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.     

K+ as a vasodilator in resting human muscle: implications for exercise hyperaemia

Aim: Potassium (K⁺) released from contracting skeletal muscle is considered a vasodilatory agent. This concept is mainly based on experiments infusing non‐physiological doses of K⁺. The aim of the present study was to investigate the role of K⁺ in blood flow regulation. Methods: We measured leg blood flow (LBF) and arterio‐venous (A‐V) O₂ difference in 13 subjects while infusing K⁺ into the femoral artery at a rate of 0.2, 0.4, 0.6 and 0.8 mmol min^?1. Results: The lowest dose increased the calculated femoral artery plasma K⁺ concentration by approx.1 mmol L^?1. Graded K⁺ infusions increased LBF from 0.39 ± 0.06 to 0.56 ± 0.13, 0.58 ± 0.17, 0.61 ± 0.11 and 0.71 ± 0.17 L min^?1, respectively, whereas the leg A‐V O₂ difference decreased from 74 ± 9 to 60 ± 12, 52 ± 11, 53 ± 9 and 45 ± 7 mL L^?1, respectively (P < 0.05). Mean arterial pressure was unchanged, indicating that the increase in LBF was associated with vasodilatation. The effect of K⁺ was totally inhibited by infusion (27 μmol min^?1) of Ba²⁺, an inhibitor of Kir2.1 channels. Simultaneous infusion of ATP and K⁺ evoked an increase in LBF equalled to the sum of their effects. Conclusions: Physiological infusions of K⁺ induce significant increases in resting LBF, which are completely blunted by inhibition of the Kir2.1 channels. The present findings in resting skeletal muscle suggest that K⁺ released from contracting muscle might be involved in exercise hyperaemia. However, the magnitude of increase in LBF observed with K⁺ infusion suggests that K⁺ only accounts for a limited fraction of the hyperaemic response to exercise.     

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.     

Thermogenic response to adrenaline during restricted blood flow in the forearm

To elucidate the underlying mechanism behind the thermogenic effect of adrenaline in human skeletal muscle, nine healthy subjects were studied during intravenous infusion of adrenaline. Restriction of blood flow to one forearm was obtained by external compression of the brachial artery, to separate a direct metabolic effect of adrenaline from an effect dependent on increased blood flow. The other arm served as the control arm. In the control arm, the forearm blood flow increased 4.7-fold (from 2.0 ± 0.3 to 9.3 ± 1.5 mL 100 g^–1 min^–1, P < 0.001) during the adrenaline infusion. Adrenaline significantly increased forearm oxygen consumption (from 4.7 ± 2.1 to 7.0 ± 3.6 μmol 100 g^–1 min^–1, P < 0.025). In the arm with restricted blood flow, the forearm blood flow increased 2.9-fold (from 1.6 ± 0.3 to 4.6 ± 0.8 mL 100 g^–1 min^–1, P < 0.002) but the forearm oxygen consumption did not increase (baseline period: 5.6 ± 2.3 μmol 100 g^–1 min^–1, adrenaline period: 6.1 ± 2.1 μmol 100 g^–1 min^–1, P = 0.54). The experimental design and the difficulties in interpretation of the result are discussed. The results give evidence for the hypothesis that the vascular system plays a key role in the thermogenic effect of adrenaline in skeletal muscle in vivo.     

Effects of adenosine on ex vivo filtragometry platelet aggregation in relation to plasma levels

The aim of the present study was to investigate the concentration effect of adenosine on unstimulated platelet aggregation in humans. Adenosine infusion was given intravenously to 12 volunteers in the antecubital vein with infusion rates increasing from 20 to 100 μg kg^?1 min^?1. Filtragometry measurements were obtained from the contralateral antecubital vein before and during 100 μg kg^?1 min^?1 or during maximal tolerable infusion rate. In another set of experiments with 10 volunteers, basal filtragometry measurements were obtained before and after infusion of various concentrations of adenosine into the filtragometer test unit. With intravenous infusion aggregation time tended to increase from 333±42 to 418±8 s (mean±SEM) and increased the venous plasma adenosine concentration from 0.42±0.09 μM to 1.52±0.38 μM . Adenosine infusion into the filtragometer tubing system dose-dependently inhibited aggregation (P<0.05). Adenosine was rapidly eliminated with a half-life of adenosine in the filtragometry tubing system calculated to be about 6 s. These data extend our knowledge from an in vitroto an ex vivo situation that adenosine dose-dependently has a platelet antiaggregatory effect.     

Exogenous endothelin-1 causes peripheral insulin resistance in healthy humans

In states of insulin resistance, increased plasma levels of endothelin-1 and a disturbed vascular reactivity have been reported. In order to investigate the effects of endothelin-1 on peripheral insulin sensitivity and the vasoactive interactions between insulin and endothelin-1, six healthy subjects were studied on two different occasions with the euglycaemic hyperinsulinaemic clamp technique combined with an intravenous infusion of either endothelin-1 (4 pmol kg^?1 min^?1) or 0.9% sodium chloride. During the endothelin-1 infusion, arterial plasma endothelin-1 levels rose 10-fold. The endothelin-1 infusion reduced insulin sensitivity as demonstrated by a 31 ± 7% decrease in whole-body glucose uptake (P < 0.05) and a 26 ± 11% fall in leg glucose uptake (P < 0.05) compared with the control protocol. During the state of hyperinsulinaemia, exogenous endothelin-1 increased mean arterial blood pressure by 8 ± 1% (P < 0.05) and decreased splanchnic and renal blood flow by 30 ± 6% (P < 0.001) and 20 ± 4% (P < 0.001), respectively. However, the endothelin-1 infusion did not lower skeletal muscle blood flow measured as leg and forearm blood flow. In summary, exogenous endothelin-1 induced insulin resistance in healthy humans by reducing insulin-dependent glucose uptake in skeletal muscle without decreasing skeletal muscle blood flow. Furthermore, endothelin-1 also preserved its vasoactive potency in the presence of hyperinsulinaemia.     

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.     

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.     

Increased insulin-stimulated glucose uptake in athletes: the importance of GLUT4 mRNA,GLUT4 protein and fibre type composition of skeletal muscle

In the present study the expression of GLUT4 and fibre type composition were examined in biopsies from skeletal muscle in seven male athletes and eight male sedentary subjects. Estimated maximal oxygen uptake was increased in the trained group when compared with the sedentary group (74.0 ± 3.9 vs. 42.9±5.1 ml kg^-1 min^-1; P < 0.01). A biopsy of vastus lateralis muscle was taken in the fasting state, 36 h after the last bout of exercise. A second muscle biopsy was obtained following 4 h of a hyperinsulinaemic (2 mU kg^-1 min^-1), euglycaemic clamp. The rate of insulin-stimulated glucose uptake was increased in the trained subjects (17.34±0.53 vs. 13.53±0.79 mg kg^-1 min^-1, P < 0.01). In parallel, the steady state levels of GLUT4 protein and mRNA per DNA were higher in muscle biopsies obtained in the basal state from athletes than in sedentary controls, 21 and 71% respectively (P < 0.05). In the total group of participants, GLUT4 protein per DNA in the basal state and insulin-stimulated glucose uptake rate correlated positively, (r = 0.51, P = 0.05). In the insulin-stimulated state we did not find any significant correlation between GLUT4 protein per DNA and glucose uptake rate (r = 0.13, n.s.). No significant relationships between GLUT4 protein abundance per DNA and muscle fibre type distribution were observed. A significantly negative correladon was found between type 2B fibre area and insulin-stimulated glucose uptake (r =–0.63, P < 0.05). In conclusion, the abundance of GLUT4 protein and mRNA, respectively, is increased in skeletal muscle from endurance trained subjects compared to sedentary subjects. However, factors other than GLUT4 immunoreactive protein abundance seem to be determinant for the increased insulin-stimulated whole body glucose uptake in endurance trained subjects.     

Effect of C-peptide administration on whole body glucose utilization in STZ-induced diabetic rats

Recent studies suggest that C-peptide stimulates glucose transport in isolated skeletal muscle. In order to determine the effect of C-peptide on whole body glucose utilization, streptozotocin (60 mg kg^-1) (STZ)-induced diabetic and normal rats were studied using the euglycaemic clamp procedure and continuous infusion of somatostatin (1.0 μg kg^-1 min^-1) in pentobarbital-anaesthetized rats. Plasma insulin levels during the 6.0- and 30.0-mU kg^-1 min^-1 insulin infusions rose to 70–90 μU mL^-1 and 500–700 μU mL^-1, respectively. Blood glucose concentrations were clamped at 7.5–7.9 mmol L^-1 in the diabetic rats and at basal levels or 7.7 mmol L^-1 in the non-diabetic (normal) rats. Biosynthetic human C-peptide (0.5 nmol kg^-1 min^-1) was infused in 12 diabetic and 11 normal rats, resulting in concentrations of 26–41 nmol L^-1. The metabolic clearance rate of glucose (MCR) for the diabetic rats receiving C-peptide (12.0±1.0 mL kg^-1 min^-1) was significantly (P<0.01) higher than that in the diabetic rats given saline (6.3±0.7 mL kg^-1 min^-1) or a randomly scrambled C-peptide (7.8±1.3 mL kg^-1 min^-1) at low-dose insulin infusion but not at the high-dose insulin infusion. In normal rats C-peptide did not significantly increase the MCR for glucose. These results thus demonstrate that C-peptide has the capacity to increase glucose utilization in STZ-induced diabetic rats.     

Acute and adaptive responses in humans to exercise in a warm, humid environment

 Acute and repeated exposure for 8–13 consecutive days to exercise in humid heat was studied. Twelve fit subjects exercised at 150 W [45% of maximum O₂ uptake (V.O₂,_max)] in ambient conditions of 35°C and 87% relative humidity which resulted in exhaustion after 45 min. Average core temperature reached 39.9 ± 0.1°C, mean skin temperature (T– _sk) was 37.9 ± 0.1°C and heart rate (HR) 152 ± 6 beats min^–1 at this stage. No effect of the increasing core temperature was seen on cardiac output and leg blood flow (LBF) during acute heat stress. LBF was 5.2 ± 0.3 l min^–1 at 10 min and 5.3 ± 0.4 l min^–1 at exhaustion (n = 6). After acclimation the subjects reached exhaustion after 52 min with a core temperature of 39.9 ± 0.1°C, T– _sk 37.7 ± 0.2°C, HR 146 ± 4 beats min^–1. Acclimation induced physiological adaptations, as shown by an increased resting plasma volume (3918 ± 168 to 4256 ± 270 ml), the lower exercise heart rate at exhaustion, a 26% increase in sweating rate, lower sweat sodium concentration and a 6% reduction in exercise V.O₂. Neither in acute exposure nor after acclimation did the rise of core temperature to near 40°C affect metabolism and substrate utilization. The physiological adaptations were similar to those induced by dry heat acclimation. However, in humid heat the effect of acclimation on performance was small due to physical limitations for evaporative heat loss. Received: 3 July 1996 / Received after revision: 26 September 1996 / Accepted: 7 January 1997     

Influence of adenosine on the vascular responses to sympathetic nerve stimulation in the canine subcutaneous adipose tissue

Adenosine appears to regulate resting blood flow in canine subcutaneous adipose tissue. Sympathetic nerve stimulation has been shown to enhance the adenosine production in this tissue. This study therefore tested the possibility that adenosine may influence the vascular responses to sympathetic nerve stimulation. Intraarterial infusion of adenosine (5–20 μM in arterial blood) increased the resting vascular conductance (from 0.048 ± 0.007 to 0.095 ± 0.013 ml ± min^-1100 g^-1± mmHg^-1) and the percental reduction in vascular conductance due to sympathetic nerve stimulation (4 Hz) by 34 per cent (p<0.05) and to i. a.noradrenaline by 27 per cent (p<0.05). The vasodilator response due to nerve stimulation after α-blockade was reduced by adenosine. Dipyridamole (0.5–1.5 μM) + EHNA (3–10 μM), which increases plasma adenosine levels, had similar effects to adenosine, while theophylline (30–80 μM) decreased the vasoconstrictor response. The vasoconstrictor escape was enhanced by EHNA alone and in combination with dipyridamole, but was reduced by theophylline. On the other hand, the poststimulatory hyperemia was unaffected by adenosine, dipyridamole and EHNA, and theophylline. The results show that adenosine does not reduce the magnitude of the initial vasoconstrictor response in proportion to the increase in resting blood flow. The autoregulatory escape in adipose tissue during nerve stimulation appears to be mediated both by adenosine and by noradrenaline acting on β-adrenoceptors. Poststimulatory hyperemia does not seem to be greatly influenced by exogenous or endogenous adenosine     

Extracellular distribution of insulin in muscle of rats exposed to long-term hyperinsulinaemia

The importance of increased capillary density for the regulation of insulin sensitivity by transcapillary delivery of insulin to muscle cells in insulin-exposed rats was investigated by direct microdialysis measurements of interstitial [¹²⁵I]insulin concentrations in the femoral muscle during an euglycaemic hyperinsulinaemic clamp. In insulin-exposed rats plasma insulin was ～25% (P<0.05) higher than that in control animals during the first 100 min and reached their maximal concentrations after 100 min. After a nitroprusside infusion given at 100 min both groups had similar concentrations of insulin in plasma as well as in muscle interstitial fluid. However, mean glucose infusion rate during the first clamp hour was 20.5±2.3 and 12.6±5.2 mg kg^-1 min^-1 (P<0.05) in insulin-exposed and control animals, respectively. During the second clamp hour the corresponding figures were 21.1±2.4 and 13.9±2.6 (P<0.05). It may be concluded that capillarization and/or nitroprusside affected plasma insulin concentrations without altering either the interstitial insulin levels or the insulin effect on glucose consumption. The data suggest that the elevated insulin sensitivity after chronic insulin exposure is dependent on other than transcapillary transport events and demonstrate the different kinetics for insulin distribution in plasma and in the interstitial fluid.     

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.     

Decreased insulin-stimulated 3–0-methylglucose transport in in vitro incubated muscle strips from type II diabetic subjects

Peripheral insulin resistance in type II diabetes mellitus has been attributed to alterations in skeletal muscle glucose metabolism. However the direct dose-response relationship between insulin and glucose transport has not yet been studied in human skeletal muscle. We investigated 3–O-methylglucose transport in in vitro incubated skeletal muscle strips from eight healthy controls (age 61 ± 6 yrs) and six lean type II diabetic patients treated with oral antidiabetic medication (age 73 ± 3 yrs). Rectus abdominis muscle samples (? 1 g), obtained during elective abdominal surgery, were clamped at their resting length in vivo, whereupon strips (20–50 mg) were prepared for in vitro incubation. Measurements of high-energy phosphates and glycogen levels revealed that the muscle strips maintained energy levels during the incubation period. Glucose transport responded to insulin in a dose-response manner in the control group, with a 2-fold increase following maximal stimulation. Muscle strips from the diabetic group demonstrated a marked decrease in the insulin dose-response curve (P < 0.01), when compared to healthy muscle strips. At a maximal insulin concentration (10,000 μU × ml^-1), the response of the diabetic muscle tissue was 50% less than that of the healthy control tissue (P < 0.05). This report demonstrates a dose-response curve for insulin stimulated 3–0-methylglucose transport in in vitro incubated human skeletal muscle strips. Furthermore, in type II diabetic muscle, our results provide evidence for one or several defects at a postreceptor level.     

Near-infrared assessments of skeletal muscle oxidative capacity in persons with spinal cord injury

After spinal cord injury (SCI) skeletal muscle decreases in size, increases in intramuscular fat, and has potential declines in mitochondrial function. Reduced mitochondrial function has been linked to the development of metabolic disease. The aim of this study was to measure mitochondrial function in persons with SCI using near-infrared spectroscopy (NIRS). Oxygen consumption of the vastus lateralis muscle was measured with NIRS during repeated short-duration arterial occlusions in nine able-bodied (AB) and nine persons with motor complete SCI. Skeletal muscle oxidative capacity (V _max) was evaluated with two approaches: (1) rate constant of the recovery of oxygen consumption after exercise and (2) extrapolated maximum oxygen consumption from a progressive work test. V _max as indicated by the rate constant (k) from the recovery kinetics test was lower in SCI compared with AB participants (k: SCI 0.7 ± 0.3 vs. AB 1.9 ± 0.4 min^?1; p < 0.001). Time constants were SCI 91.9 ± 37.8 vs. AB 33.6 ± 8.3 s. V _max from the progressive work test approached a significant difference between groups (SCI 5.1 ± 2.9 vs. AB 9.8 ± 5.5 % Hb-Mb/s; p = 0.06). NIRS measurements of V _max suggest a deficit of 50–60 % in participants with SCI compared with AB controls, consistent with previous studies using ³¹P-MRS and muscle biopsies. NIRS measurements can assess mitochondrial capacity in people with SCI and potentially other injured/diseased populations.     

Reduced coronary reserve in response to short-term ischaemia and vasoactive drugs in ex vivo hearts from diabetic mice

Aim: The aim of the present study was to compare the coronary flow (CF) reserve of ex vivo perfused hearts from type 2 diabetic (db/db) and non‐diabetic (db/+) mice. Methods: The hearts were perfused in the Langendorff mode with Krebs–Henseleit bicarbonate buffer (37 °C, pH 7.4) containing 11 mmol L^?1 glucose as energy substrate. The coronary reserve was measured in response to three different interventions: (1) administration of nitroprusside (a nitric oxide donor), (2) administration of adenosine and (3) production of reactive hyperaemia by short‐term ischaemia. Results: Basal CF was approximately 15% lower in diabetic when compared with non‐diabetic hearts (2.1 ± 0.1 vs. 2.6 ± 0.2 mL min^?1). The maximum increase in CF rate in response to sodium nitroprusside and adenosine was significantly lower in diabetic (0.6 ± 0.1 and 0.9 ± 0.1 mL min^?1 respectively) than in non‐diabetic hearts (1.2 ± 0.1 and 1.4 ± 0.1 mL min^?1 respectively). Also, there was a clear difference in the rate of return to basal CF following short‐term ischaemia between diabetic and non‐diabetic hearts. Thus, basal tone was restored 1–2 min after the peak hyperaemic response in non‐diabetic hearts, whereas it took approximately 5 min in diabetic hearts. Conclusion: These results show that basal CF, as well as the CF reserve, is impaired in hearts from type 2 diabetic mice. As diabetic and non‐diabetic hearts were exposed to the same (maximum) concentrations of NO or adenosine, it is suggested that the lower coronary reserve in type 2 diabetic hearts is, in part, because of a defect in the intracellular pathways mediating smooth muscle relaxation.     

Plasma potassium concentration as a determinant of proximal tubular NaCl and NaHCO3 reabsorption in dog kidneys

To examine whether an acute increase in plasma potassium concentration ([K]_p) may inhibit proximal tubular transport, clearance studies were performed in seven anaesthetized, volume expanded dogs treated with amiloride (1 mg kg^-1body wt) to block tubular potassium secretion, and with bumetanide (30 μg kg^-1body wt) to inhibit NaCl reabsorption in Henle's loop. As [K]_p was raised in steps from 2.6 ± 0.2 to 7.9 ± 0.2 mm , bicarbonate, chloride, and sodium reabsorption decreased by 232 ± 56, 520 ± 59 and 958 ± 112 μmol min^-1, respectively, at constant glomerular filtration rate (GFR). On average, the molar ratio between the inhibitory effects on bicarbonate and chloride reabsorption were 1:2.2–2.4. Reabsorption was calculated at GFR 100 ml min^-1: (reabsorption 100/GFR (mmol min^-1). It was inversely correlated to In [K]_p with r=–0.82 for bicarbonate and with r =–0.89 for chloride. Fractional potassium reabsorption remained constant at 0.31 ± 0.03. Administration of acetazolamide (100 mg kg^-1body wt) in eight dogs at [K]_p 8 mm reduced fractional reabsorption of bicarbonate, chloride and sodium as much as in previous studies on normokalaemic dogs. We conclude that acute elevation of [K]_p inhibits NaHCO₃ transport and passive proximal tubular NaCl reabsorption. This inhibition is not related to changes in potassium secretion and carbonic anhydrase activity, but may be secondary to depolarization of the basolateral membrane.     

Effects of dipyridamole on adenosine concentration,insulin sensitivity and glucose utilisation in soleus muscle of the rat

Adenosine has been shown to modulate the sensitivity of skeletal muscle to insulin (Budohoski et al. 1984). In an attempt to further characterize the modulatory action of adenosine on insulin sensitivity inskeletal muscle we have investigated the effect of the nucleoside transport inhibitor dipyridamole in isolated incubated soleus muscle strips. At a concentration of 50 M, dipyridamole increased the concentration of adenosine in the soleus muscle by 36% and in the incubation medium by 32%. At this concentration of dipyridamole, the basal rates (in the presence of 1 unit of insulin/ml) of lactate formation, 2-deoxy [2,6-³H]glucose phosphorylation and glucose oxidation were decreased by 48%, 43% and 47% respectively, whilst the rate of glycogen synthesis was unaffected. Insulin-stimulated rates (in the presence of 10000 unit of insulin/ml) of lactate formation, 2-deoxy [2,6-³H] glucose phosphorylation, glycogen synthesis and glucose oxidation were decreased by 70%, 30%, 26% and 20% respectively in the presence of 50 M dipyridamole. Although 50 M dipyridamole was required to exert a significant effect on medium and soleus muscle adenosine concentrations, statistically significant effects on glycolytic rate were observed at concentrations as low as 2 M dipyridamole.It is concluded that the results are not consistent with dipyridamole exerting an effect on skeletal muscle carbohydrate metabolism solely through elevation of the intracellular or interstial adenosine concentration, but strongly suggest that dipyridamole inhibits glucose transport and/or phosphorylation in skeletal muscle.     

Haemodynamic responses to dehydration in the resting and exercising human leg

Dehydration and hyperthermia reduces leg blood flow (LBF), cardiac output ( $ \dot{Q} $ ) and arterial pressure during whole-body exercise. It is unknown whether the reductions in blood flow are associated with dehydration-induced alterations in arterial blood oxygen content (C _aO₂) and O₂-dependent signalling. This study investigated the impact of dehydration and concomitant alterations in C _aO₂ upon LBF and $ \dot{Q} $ . Haemodynamics, arterial and femoral venous blood parameters and plasma [ATP] were measured at rest and during one-legged knee-extensor exercise in 7 males in four conditions: (1) control, (2) mild dehydration, (3) moderate dehydration, and (4) rehydration. Relative to control, C _aO₂ and LBF increased with dehydration at rest and during exercise (C _aO₂: from 199 ± 1 to 208 ± 2, and 202 ± 2 to 210 ± 2 ml L^?1 and LBF: from 0.38 ± 0.04 to 0.77 ± 0.09, and 1.64 ± 0.09 to 1.88 ± 0.1 L min^?1, respectively). Similarly, $ \dot{Q} $ was unchanged or increased with dehydration at rest and during exercise, whereas arterial and leg perfusion pressures declined. Following rehydration, C _aO₂ declined (to 193 ± 2 mL L^?1) but LBF remained elevated. Alterations in LBF were unrelated to C _aO₂ (r ² = 0.13–0.27, P = 0.48–0.64) and plasma [ATP]. These findings suggest dehydration and concomitant alterations in C _aO₂ do not compromise LBF despite reductions in plasma [ATP]. While an additive or synergistic effect cannot be excluded, reductions in LBF during exercise with dehydration may not necessarily be associated with alterations in C _aO₂ and/or intravascular [ATP].