Role of adenosine in exercise‐induced human skeletal muscle vasodilatation

The role of adenosine in exercise‐induced human skeletal muscle vasodilatation remains unknown. We therefore evaluated the effect of theophylline‐induced adenosine receptor blockade in six subjects and the vasodilator potency of adenosine infused in the femoral artery of seven subjects. During one‐legged, knee‐extensor exercise at ～48% of peak power output, intravenous (i.v.) theophylline decreased (P < 0.003) femoral artery blood flow (F_aBF) by ～20%, i.e. from 3.6 ± 0.5 to 2.9 ± 0.5 L min^?1, and leg vascular conductance (VC) from 33.4 ± 9.1 to 27.7 ± 8.5 mL min^?1 mmHg^?1, whereas heart rate (HR), mean arterial pressure (MAP), leg oxygen uptake and lactate release remained unaltered (P = n.s.). Bolus injections of adenosine (2.5 mg) at rest rapidly increased (P < 0.05) F_aBF from 0.3 ± 0.03 L min^?1 to a 15‐fold peak elevation (P < 0.05) at 4.1 ± 0.5 L min^?1. Continuous infusion of adenosine at rest and during one‐legged exercise at ～62% of peak power output increased (P < 0.05) F_aBF dose‐dependently to level off (P = ns) at 8.3 ± 1.0 and 8.2 ± 1.4 L min^?1, respectively. One‐legged exercise alone increased (P < 0.05) F_aBF to 4.7 ± 1.7 L min^?1. Leg oxygen uptake was unaltered (P = n.s.) with adenosine infusion during both rest and exercise. The present findings demonstrate that endogenous adenosine controls at least ～20% of the hyperaemic response to submaximal exercise in skeletal muscle of humans. The results also clearly show that arterial infusion of exogenous adenosine has the potential to evoke a vasodilator response that mimics the increase in blood flow observed in response to exercise.     

No role of interstitial adenosine in insulin-mediated vasodilation

The mechanisms behind the vasodilatory effect of insulin are not fully understood, but nitric oxide plays an important role. We have investigated the possibility that insulin mediates vasodilatation in the human skeletal muscle via an increase in extracellular adenosine concentrations. In eight healthy subjects (H) and in four subjects with a complete, high (C5–C6/7) spinal cord injury (SCI) a hyperinsulinaemic (480 mU min^–1 kg^–1), isoglycaemic clamp was performed. SCI subjects were included as it has been proposed that adenosine and adenine nucleotides may be released from nerve endings in the skeletal muscle. Adenosine concentrations in the extracellular fluid (ECF) of skeletal muscle in the thigh were measured by means of the microdialysis technique. Leg blood flow (LBF) was measured by termodilution. In response to insulin infusion, LBF always increased (P < 0.05) (from 228 ± 25 and 318 ± 18 mL min^–1 to 451 ± 41 and 530 ± 29 mL min^–1, SCI and H, respectively [mean ± SEM]). Concentrations of adenosine in the muscle ECF did not change with infusion of insulin and did not differ between groups (before: 147 ± 55 [SCI] and 207 ± 108 [H] nmol L^–1; during: 160 ± 36 [SCI] and 165 ± 74 [H] nmol L^–1). No significant correlation between concentrations of adenosine and corresponding LBF rates was achieved (LBF=[–0.0936 · Adenosine] + 475. R=–0.092, P=0.22, number of samples=181, number of subjects=12). Conclusion: the mechanism by which insulin mediates an increase in skeletal muscle blood flow is not associated with adenosine in the ECF.     

Epidermal growth factor and insulin short‐term increase hPepT1‐mediated glycylsarcosine uptake in Caco‐2 cells

Aims: Little is known about the physiological regulation of the human intestinal di/tri‐peptide transporter, hPepT1. In the present study we evaluated the effects of epidermal growth factor (EGF) and insulin on hPepT1‐mediated dipeptide uptake in the intestinal cell line Caco‐2. Methods: Caco‐2 cells were grown on filters for 23–27 days. Apical dipeptide uptake was measured using [¹⁴C]glycylsarcosine([¹⁴C]Gly‐Sar). HPepT1 mRNA levels were investigated using RT‐PCR, cytosolic pH was determined using the pH‐sensitive fluorescent probe BCECF. Results: Basolateral application of EGF increased [¹⁴C]Gly‐Sar uptake with an ED₅₀ value of 0.77 ± 0.25 ng mL^?1 (n = 3?6) and a maximal stimulation of 33 ± 2% (n = 3?6). Insulin stimulated [¹⁴C]Gly‐Sar uptake with an ED₅₀ value of 3.5 ± 2.0 ng mL^?1 (n = 3?6) and a maximal stimulation of approximately 18% (n = 3?6). Gly‐Sar uptake followed simple Michaelis‐Menten kinetics. K_m in control cells was 0.98 ± 0.11 mM (n = 8) and V_max was 1.86 ± 0.07 nmol cm^?2 min^?1 (n = 8). In monolayers treated with 200 ng mL^?1 of EGF, K_m was 1.11 ± 0.05 mM (n = 5) and V_max was 2.79 ± 0.05 nmol cm^?2 min^?1 (n = 5). In monolayers treated with 50 ng mL^?1 insulin, K_m was 1.03 ± 0.08 mM and V_max was 2.19 ± 0.06 nmol cm^?2 min^?1 (n = 5). Kinetic data thus indicates an increase in the number of active transporters, following stimulation. The incrased Gly‐Sar uptake was not accompanied by changes in hPepT1 mRNA, nor by measurable changes in cytosolic pH. Conclusions: Short‐term stimulation with EGF and insulin caused an increase in hPepT1‐mediated uptake of Gly‐Sar in Caco‐2 cell monolayers, which could not be accounted for by changes in hPepT1 mRNA or proton‐motive driving force.     

Contribution of pH,diprotonated phosphate and potassium for the reflex increase in blood pressure during handgrip

The relative importance of pH, diprotonated phosphate (H₂PO₄^?) and potassium (K⁺) for the reflex increase in mean arterial pressure (MAP) during exercise was evaluated in seven subjects during rhythmic handgrip at 15 and 30% maximal voluntary contraction (MVC), followed by post-exercise muscle ischaemia (PEMI). During 15% MVC, MAP rose from 92 ± 1 to 103 ± 2 mmHg, [K⁺] from 4.1 ± 0.1 to 5.1 ± 0.1 mmol L^?1, while the intracellular (7.00 ± 0.01 to 6.80 ± 0.06) and venous pH fell (7.39 ± 0.01 to 7.30 ± 0.01) (P < 0.05). The intracellular [H₂PO₄^?] increased 8.4 ± 2 mmol kg^?1 and the venous [H₂PO₄^?] from 0.14 ± 0.01 to 0.16 ± 0.01 mmol L^?1 (P < 0.05). During PEMI, MAP remained elevated along with the intracellular [H₂PO₄^?] as well as a low intracellular and venous pH. However, venous [K⁺] and [H₂PO₄^?] returned to the level at rest. During 30% MVC handgrip, MAP rose to 130 ± 3 mmHg, [K⁺] to 5.8 ± 0.2 mmol L^?1, the intracellular and extracellular [H₂PO₄^?] by 20 ± 5 mmol kg^?1 and to 0.20 ± 0.02 mmol L^?1, respectively, while the intracellular (6.33 ± 0.06) and venous pH fell (7.23 ± 0.02) (P < 0.05). During post-exercise muscle ischaemia all variables remained close to the exercise levels. Analysis of each variable as a predictor of blood pressure indicated that only the intracellular pH and diprotonated phosphate were linked to the reflex elevation of blood pressure during handgrip.     

Effect of endurance training on muscle microvascular filtration capacity and vascular bed morphometry in the elderly

Aim: Exercise training is a strong stimulus for vascular remodelling and could restore age‐induced vascular alterations. The purpose of the study was to test the hypothesis that an increase in vascular bed filtration capacity would corroborate microvascular adaptation with training. Methods: We quantified (1) microvascularization from vastus lateralis muscle biopsy to measure the capillary to fibre interface (LC/PF) and (2) the microvascular filtration capacity (K_f) in lower limbs through a venous congestion plethysmography procedure. Twelve healthy older subjects (74 ± 4 years) were submitted to a 14‐week training programme during which lower‐limbs were trained for endurance exercise. Results: The training programme induced a significant increase in the aerobic exercise capacity of lower limbs (+11%Vo _2peak; P < 0.05; +28% Citrate Synthase Activity; P < 0.01). K_f was largely increased (4.3 ± 0.9 10^?3 mL min^?1 mmHg^?1 100 mL^?1 post‐training vs. 2.4 ± 0.8 pre‐training, mean ± SD; P < 0.05) and microvascularization developed as shown by the rise in LC/PF (0.29 ± 0.06 post‐ vs. 0.23 ± 0.06 pre‐training; P < 0.05). Furthermore, K_f and LC/PF were correlated (r = 0.65, P < 0.05). Conclusion: These results demonstrated the microvascular adaptation to endurance training in the elderly. The increase in K_f with endurance training was probably related to a greater surface of exchange with an increased microvessel/fibre interface area. We conclude that measurement of the microvascular filtration rate reflects the change in the muscle exchange area and is influenced by exercise training.     

Systemic effects of angiotensin III in conscious dogs during acute double blockade of the renin-angiotensin-aldosterone-system

Aims: The study was designed to determine (i) whether the effects of angiotensin III (AngIII) are similar to those of angiotensin II (AngII) at identical plasma concentrations and (ii) whether AngIII operates solely through AT₁‐ receptors. Methods: Angiotensin II (3 pmol kg^?1 min^?1–3.1 ng kg^?1 min^?1) or AngIII (15 pmol kg^?1 min^?1–14 ng kg^?1 min^?1) was infused i.v. during acute inhibition of angiotensin converting enzyme (enalaprilate; 2 mg kg^?1) and of aldosterone (canrenoate; 6 mg kg^?1 plus 1 mg kg^?1 h^?1). Arterial plasma concentrations of angiotensins were determined by radioimmunoassay using a cross‐reacting antibody to AngII. During ongoing peptide infusion, candesartan (2 mg kg^?1) was administered to block the AT₁‐receptors. Results: Angiotensin immunoactivity in plasma increased to 60 ± 10 pg mL^?1 during infusion of AngII or infusion of AngIII. AngII significantly increased mean arterial blood pressure (+14 ± 4 mmHg) and plasma aldosterone by 79% (+149 ± 17 pg mL^?1) and reduced plasma renin activity and sodium excretion (?41 ± 16 mIU L^?1 and ?46 ± 6 μmol min^?1 respectively). AngIII mimicked these effects and the magnitude of AngIII responses was statistically indistinguishable from those of AngII. All measured effects of both peptides were blocked by candesartan. Conclusion: At the present arterial plasma concentrations, AngIII is equipotent to AngII with regard to effects on blood pressure, aldosterone secretion and renal functions, and these AngIII effects are mediated through AT₁‐ receptors. The metabolic clearance rate of AngIII is five times that of AngII.     

Effect of verapamil on restoration of cardiac performance in raised [K+]0 by adrenergic stimulation in the rabbit

Modulation of the L-type calcium channel by catecholamines improves action potential parameters in single ventricular myocytes depolarized by high [K⁺]₀ Tyrode. Whether this modulation is important in offsetting the negative effects of hyperkalaemia in the whole heart is not known. We tested the effects of the calcium channel antagonist, verapamil, on restoration of cardiac performance by adrenergic stimulation in high [K⁺]₀ in anaesthetized rabbits and isolated perfused working rabbit hearts. Raised [K⁺]₀ decreased SBP, LVP and LVdP/dt_maxin vivo ([K⁺]_a 8.6 ± 0.2 mM; n= 10) and aortic flow (AF) in the isolated heart (8 mM [K⁺]₀ Tyrode; n= 25). However, the negative effects of raised [K⁺]_a were offset by isoprenaline (Iso, 1 μg kg^-1 min^-1 i.v.) in vivo and by noradrenaline (NA, 80 nM) in the isolated heart. Verapamil (0.15 mg kg^-1 iv.; 15 nM isolated heart) markedly potentiated the negative inotropic effects of raised [K⁺]_n in both preparations. Verapamil attenuated the effect of isoprenaline in vivo but in the isolated heart, the protective effect of NA in 8 mM [K⁺] Tyrode (AF 97 ± 10 mL min¹ in 8 mM [K⁺]₀ compared with AF 141 ± 8.5 mL min^-1 in 8 mM [K⁺]₀+ NA) was offset by the drug (90±8mL min^-1 in 8 mM [K⁺]₀+ NA + V). Furthermore, verapamil abolished aortic flow in 8 mM [K⁺]₀ alone. These findings suggest that the heart may be critically dependent on modulation of intracellular calcium in order to tolerate concentrations of K⁴ similar to those seen during a short burst of intensive exercise ([K⁺]_a 8.6 mM).     

Serial effects of high-resistance and prolonged endurance training on Na+–K+ pump concentration and enzymatic activities in human vastus lateralis

The purpose of this study was to compare two contrasting training models, namely high-resistance training and prolonged submaximal training on the expression of Na⁺–K+ ATPase and changes in the potential of pathways involved in energy production in human vastus lateralis. The high-resistance training group (VO _2peak = 45.3 ± 1.9 mL kg^?1 min^?1, mean ± SE, n = 9) performed three sets of six to eight repetitions maximal, each of squats, leg presses and leg extensions, three times per week for 12 weeks, while the prolonged submaximal training group (VO _2peak = 44.4 ± 6.6 mL kg^?1 min^?1, n = 7) cycled 5–6 times per week for 2 h day^?1 at 68% VO _2peak for 11 weeks. In the HRT group, Na⁺–K⁺ ATPase (pmol g^?1 wet wt), measured with the ³H-ouabain binding technique, showed no change from 0 (289 ± 22) to 4 weeks (283 ± 15), increased (P < 0.05) by 16% at 7 weeks and remained stable until 12 weeks (319 ± 19). For prolonged submaximal training, a 22% increase (P < 0.05) was observed from 0 (278 ± 31) until 3 weeks (339 ± 29) with no further changes observed at either 9 weeks (345 ± 25) or 11 weeks (359 ± 34). In contrast to high-resistance training, where a 15% increase (P < 0.05) was observed, only in the maximal activity of phosphorylase, prolonged submaximal training resulted in increases in malate dehydrogenase, β-hydroxyl-CoA dehydrogenase, hexokinase and phosphofructokinase. In contrast to high-resistance training which failed to result in an increase in VO _2peak, prolonged submaximal training increased VO _2peak by ≈15%. Only for prolonged exercise training was a relationship observed for VO _2peak and Na⁺–K⁺-ATPase (r = 0.59; P < 0.05). Correlations between VO _2peak and mitochondrial enzyme activities were not significant (P > 0.05) for either training programme. It is concluded that although both training programmes stimulate an up-regulation in Na⁺–K⁺ ATPase concentration, only the prolonged submaximal training programme enhances the potential for β-oxidation, oxidative phosphorylation and glucose phosphorylation.     

Antisense oligodeoxynucleotides of sulfonylurea receptors inhibit ATP-sensitive K+ channels in cultured neonatal rat ventricular cells

 To identify the functional sulfonylurea receptor (SUR), a subunit of the adenosine 5′-triphosphate (ATP)-sensitive K⁺ (K_ATP) channels, in neonatal rat ventricular cells, such cells in primary culture were treated for 6 days with antisense (AS) oligodeoxynucleotides (ODNs) complementary to the mRNA for SURs. For quantification, single-channel (inside-out patches) and whole-cell currents were measured using the patch-clamp technique. The maximal K_ATP currents (at 0 mV) induced by metabolic inhibition were 48.9±2.8 pA/pF in control (n=48), 34.3±3.5 pA/pF in AS-SUR1 (n=21, P<0.05 vs control), and 23.5±3.4 pA/pF in AS-SUR2 (n=17, P<0.01 vs control). As a control, scramble oligonucleotides had no effect. The fast Na⁺ current and inward-rectifying K⁺ current were not affected by AS-SURs. Treatment with both AS-SUR1 and AS-SUR2 had no additive effects on inhibition of K_ATP currents compared with AS-SUR2 alone. The single-channel conductance, open probability, and kinetics (in ATP-free solution) were not significantly different between control, AS-SUR1, and AS-SUR2. These results suggest that treatment with AS-ODN for SUR1 or SUR2 reduced the number of functional K_ATP channels. Furthermore, in four out of seven control cells tested, outward K⁺ currents were stimulated by diazoxide, which is a potent K⁺ channel-opening drug for the constructed SUR1/Kir6.2 and SUR2B/Kir6.2 channels, but not for the SUR2A/Kir6.2 channel. Therefore, in neonatal rat ventricular cells, both SUR2 and SUR1 subtypes could be integral components of the functional K_ATP channels. The larger population of K_ATP channels may be constructed with SUR2, whereas a smaller population may be constructed with a combination of SUR1 and SUR2. Received: 29 May 1998 / Received after revision: 8 September 1998 / Accepted: 13 October 1998     

The effects of P2X receptor agonists on renal sodium and water excretion in anaesthetized rats

Aim: To investigate in vivo effects of P2X receptor activation on sodium and water excretion in urine. Methods: The clearance experiments were carried out in anaesthetized rats during intravenous infusion (2 μmol kg^?1 + 20 nmol (kg min)^?1, v = 40 μL min^?1) of P2X receptors agonists: α,β‐methylene ATP (α,β‐meATP) and β,γ‐methylene ATP (β,γ‐meATP). Cortical blood flow (CBF) was estimated by laser Doppler flux during intrarenal artery infusion of β,γ‐meATP (20 nmol (kg min)^?1, v = 2 μL min^?1). Influence of α,β‐meATP and β,γ‐meATP on the activity of Na‐K‐ATPase was investigated in isolated proximal tubules. Results: Intravenous infusion of β,γ‐meATP resulted in a marked, progressively increasing diuresis and this effect was accompanied by a progressive increase in the sodium excretion rate. The glomerular filtration rate was unaffected. The effects of β,γ‐meATP were abolished by P2 receptor antagonist PPADS (70 nmol (kg min)^?1). CBF increased by 16 ± 2% during renal artery infusion of β,γ‐meATP. Furthermore, α,β‐meATP and β,γ‐meATP increased 1.5‐fold lithium clearance (C_Li). Sodium excretion, expressed as a fraction of the distal delivery (C_NaC_Li^?1), increased 1.5‐fold during infusion of α,β‐meATP or β,γ‐meATP. Both agonists at 10^?6 m produced a statistical significant decrement in the ouabain‐sensitive ATPase activity about 16–20% and these effects were blocked in the presence of PPADS. Conclusion: Activation of P2X receptors increased renal sodium and water excretion. Mechanistically, P2X agonists increased renal perfusion and inhibited sodium reabsorption via an Na‐K‐ATPase‐dependent mechanism.     

Non‐invasive and quantitative evaluation of peripheral vascular resistances in rats by combined NMR measurements of perfusion and blood pressure using ASL and dynamic angiography

The in vivo determination of peripheral vascular resistances (VR) is crucial for the assessment of arteriolar function. It requires simultaneous determination of organ perfusion (F) and arterial blood pressure (BP). A fully non‐invasive method was developed to measure systolic and diastolic BP in the caudal artery of rats based on dynamic NMR angiography. A good agreement was found between the NMR approach and the gold standard techniques (linear regression slope = 0.98, R² = 0.96). This method and the ASL‐MRI measurement of skeletal muscle perfusion were combined into one single NMR experiment to quantitatively evaluate the local vascular resistances in the calf muscle of anaesthetized rats, in vivo and non‐invasively 1) at rest: VR = 7.0 ± 1.0 mmHg·min 100 g·ml^?1, F = 13 ± 3 ml min^?1.100 g^?1 and mean BP (MBP) = 88 ± 10 mmHg; 2) under vasodilator challenge (milrinone): VR = 3.7 ± 1.1 mmHg min.100 g ml^?1, F = 21 ± 4 ml min^?1.100 g^?1 and MBP = 75 ± 14 mmHg; 3) under vasopressor challenge (norepinephrine): VR = 9.8 ± 1.2 mmHg min 100 g ml^?1, F = 14 ± 3 ml min^?1.100 g^?1 and MBP = 137 ± 2 mmHg. Copyright © 2009 John Wiley & Sons, Ltd.     

K+ channel modulation in arterial smooth muscle

Potassium channels play an essential role in the membrane potential of arterial smooth muscle, and also in regulating contractile tone. Four types of K⁺ channel have been described in vascular smooth muscle: Voltage-activated K⁺ channels (K_V) are encoded by the Kv gene family, Ca²⁺-activated K⁺ channels (BK_Ca) are encoded by the slogene, inward rectifiers (K_IR) by Kir2.0, and ATP-sensitive K⁺ channels (K_ATP) by Kir6.0 and sulphonylurea receptor genes. In smooth muscle, the channel subunit genes reported to be expressed are: Kv1.0, Kv1.2, Kv1.4–1.6, Kv2.1, Kv9.3, Kvβ1–β4, slo α and β, Kir2.1, Kir6.2, and SUR1 and SUR2. Arterial K⁺ channels are modulated by physiological vasodilators, which increase K⁺ channel activity, and vasoconstrictors, which decrease it. Several vasodilators acting at receptors linked to cAMP-dependent protein kinase activate K_ATP channels. These include adenosine, calcitonin gene-related peptide, and β-adrenoceptor agonists. β-adrenoceptors can also activate BK_Ca and K_V channels. Several vasoconstrictors that activate protein kinase C inhibit K_ATP channels, and inhibition of BK_Ca and K_V channels through PKC has also been described. Activators of cGMP-dependent protein kinase, in particular NO, activate BK_Ca channels, and possibly K_ATP channels. Hypoxia leads to activation of K_ATP channels, and activation of BK_Ca channels has also been reported. Hypoxic pulmonary vasoconstriction involves inhibition of K_V channels. Vasodilation to increased external K⁺ involves K_IR channels. Endothelium-derived hyperpolarizing factor activates K⁺ channels that are not yet clearly defined. Such K⁺ channel modulations, through their effects on membrane potential and contractile tone, make important contributions to the regulation of blood flow.     

Physiological role of inward rectifier K+ channels in vascular smooth muscle cells

K⁺ channels play indispensable roles in establishing the membrane potential and in regulating the contractile tone of arterial smooth muscle cells. There are four types of K⁺ channels in arterial smooth muscle: voltage-dependent K⁺ (K_V), Ca²⁺-dependent K⁺ (BK_Ca), ATP-dependent K⁺ (K_ATP), and inward rectifier K⁺ (Kir2) channels. Comparatively few physiological studies have focused on Kir2 channels because they are present only in certain small-diameter cerebral and submucosal arterioles and in coronary arterial smooth muscle. Here, we review the characteristics and regulation of Kir2 channels in vascular arterial smooth muscle. Current knowledge of the predominant Kir2 channel subtype is Kir2.1, not Kir2.2 and 2.3. Electrophysiological measurements to determine the current–voltage relationship in arterial smooth muscle revealed inward rectification with a single-channel conductance of 21 pS. Kir2 channels were found to influence the resting tone of cerebral and coronary arteries based on the fact that barium (Ba²⁺) induces the constriction of these arteries at resting tone. Kir2 channels are also highly responsive to vasoconstrictors and vasodilators. For example, the vasoconstrictors endothelin-1 and angiotensin II inhibit Kir2 channel function by activating protein kinase C (PKC), and the vasodilator adenosine stimulates Kir2 channel function by increasing the level of cAMP, which subsequently activates protein kinase A (PKA). Certain pathological conditions such as left ventricular hypertrophy are associated with a decrease in Kir2 channel expression. Although our understanding of the physiological role and regulation of Kir2 channels is incomplete, it is believed that Kir2 channels contribute to the control of vascular tone in small-diameter vessels via various intracellular signalling pathways that regulate cell membrane potential.     

Middle cerebral artery blood velocity during exercise with β‐1 adrenergic and unilateral stellate ganglion blockade in humans

A reduced ability to increase cardiac output (CO) during exercise limits blood flow by vasoconstriction even in active skeletal muscle. Such a flow limitation may also take place in the brain as an increase in the transcranial Doppler determined middle cerebral artery blood velocity (MCA V_mean) is attenuated during cycling with β‐1 adrenergic blockade and in patients with heart insufficiency. We studied whether sympathetic blockade at the level of the neck (0.1% lidocain; 8 mL; n=8) affects the attenuated exercise – MCA V_mean following cardio‐selective β‐1 adrenergic blockade (0.15 mg kg^?1 metoprolol i.v.) during cycling. Cardiac output determined by indocyanine green dye dilution, heart rate (HR), mean arterial pressure (MAP) and MCA V_mean were obtained during moderate intensity cycling before and after pharmacological intervention. During control cycling the right and left MCA V_mean increased to the same extent (11.4 ± 1.9 vs. 11.1 ± 1.9 cm s^?1). With the pharmacological intervention the exercise CO (10 ± 1 vs. 12 ± 1 L min^?1; n=5), HR (115 ± 4 vs. 134 ± 4 beats min^?1) and ΔMCA V_mean (8.7 ± 2.2 vs. 11.4 ± 1.9 cm s^?1) were reduced, and MAP was increased (100 ± 5 vs. 86 ± 2 mmHg; P < 0.05). However, sympathetic blockade at the level of the neck eliminated the β‐1 blockade induced attenuation in ΔMCA V_mean (10.2 ± 2.5 cm s^?1). These results indicate that a reduced ability to increase CO during exercise limits blood flow to a vital organ like the brain and that this flow limitation is likely to be by way of the sympathetic nervous system.     

Effect of acute hypoxia on muscle blood flow, VO2p, and [HHb] kinetics during leg extension exercise in older men

The adjustment of pulmonary oxygen uptake (VO_2p), heart rate (HR), limb blood flow (LBF), and muscle deoxygenation [HHb] was examined during the transition to moderate-intensity, knee-extension exercise in six older adults (70 ± 4 years) under two conditions: normoxia (FIO₂ = 20.9 %) and hypoxia (FIO₂ = 15 %). The subjects performed repeated step transitions from an active baseline (3 W) to an absolute work rate (21 W) in both conditions. Phase 2 VO_2p, HR, LBF, and [HHb] data were fit with an exponential model. Under hypoxic conditions, no change was observed in HR kinetics, on the other hand, LBF kinetics was faster (normoxia 34 ± 3 s; hypoxia 28 ± 2), whereas the overall [HHb] adjustment ( $ \tau^{\prime } = {\text{TD}} + \tau $ ) was slower (normoxia 28 ± 2; hypoxia 33 ± 4 s). Phase 2 VO_2p kinetics were unchanged (p < 0.05). The faster LBF kinetics and slower [HHb] kinetics reflect an improved matching between O₂ delivery and O₂ utilization at the microvascular level, preventing the phase 2 VO_2p kinetics from become slower in hypoxia. Moreover, the absolute blood flow values were higher in hypoxia (1.17 ± 0.2 L min^?1) compared to normoxia (0.96 ± 0.2 L min^?1) during the steady-state exercise at 21 W. These findings support the idea that, for older adults exercising at a low work rate, an increase of limb blood flow offsets the drop in arterial oxygen content (CaO₂) caused by breathing an hypoxic mixture.     

Muscle fatigue and excitation-contraction coupling responses following a session of prolonged cycling

Aim: The mechanisms underlying the fatigue that occurs in human muscle following sustained activity are thought to reside in one or more of the excitation–contraction coupling (E–C coupling) processes. This study investigated the association between the changes in select E–C coupling properties and the impairment in force generation that occurs with prolonged cycling. Methods: Ten volunteers with a peak aerobic power () of 2.95 ± 0.27 L min^?1 (mean ± SE), exercised for 2 h at 62 ± 1.3%. Quadriceps function was assessed and tissue properties (vastus lateralis) were measured prior to (E1‐pre) and following (E1‐post) exercise and on three consecutive days of recovery (R1, R2 and R3). Results: While exercise failed to depress the maximal activity (V_max) of the Na⁺,K⁺‐ATPase (P = 0.10), reductions (P < 0.05) were found at E1‐post in V_max of sarcoplasmic reticulum Ca²⁺‐ATPase (?22%), Ca²⁺‐uptake (?26%) and phase 1(?33%) and 2 (?38%) Ca²⁺‐release. Both V_max and Ca²⁺‐release (phase 2) recovered by R1, whereas Ca²⁺‐uptake and Ca²⁺‐release (phase 1) remained depressed (P < 0.05) at R1 and at R1 and R2 and possibly R3 (P < 0.06) respectively. Compared with E1‐pre, fatigue was observed (P < 0.05) at 10 Hz electrical stimulation at E1‐post (?56%), which persisted throughout recovery. The exercise increased (P < 0.05) overall content of the Na⁺,K⁺‐ATPase (R1, R2 and R3) and the isoforms β2 (R1, R2 and R3) and β3 (R3), but not β1 or the α‐isoforms (α1, α2 and α3). Conclusion: These results suggest a possible direct role for Ca²⁺‐release in fatigue and demonstrate a single exercise session can induce overlapping perturbations and adaptations (particularly to the Na⁺,K⁺‐ATPase).     

Dopamine‐induced inhibition of Na+‐K+‐ATPase activity requires integrity of actin cytoskeleton in opossum kidney cells

The present study evaluated the importance of the association between Na⁺‐K⁺‐ATPase and the actin cytoskeleton on dopamine‐induced inhibition of Na⁺‐K⁺‐ATPase activity. The approach used measures the transepithelial transport of Na⁺ in monolayers of opossum kidney (OK) cells, when the Na⁺ delivered to Na⁺‐K⁺‐ATPase was increased at the saturating level by amphotericin B. The maximal amphotericin B (1.0 μg mL^–1) induced increase in short‐circuit current (I_sc) was prevented by ouabain (100 μM ) or removal of apical Na⁺. Dopamine (1 μM ) applied from the apical side significantly decreased (29 ± 5% reduction) the amphotericin B‐induced increase in I_sc, this being prevented by the D₁‐like receptor antagonist SKF 83566 (1 μM ) and the protein kinase C (PKC) inhibitor chelerythrine (1 μM ). Exposure of OK cells to cytochalasin B (1 μM ) or cytochalasin D (1 μM ), inhibitors of actin polymerization, from both cell sides reduced by 31 ± 4% and 36 ± 3% the amphotericin B‐induced increase in I_sc and abolished the inhibitory effect of apical dopamine (1 μM ), but not that of the PKC activator phorbol‐12,13‐dibutyrate (PDBu; 100 nM ). Colchicine (1 μM ) failed to alter the inhibitory effects of dopamine. The relationship between Na⁺‐K⁺‐ATPase and the concentration of extracellular Na⁺ showed a Michaelis–Menten constant (K_m) of 44.1 ± 13.7 mM and a V_max of 49.6 ± 4.8 μA cm^–2 in control monolayers. In the presence of apical dopamine (1 μM ) or cytochalasin B (1 μM ) V_max values were significantly (P < 0.05) reduced without changes in K_m values. These results are the first, obtained in live cells, showing that the PKC‐dependent inhibition of Na⁺‐K⁺‐ATPase activity by dopamine requires the integrity of the association between actin cytoskeleton and Na⁺‐K⁺‐ATPase.     

Natriuresis in conscious dogs caused by increased carotid [Na+] during angiotensin II and aldosterone blockade

The renal response to a selective increase in the Na⁺ concentration of the blood perfusing the central nervous system was investigated in conscious dogs treated with the converting enzyme inhibitor enalaprilat and the aldosterone antagonist canrenoate. In split-infusion experiments the plasma [Na⁺] of carotid blood was increased (approx. 6 mM) by bilateral infusion of hypertonic NaCl. Concomitantly distilled water was infused into the v. cava making the sum of the infusions isotonic. In control experiments isotonic saline was infused at identical rates into all three catheters. Na⁺ excretion increased markedly in both series, 103 ± 14 to 678± 84 μmol min^-1 during split-infusion and 90 ± 14 to 496 ± 74 μmol min^-1 during the isotonic volume expansion. Peak rate of excretion, peak fractional sodium excretion, and cumulative sodium excretion were all significantly higher (P < 0.05) during split-infusion than during control experiments. Plasma vasopressin increased only during split-infusion (0.68 ± 0.11 to 2.4 ± 0.8 pg ml^-1) while the increases in plasma atrial natriuretic peptide were similar in the two series. Urinary excretion of urodilatin (ANP95-126) increased significantly more during split-infusion (46 ±11 to 152 ±28 fmol min^-1) than during the isotonic volume expansion (45 ± 14 to 84 ± 16 fmol min^-1) (P < 0.05). It is concluded that the natriuretic mechanisms activated by a selective increase in the Na⁺ concentration of carotid blood and associated with increased excretion of urodilatin cannot be eliminated by blockade of the renin-angiotensin-aldosterone system.     

Global cerebral blood flow during infusion of adenosine in humans: assessment by magnetic resonance imaging and positron emission tomography

Adenosine, an endogenous vasodilator, induces a cerebral vasodilation at hypotensive infusion rates in anaesthetized humans. At lower doses (< 100 μg kg^?1 min^?1), adenosine has shown to have an analgesic effect. This study was undertaken to investigate whether a low dose, causing tolerable symptoms of peripheral vasodilation affects the global cerebral blood flow (CBF). In nine healthy volunteers CBF measurements were made using axial magnetic resonance (MR) phase images of the internal carotid and vertebral arteries at the level of C2–3. Quantitative assessment of CBF was also obtained with positron emission tomography (PET) technique, using intravenous bolus []> ¹⁵O]butanol as tracer in four of the subject at another occasion. During normoventilation (5.4 ± 0.2 kPa, mean ± s.e.m.), the cerebral blood flow measured by magnetic resonance imaging technique, as the sum of the flows in both carotid and vertebral arteries, was 863 ± 66 mL min^?1, equivalent to about 64 ± 5 mL 100 g^?1 min^?1. The cerebral blood flow measured by positron emmission tomography technique, was 59 ± 4 mL 100 g^?1 min^?1. All subjects had a normal CO₂ reactivity. When adenosine was infused (84 ± 7 μg kg^?1 min^?1) the cerebral blood flow, measured by magnetic resonance imaging was 60 ± 5 mL 100 g^?1 min^?1. The end tidal CO₂ level was slightly lower (0.2 ± 0.1 kPa) during adenosine infusion than during normoventilation. In the subgroup there was no difference in cerebral blood flow as measured by magnetic resonance imaging or positron emission tomography. In conclusion, adenosine infusion at tolerable doses in healthy volunteers does not affect global cerebral blood flow in unanaesthetized humans.     

Reducing plasma free fatty acids by acipimox improves glucose tolerance in high‐fat fed mice

To study whether free fatty acids (FFAs) contribute to glucose intolerance in high‐fat fed mice, the derivative of nicotinic acid, acipimox, which inhibits lipolysis, was administered intraperitoneally (50 mg kg^?1) to C57BL/6J mice which had been on a high‐fat diet for 3 months. Four hours after administration of acipimox, plasma FFA levels were reduced to 0.46 ± 0.06 mmol L^?1 compared with 0.88 ± 0.10 mmol L^?1 in controls (P < 0.001). At this point, the glucose elimination rate after an intravenous glucose load (1 g kg^?1) was markedly improved. Thus, the elimination constant (K_G) for the glucose disposal between 1 and 50 min after the glucose challenge was increased from 0.54 ± 0.01% min^?1 in controls to 0.66 ± 0.01% min^?1 by acipimox (P < 0.001). In contrast, the acute insulin response to glucose (1–5 min) was not significantly different between the groups, although the area under the insulin for the entire 50‐min period after glucose administration was significantly reduced by acipimox from 32.1 ± 2.9 to 23.9 ± 1.2 nmol L^?1 × 50 min (P=0.036). This, however, was mainly because of lower insulin levels at 20 and 50 min because of the lowered glucose levels. In contrast, administration of acipimox to mice fed a normal diet did not affect plasma levels of FFA or the glucose elimination or insulin levels after the glucose load. It is concluded that reducing FFA levels by acipimox in glucose intolerant high‐fat fed mice improves glucose tolerance mainly by improving insulin sensitivity making the ambient islet function adequate, suggesting that increased FFA levels are of pathophysiological importance in this model of glucose intolerance.