diabetic individuals as well. Nine sedentary participants with Type 2 diabetes (DS) and 10 obese nondiabetic controls (CS) were studied. Prior to the initiation of aerobic training, dorsal foot cutaneous perfusion was measured noninvasively by continuous laser Doppler assessment at baseline and during localized heating to 44 degrees C. These measurements were repeated 48-72 h following 10 weeks of moderate aerobic training performed 3 days per week. Interstitial nitric oxide (NO) levels were measured concurrently in the contralateral foot dorsum. Aerobic training did not significantly enhance baseline skin perfusion, nor were interstitial NO levels different under any condition. At baseline, groups differed only in glycated hemoglobin (HbA1c), fasting serum glucose, HDL-cholesterol, and insulin resistance. At rest, cutaneous perfusion during local heating to 44 degrees C was significantly lower in DS before training, but did not differ significantly from CS afterward. Neither group, however, experienced significant increases in dorsal foot perfusion during local responsiveness to heating to 44 degrees C following 10 weeks of moderate aerobic training, despite slightly lower perfusion in DS before training; these findings were independent of interstitial NO levels. Thus, moderate aerobic training for only 10 weeks does not appear to reverse the impairment in cutaneous perfusion of the foot dorsum in response local heating in a Type 2 diabetic population.

Radionuclide plethysmography for noninvasive evaluation of peripheral arterial blood flow

We validated a noninvasive radionuclide plethysmography technique to evaluate peripheral arterial blood flow during reactive hyperemia. This method, based on the measurement of blood volume variations during repetitive venous occlusions, was compared with strain-gauge venous impedance plethysmography. The technique uses 99mTc-labeled autologous red blood cells scintigraphy to determine the rate of change of forearm scintigraphic counts during venous occlusion. Thirteen subjects were simultaneously evaluated with radionuclide and impedance plethysmography. Six baseline flow measurements were performed to evaluate the reproducibility of each method. Twenty-seven serial measurements were then made to evaluate flow variation during forearm reactive hyperemia. After 30 min of recovery, resting forearm blood flows were again evaluated. Impedance and radionuclide methods showed excellent reproducibility with intraclass correlation coefficients of 0.96 and 0.93, respectively. There was also good correlation of flows between both methods during reactive hyperemia (r = 0.87). Resting flows at 30 min after reactive hyperemia were slightly lower than at baseline with both methods. We conclude that radionuclide plethysmography could be used for the noninvasive evaluation of forearm blood flow and its dynamic variations during reactive hyperemia.

Quantitative hyperemic reactivity in opposed limbs during myocardial perfusion imaging: a new marker of coronary artery disease

OBJECTIVES

We sought to evaluate the feasibility and validity of a new method to quantify the hyperemic response of the forearms that can be incorporated into a rest myocardial perfusion protocol.

BACKGROUND

Evaluation of the hyperemic response could provide useful clinical information in the detection and risk stratification of atherosclerotic vascular disease.

METHODS

Patients with proven coronary artery disease (CAD) (n = 46) were compared with low-risk subjects without such evidence (n = 47). A regular dose of Myoview was injected after 5 min of right arm ischemia. Three dimensionless parametric ratios (right/left) were derived from the analysis of activity-time curves of the hyperemic right forearm and that of the contralateral left forearm.

RESULTS

The maximal ingress upslope ratio was 40% lower in the CAD group (3.0 +/- 0.2 vs. 4.2 +/- 0.3, p < 0.0005), and the integral to peak ratio was also lower (23 +/- 4 vs. 52 +/- 11, p < 0.01), whereas the peak activity ratio was nonsignificantly lower (3.0 +/- 0.3 vs. 3.8 +/- 0.3, p = 0.07). Using a value of 3.55 for the maximal upslope ratio, this approach could predict the presence of CAD with a sensitivity of 0.70 and a specificity of 0.60.

CONCLUSIONS

This simple and noninvasive method is feasible and can discriminate between patients with known CAD and those at low risk of atherosclerosis. Refinements of this approach and its inclusion in larger clinical trials are needed to determine whether it could provide additional value to myocardial scintigraphic imaging.

Postexercise hypotension is not explained by a prostaglandin-dependent peripheral vasodilation

In normally active individuals, postexercise hypotension after a single bout of aerobic exercise occurs due to an unexplained peripheral vasodilation. Prostaglandin production has been suggested to contribute to the increases in blood flow during and after exercise; however, its potential contribution to postexercise hypotension has not been assessed. The purpose of this study was to determine the potential contribution of a prostaglandin-dependent vasodilation to changes in systemic vascular conductance underlying postexercise hypotension; this was done by inhibiting production of prostaglandins with the cyclooxygenase inhibitor ibuprofen. We studied 11 healthy normotensive men (aged 23.7 +/- 4.2 yr) before and during the 90 min after a 60-min bout of cycling at 60% peak O(2) uptake on a control and a cyclooxygenase inhibition day (randomized). Subjects received 10 mg/kg of oral ibuprofen on the cyclooxygenase inhibition day. On both study days, arterial blood pressure (automated auscultation) and cardiac output (acetylene uptake) were measured, and systemic vascular conductance was calculated. Inhibition of cyclooxygenase had no effect on baseline values of mean arterial pressure or systemic vascular conductance (P > 0.2). After exercise on both days, mean arterial pressure was reduced (-2.2 +/- 1.0 mmHg change with the control condition and -3.8 +/- 1.5 mmHg change with the ibuprofen condition, both P < 0.05 vs. preexercise) and systemic vascular conductance was increased (5.2 +/- 5.0% change with the control condition and 8.7 +/- 4.1% change with the ibuprofen condition, both P < 0.05 vs. preexercise). There were no differences between study days (P > 0.6). These data suggest that prostaglandin-dependent vasodilation does not contribute to the increased systemic vascular conductance underlying postexercise hypotension.