Thyroid status and endothelium-dependent vasodilation in skeletal muscle

Cardiovascular dysfunction is characteristic of both hypo- and hyperthyroidism. Endothelium-dependent dilation of conductance vessels is impaired in hypothyroidism but augmented in hyperthyroidism. We hypothesized that these alterations in dilation extend into the resistance vasculature of skeletal muscle. To test this hypothesis, rats were made hypothyroid with propylthiouracil (Hypo; n = 13) or hyperthyroid with triiodothyronine (Hyper; n = 9) over 3-4 mo. Compared with euthyroid controls (Eut; n = 14), Hypo rats were characterized by reduced skeletal muscle oxidative capacity and blunted growth; Hyper rats exhibited increased muscle oxidative capacity and left ventricular hypertrophy (P < 0.05 for all effects). Vasodilation to the endothelium-dependent agent acetylcholine ( approximately 2 x 10(-4) M) in skeletal muscle was determined in situ. Conductance in certain muscles increased from control [e.g., soleus: 0.98 +/- 0.15 (Eut), 0.79 +/- 0.14 (Hypo), and 1.06 +/- 0.24 ml.min(-1).100 g(-1).mmHg(-1) (Hyper); not significant among groups] to acetylcholine [1.91 +/- 0.21 (Eut), 2.28 +/- 0.26 (Hypo), and 2.15 +/- 0.33 ml.min(-1).100 g(-1).mmHg(-1) (Hyper); P < 0.05 vs. control values for all groups] but did not differ among groups. Expression of mRNA for the endothelial isoform of nitric oxide synthase in resistance vessels isolated from various muscles was similarly unchanged with alterations in thyroid status [e.g., soleus 1A arterioles: 33.15 +/- 0.58 (Eut), 32.73 +/- 0.27 (Hypo), and 32.80 +/- 0.54 (Hyper) cycles at threshold; not significant]. These data suggest that endothelium-dependent dilation of resistance vasculature in skeletal muscle is unchanged in both hypo- and hyperthyroidism. These data also emphasize the importance of examining resistance vasculature to improve understanding of effects of chronic disease on integrated cardiovascular function.     

Thyroid status influences baroreflex function and autonomic contributions to arterial pressure and heart rate

The effect of thyroid status on arterial baroreflex function and autonomic contributions to resting blood pressure and heart rate (HR) were evaluated in conscious rats. Rats were rendered hyperthyroid (Hyper) or hypothyroid (Hypo) with triiodothyronine and propylthiouracil treatments, respectively. Euthyroid (Eut), Hyper, and Hypo rats were chronically instrumented to measure mean arterial pressure (MAP), HR, and lumbar sympathetic nerve activity (LSNA). Baroreflex function was evaluated with the use of a logistic function that relates LSNA or HR to MAP during infusion of phenylephrine and sodium nitroprusside. Contributions of the autonomic nervous system to resting MAP and HR were assessed by blocking autonomic outflow with trimethaphan. In Hypo rats, the arterial baroreflex curve for both LSNA and HR was shifted downward. Hypo animals exhibited blunted sympathoexcitatory and tachycardic responses to decreases in MAP. Furthermore, the data suggest that in Hypo rats, the sympathetic influence on HR was predominant and the autonomic contribution to resting MAP was greater than in Eut rats. In Hyper rats, arterial baroreflex function generally was similar to that in Eut rats. The autonomic contribution to resting MAP was not different between Hyper and Eut rats, but predominant parasympathetic influence on HR was exhibited in Hyper rats. The results demonstrate baroreflex control of LSNA and HR is attenuated in Hypo but not Hyper rats. Thyroid status alters the balance of sympathetic to parasympathetic tone in the heart, and the Hypo state increases the autonomic contributions to resting blood pressure.     

Differential sympathetic outflow and vasoconstriction responses at kidney and skeletal muscles during fictive locomotion

We compared sympathetic and circulatory responses between kidney and skeletal muscles during fictive locomotion evoked by electrical stimulation of the mesencephalic locomotor region (MLR) in decerebrate and paralyzed rats (n = 8). Stimulation of the MLR for 30 s at 40-microA current intensity significantly increased arterial pressure (+38 +/- 6 mmHg), triceps surae muscle blood flow (+17 +/- 3%), and both renal and lumbar sympathetic nerve activities (RSNA +113 +/- 16%, LSNA +31 +/- 7%). The stimulation also significantly decreased renal cortical blood flow (-18 +/- 6%) and both renal cortical and triceps surae muscle vascular conductances (RCVC -38 +/- 5%, TSMVC -17 +/- 3%). The sympathetic and vascular conductance changes were significantly dependent on current intensity for stimulation at 20, 30, and 40 microA. The changes in LSNA and TSMVC were significantly less than those in RSNA and RCVC, respectively, at all current intensities. At the early stage of stimulation (0-10 s), decreases in RCVC and TSMVC were significantly correlated with increases in RSNA and LSNA, respectively. These data demonstrate that fictive locomotion induces less vasoconstriction in skeletal muscles than in kidney because of less sympathetic activation. This suggests that a neural mechanism mediated by central command contributes to blood flow distribution by evoking differential sympathetic outflow during exercise.     

Endothelin-1-mediated vasoconstriction at rest and during dynamic exercise in healthy humans

It is now generally accepted that alpha-adrenoreceptor-mediated vasoconstriction is attenuated during exercise, but the efficacy of nonadrenergic vasoconstrictor pathways during exercise remains unclear. Thus, in eight young (23 +/- 1 yr), healthy volunteers, we contrasted changes in leg blood flow (ultrasound Doppler) before and during intra-arterial infusion of the alpha(1)-adrenoreceptor agonist phenylephrine (PE) with that of the nonadrenergic endothelin A (ET(A))/ET(B) receptor agonist ET-1. Heart rate, arterial blood pressure, common femoral artery diameter, and mean blood velocity were measured at rest and during knee-extensor exercise at 20%, 40%, and 60% of maximal work rate (WR(max)). Drug infusion rates were adjusted for blood flow to maintain comparable doses across all subjects and conditions. At rest, PE infusion (8 ng x ml(-1) x min(-1)) provoked a rapid and significant decrease in leg blood flow (-51 +/- 3%) within 2.5 min. Resting ET-1 infusion (40 pg x ml(-1) x min(-1)) significantly decreased leg blood flow within 5 min, reaching a maximal vasoconstriction (-34 +/- 3%) after 25-30 min of continuous infusion. Compared with rest, an exercise intensity-dependent attenuation to PE-mediated vasoconstriction was observed (-18 +/- 5%, -7 +/- 2%, and -1 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). Vasoconstriction in response to ET-1 was also blunted in an exercise intensity-dependent manner (-13 +/- 3%, -7 +/- 4%, and 2 +/- 3% change in leg blood flow at 20%, 40%, and 60% of WR(max), respectively). These findings support a significant contribution of ET-1 and alpha-adrenergic receptors in the regulation of skeletal muscle blood flow in the human leg at rest and suggest a similar, intensity-dependent "lysis" of peripheral ET and alpha-adrenergic vasoconstriction during dynamic exercise.     

Nonuniform effects of endurance exercise training on vasodilation in rat skeletal muscle.

Endurance exercise training (Ex) has been shown to increase maximal skeletal muscle blood flow. The purpose of this study was to test the hypothesis that increased endothelium-dependent vasodilation is associated with the Ex-induced increase in muscle blood flow. Furthermore, we hypothesized that enhanced endothelium-dependent dilation is confined to vessels in high-oxidative muscles that are recruited during Ex. To test these hypotheses, sedentary (Sed) and rats that underwent Ex (30 m/min x 10% grade, 60 min/day, 5 days/wk, 8-12 wk) were studied using three experimental approaches. Training effectiveness was evidenced by increased citrate synthase activity in soleus and vastus lateralis (red section) muscles (P < 0.05). Vasodilatory responses to the endothelium-dependent agent acetylcholine (ACh) in situ tended to be augmented by training in the red section of gastrocnemius muscle (RG; Sed: control, 0.69 +/- 0.12; ACh, 1.25 +/- 0.15; Ex: control, 0.86 +/- 0.17; ACh, 1.76 +/- 0.27 ml x min(-1) x 100 g(-1) x mmHg(-1); 0.05 < P < 0.10 for Ex vs. Sed during ACh). Responses to ACh in situ did not differ between Sed and Ex for either the soleus muscle or white section of gastrocnemius muscle (WG). Dilatory responses of second-order arterioles from the RG in vitro to flow (4-8 microl/min) and sodium nitroprusside (SNP; 10(-7) through 10(-4) M), but not ACh, were augmented in Ex (vs. Sed; P < 0.05). Dilatory responses to ACh, flow, and SNP of arterioles from soleus and WG muscles did not differ between Sed and Ex. Content of the endothelial isoform of nitric oxide synthase (eNOS) was increased in second-order, fourth-order, and fifth-order arterioles from the RG of Ex; eNOS content was similar between Sed and Ex in vessels from the soleus and WG muscles. These findings indicate that Ex induces endothelial adaptations in fast-twitch, oxidative, glycolytic skeletal muscle. These adaptations may contribute to enhanced skeletal muscle blood flow in endurance-trained individuals.     

Effects of arterial oxygen content on peripheral locomotor muscle fatigue.

The effect of arterial O2 content (Ca(O2)) on quadriceps fatigue was assessed in healthy, trained male athletes. On separate days, eight participants completed three constant-workload trials on a bicycle ergometer at fixed workloads (314 +/- 13 W). The first trial was performed while the subjects breathed a hypoxic gas mixture [inspired O2 fraction (Fi(O2)) = 0.15, Hb saturation = 81.6%, Ca(O2) = 18.2 ml O2/dl blood; Hypo] until exhaustion (4.5 +/- 0.4 min). The remaining two trials were randomized and time matched with Hypo. The second and third trials were performed while the subjects breathed a normoxic (Fi(O2) = 0.21, Hb saturation = 95.0%, Ca(O2) = 21.3 ml O2/dl blood; Norm) and a hyperoxic (Fi(O2) = 1.0, Hb saturation = 100%, Ca(O2) = 23.8 ml O2/dl blood; Hyper) gas mixture, respectively. Quadriceps muscle fatigue was assessed via magnetic femoral nerve stimulation (1-100 Hz) before and 2.5 min after exercise. Myoelectrical activity of the vastus lateralis was obtained from surface electrodes throughout exercise. Immediately after exercise, the mean force response across 1-100 Hz decreased from preexercise values (P < 0.01) by -26 +/- 2, -17 +/- 2, and -13 +/- 2% for Hypo, Norm, and Hyper, respectively; each of the decrements differed significantly (P < 0.05). Integrated electromyogram increased significantly throughout exercise (P < 0.01) by 23 +/- 3, 10 +/- 1, and 6 +/- 1% for Hypo, Norm, and Hyper, respectively; each of the increments differed significantly (P < 0.05). Mean power frequency fell more (P < 0.05) during Hypo (-15 +/- 2%); the difference between Norm (-7 +/- 1%) and Hyper (-6 +/- 1%) was not significant (P = 0.32). We conclude that deltaCa(O2) during strenuous systemic exercise at equal workloads and durations affects the rate of locomotor muscle fatigue development.     

Endothelin-1 influences the efficacy of inhaled nitric oxide in experimental acute lung injury

Beneficial effects of inhaled nitric oxide (iNO) on arterial oxygenation in acute lung injury (ALI) suggest the presence of vasoconstriction in ventilated lung regions and this may be influenced by endothelin-1 (ET-1). We studied a possible interaction between ET-1 and iNO in experimental ALI. Sixteen piglets were anesthetized and mechanically ventilated (inspired O2 fraction, 1.0). After induction of ALI by surfactant depletion, animals were randomly assigned to either inhale 30 ppm NO (iNO group, n = 8), or to receive no further intervention (controls, n = 8). Measurements were performed during the following 4 hrs. In all animals, induction of ALI significantly decreased arterial oxygen tension (PaO2) from 569 +/- 15 (prelavage) to 58 +/- 3 mm Hg. Inhaled NO significantly increased PaO2 when compared with controls (iNO group: 265 +/- 51 mm Hg; controls: 50 +/- 4 mm Hg, values at 4 hrs, P < 0.01). Prelavage ET-1 plasma levels were comparable between groups (iNO: 0.74 +/- 0.03, controls: 0.71 +/- 0.03 fmol/ml, NS). During the protocol, the ET-1 levels increased and were different at 3 hrs (iNO: 0.93 +/- 0.06, controls: 1.25 +/- 0.09 fmol/ml; P < 0.05). PaO2 changes induced by iNO revealed a moderate and significant correlation with ET-1 plasma levels (R = 0.548, P = 0.001). Our data suggest that endogenous ET-1 production influences the efficacy of iNO in ALI. Furthermore, iNO reduced ET-1 plasma levels, possibly indicating anti-inflammatory properties of iNO in the early phase of ALI.     

Effect of nephrectomy and of adrenergic receptor blockers on the cardiorenal actions of endothelin

The cardiorenal actions of endothelin-1 (ET-1) were evaluated in rats following nephrectomy, in rats during alpha-adrenergic blockade with phentolamine, and in rats during beta-adrenergic blockade with propranolol. Female rats were anesthetized with pentobarbital and, following surgery, were allowed 60 min to stabilize before 3 x 20 min-control clearances were collected. ET-1 was then infused at a rate of 100 ng kg-1 min-1 for 30 min, the infusion was stopped, and three additional clearances were collected. Four groups of rats were studied: in Group 1 (n = 10), ET-1 was infused; in Group 2 (n = 5), a bilateral nephrectomy was performed 120 min before infusing ET-1; in Group 3 (n = 5), ET-1 was infused into rats treated with phentolamine (0.015 mg kg-1 min-1); and in Group 4 (n = 5), ET-1 was infused into rats treated with propranolol (0.015 mg kg-1 min-1). At 30 min during infusion of ET-1 into Group 1 rats, mean arterial blood pressure had increased (P less than 0.01) by 27 +/- 2% (SE) and the glomerular filtration rate had decreased (P less than 0.01) by 71 +/- 6% of baseline values. Nephrectomy potentiated and prolonged the ET-1-induced systemic vasoconstriction. Phentolamine had no effect on the cardiorenal actions of ET-1 whereas propranolol enhanced ET-1-induced changes in mean arterial blood pressure; mean arterial blood pressure increased 38 +/- 2% at 30 min during ET-1 + propranolol infusion (P less than 0.01 versus value with ET-1 alone). These data indicate that the kidney affects ET-1-induced systemic vasoconstriction and that beta-adrenergic (but not alpha-adrenergic) receptors are activated during infusion of ET-1 with a resultant attenuation of ET-1-induced changes in systemic blood pressure.     

Insulin resistance does not impair contractile responses of cerebral arteries

Insulin resistance (IR) impairs endothelium-mediated vasodilation in cerebral arteries as well as K+ channel function in vascular smooth muscle. Peripheral arteries also show an impaired endothelium-dependent vasodilation in IR and concomitantly show an enhanced contractile response to endothelin-1 (ET-1). However, the contractile responses of the cerebral arteries in IR have not been examined systematically. This study examined the contractile responses of pressurized isolated middle cerebral arteries (MCAs) in fructose-fed IR and control rats. IR MCAs showed no difference in pressure-mediated (80 mmHg) vasoconstriction compared to controls, either in time to develop spontaneous tone (control: 61+/-3 min, n=30; IR: 63+/-2 min, n=26) or in the degree of that tone (control: 60 min: 33+/-2%, n=22 vs. IR 60 min: 34+/-3%, n=17). MCAs treated with ET-1 (10(-8.5) M) constrict similarly in control (53+/-3%, n=14) and IR (53+/-3%, n=14) arteries. Constrictor responses to U46619 (10(-6) M) are also similar in control (48+/-9%, n=8) and IR (42+/-5%, n=6) MCAs as are responses to extraluminal uridine 5'-triphosphate (UTP; 10(-4.5) M) (control: 35+/-7%, n=11 vs. IR: 38+/-3%, n=10). These findings demonstrate that constrictor responses remain intact in IR despite a selective impairment of dilator responses and endothelial and vascular smooth muscle K+ channel function in cerebral arteries. Thus, it appears that the increased susceptibility to cerebrovascular abnormalities associated with IR and diabetes (including cerebral ischemia, stroke, vertebrobasilar transient ischemic attacks) is not due to an enhanced vasoreactivity to constrictor agents.     

Exogenous NO administration and alpha-adrenergic vasoconstriction in human limbs.

Nitric oxide (NO) is capable of blunting alpha-adrenergic vasoconstriction in contracting skeletal muscles of experimental animals (functional sympatholysis). We therefore tested the hypothesis that exogenous NO administration can blunt alpha-adrenergic vasoconstriction in resting human limbs by measuring forearm blood flow (FBF; Doppler ultrasound) and blood pressure in eight healthy males during brachial artery infusions of three alpha-adrenergic constrictors (tyramine, which evokes endogenous norepinephrine release; phenylephrine, an alpha1-agonist; and clonidine, an alpha2-agonist). To simulate exercise hyperemia, the vasoconstriction caused by the alpha-agonists was compared during adenosine-mediated (>50% NO independent) and sodium nitroprusside-mediated (SNP; NO donor) vasodilation of the forearm. Both adenosine and SNP increased FBF from approximately 35-40 to approximately 200-250 ml/min. All three alpha-adrenergic constrictor drugs caused marked reductions in FBF and calculated forearm vascular conductance (P < 0.05). The relative reductions in forearm vascular conductance caused by the alpha-adrenergic constrictors during SNP infusion were similar (tyramine, -74 +/- 3 vs. -65 +/- 2%; clonidine, -44 +/- 6 vs. -44 +/- 6%; P > 0.05) or slightly greater (phenylephrine, -47 +/- 6 vs. -33 +/- 6%; P < 0.05) compared with the responses during adenosine. In conclusion, these results indicate that exogenous NO sufficient to raise blood flow to levels simulating those seen during exercise does not blunt alpha-adrenergic vasoconstriction in the resting human forearm.     

Endothelium-dependent vasodilation in different rat hindlimb skeletal muscles.

Few studies have examined potential for endothelium-dependent vasodilation in skeletal muscles of different fiber-type composition. We hypothesized that muscles composed of slow oxidative (SO)- and/or fast oxidative glycolytic (FOG)-type fibers have greater potential for endothelium-dependent vasodilation than muscles composed of fast glycolytic (FG)-type fibers. To test this hypothesis, the isolated perfused rat hindlimb preparation was used with a constant-flow, variable-pressure approach. Perfusion pressure was monitored continuously, and muscle-specific flows were determined by using radiolabeled microspheres at four time points: control, at peak effect of acetylcholine (ACh I; 1-2 x 10(-4) M), at peak effect of ACh after infusion of an endothelial inhibitor (ACh II), and at peak effect of sodium nitroprusside (SNP; 4-5 x 10(-4) M). Conductance was calculated by using pressure and flow data. In the SO-type soleus muscle, conductance increased with ACh and SNP, but the increase in conductance with ACh was partially abolished by the endothelial inhibitor N(G)-nitro-l-arginine methyl ester (control, 0.87 +/- 0.19; ACh I, 2.07 +/- 0.29; ACh II, 1.32 +/- 0.15; SNP, 1.76 +/- 0.19 ml. min(-1). 100 g(-1). mmHg(-1); P < 0.05, ACh I and SNP vs. control). In the FOG-type red gastrocnemius muscle, similar findings were obtained (control, 0.64 +/- 0.11; ACh I, 1.36 +/- 0.21; ACh II, 0.73 +/- 0.16; SNP, 1.30 +/- 0.21 ml. min(-1). 100 g(-1). mmHg; P < 0.05, ACh I and SNP vs. control). In the FG-type white gastrocnemius muscle, neither ACh nor SNP increased conductance. Similar findings were obtained when muscles were combined into high- and low-oxidative muscle groups. Indomethacin had no effect on responses to ACh. These data indicate that endothelium-dependent vasodilation is exhibited by high-oxidative, but not low-oxidative, rat skeletal muscle. Furthermore, endothelium-dependent vasodilation in high-oxidative muscle appears to be primarily mediated by nitric oxide.     

Coronary metabolic adaptation restricted by endothelin in the dog heart.

Endothelin elicits long-lasting vasoconstriction in the coronary bed. This remarkable spastic response raises the question whether or not the metabolic adaptive mechanisms of the coronaries are activated under endothelin effect. The role of the compensatory mediators adenosine and inosine was investigated before and after intracoronary (i.c.) administration of endothelin-1 (ET-1, 1.0 nmol) using 1-min reactive hyperemia (RH) tests on in situ dog hearts (n=15) with or without blocking the ATP-sensitive potassium (K+(ATP)) channels by glibenclamide (GLIB, 1.0 micromol min(-1), i.c.). The release of adenosine and inosine via the coronary sinus was measured by HPLC during the first minute of RH. Endothelin-1 reduced baseline coronary blood flow (CBF) and RH response (hyperemic excess flow (EF) control vs. ET-1: 81.7+/-13.6 vs. 43.4+/-10.9 ml, P<0.01), while it increased the net nucleoside release (adenosine, control vs. ET-1: 58.9+/-20.4 vs. 113.7+/-39.4 nmol, P<0.05; inosine: 242.1+/-81.8 vs. 786.9+/-190.8 nmol, P<0.05). GLIB treatment alone did not change baseline CBF but also reduced RH significantly and increased nucleoside release (EF control vs. GLIB: 72.1+/-11.7 vs. 31.9+/-5.5 ml, P<0.01; adenosine: 18.8+/-4.6 vs. 63.0+/-24.8 nmol, P<0.05; inosine: 113.0+/-37.2 vs. 328.2+/-127.5 nmol, P<0.05). Endothelin-1 on GLIB-treated coronaries further diminished RH and increased nucleoside release (EF: 21.5+/-8.0 ml, P<0.05 vs. GLIB; adenosine: 75.3+/-28.1 nmol, NS; inosine: 801.9+/-196.6 nmol, P<0.05 vs. GLIB). The data show that ET-1 reduces metabolic adaptive capacity of the coronaries, and this phenomenon is due to decreased vascular responsiveness and not to the blockade of ischemic mediator release from the myocardium. The coronary effect of ET-1 may partially be dependent on K+(ATP) channels.     

Cold-induced changes in thyroid function in a poikilothermic mammal, the naked mole-rat

Cold acclimation induces very divergent responses in thyroid function in reptiles and mammals reflective of their different thermoregulatory modes. Naked mole-rats, unlike other small mammals, are unable to effectively employ endothermy and are operatively poikilotherms. We therefore investigated changes in their thyroid status with chronic cold exposure. Under simulated burrow conditions, free thyroxine (T(4); 0.39 +/- 0.09 ng/dl) and thyroid stimulating hormone (TSH; 1.12 +/- 0.56 microIU/ml) levels fell within the reptilian range, one order of magnitude lower than mammalian levels. However, cold induced typical mammalian responses: free T(4) levels (0.55 +/- 0.09 ng/dl) and thyroid follicular cell height were significantly greater. Although TSH levels (1.28 +/- 0.83 microIU/ml) were not significantly elevated, thyrotrophs exhibited ultrastructural signs of increased secretory activity. Low thyroid hormone concentrations may contribute substantially to the unusual thermoregulatory mode exhibited by naked mole-rats.     

Enhanced endothelin-1 system activity with overweight and obesity

Endothelin (ET)-1-mediated vasoconstrictor tone contributes to the development and progression of several adiposity-related conditions, including hypertension and atherosclerotic vascular disease. The aims of the present study were to determine 1) whether endogenous ET-1 vasoconstrictor activity is elevated in overweight and obese adults, and, if so, 2) whether increased ET-1-mediated vasoconstriction contributes to the adiposity-related impairment in endothelium-dependent vasodilation. Seventy-nine adults were studied: 34 normal weight [body mass index (BMI) < 25 kg/m(2)], 22 overweight (BMI ≥ 25 and < 30 kg/m(2)), and 23 obese (BMI ≥ 30 kg/m(2)). Forearm blood flow (FBF) responses to intra-arterial infusion of ET-1 (5 pmol/min for 20 min) and selective ET-1 receptor blockade (BQ-123, 100 nmol/min for 60 min) were determined. In a subset of the study population, FBF responses to ACh (4.0, 8.0, and 16.0 μg·100 ml tissue(-1)·min(-1)) were measured in the absence and presence of selective ET-1 receptor blockade. The vasoconstrictor response to ET-1 was significantly blunted in overweight and obese adults (～ 70%) compared with normal weight adults. Selective ET-1 receptor blockade elicited a significant vasodilator response (～ 20%) in overweight and obese adults but did not alter FBF in normal weight adults. Coinfusion of BQ-123 did not affect FBF responses to ACh in normal weight adults but resulted in an ～ 20% increase (P < 0.05) in ACh-induced vasodilation in overweight and obese adults. These results demonstrate that overweight and obesity are associated with enhanced ET-1-mediated vasoconstriction that contributes to endothelial vasodilator dysfunction and may play a role in the increased prevalence of hypertension with increased adiposity.     

Do P2X purinergic receptors regulate skeletal muscle blood flow during exercise?

Although there is evidence that sympathetic nerves release ATP as a neurotransmitter to produce vasoconstriction via P2X purinergic receptors, the role of these receptors in the regulation of blood flow to exercising skeletal muscle has yet to be determined. We hypothesized that there is tonic P2X receptor-mediated vasoconstriction in exercising skeletal muscle. To test this hypothesis, the effect of P2X receptor blockade on skeletal muscle blood flow was examined in six exercising mongrel dogs. P2X receptor antagonism was accomplished with pyridoxal-phosphate-6-azophenyl-2'4'-disulfonic acid (PPADs). Animals were instrumented chronically with flow probes on the external iliac arteries of both hindlimbs and a catheter in one femoral artery. PPADs (40 mg) was infused as a bolus into the femoral artery catheter during steady-state exercise at 6 miles/h. Intra-arterial infusion of PPADs increased iliac blood flow from 542 +/- 55 to 677 +/- 69 ml/min (P < 0.05) and iliac vascular conductance from 5.17 +/- 0.62 to 6.53 +/- 0.80 ml.min(-1).mmHg(-1). The PPADs infusion did not affect blood flow in the contralateral iliac artery. These data support the hypothesis that P2X purinergic receptors produce vasoconstriction in exercising skeletal muscle.     

Sympathetic vascular transduction is augmented in young normotensive blacks.

The purpose of the present study was to determine sympathetic vascular transduction in young normotensive black and white adults. We hypothesized that blacks would demonstrate augmented transduction of muscle sympathetic nerve activity (MSNA) into vascular resistance. To test this hypothesis, MSNA, forearm blood flow, heart rate, and arterial blood pressure were measured during lower body negative pressure (LBNP). At rest, no differences existed in arterial blood pressure, heart rate, forearm blood flow, and forearm vascular resistance (FVR). Likewise, LBNP elicited comparable responses of these variables for blacks and whites. Baseline MSNA did not differ between blacks and whites, but whites demonstrated greater increases during LBNP (28 +/- 7 vs. 55 +/- 18%, 81 +/- 21 vs. 137 +/- 42%, 174 +/- 81 vs. 556 +/- 98% for -5, -15, and -40 mmHg LBNP, respectively; P < 0.001). Consistent with smaller increases in MSNA but similar FVR responses during LBNP, blacks demonstrated greater sympathetic vascular transduction (%FVR/%MSNA) than whites (0.95 +/- 0.07 vs. 0.82 +/- 0.07 U; 0.82 +/- 0.11 vs. 0.64 +/- 0.09 U; 0.95 +/- 0.37 vs. 0.35 +/- 0.09 U; P < 0.01). In summary, young whites demonstrate greater increases in MSNA during baroreceptor unloading than age-matched normotensive blacks. However, more importantly, for a given increase in MSNA, blacks demonstrate greater forearm vasoconstriction than whites. This finding may contribute to augmented blood pressure reactivity in blacks.     

Net lactate uptake during progressive steady-level contractions in canine skeletal muscle

The purpose of this study was to determine the changes in net lactate uptake (L) by skeletal muscle with a constant elevated blood lactate concentration during steady-level contractions of increasing intensity. The gastrocnemius-plantaris muscle group was isolated in situ in 11 anesthetized dogs. An infusion of lactate/lactic acid at a pH of 3.5-3.7 established a blood lactate concentration of approximately 9 mM while maintaining normal blood gas/pH status. L was measured during three consecutive 30-min periods during which the muscles 1) rested, 2) contracted at 1 Hz, and 3) contracted at 4 Hz. L was always positive, indicating net uptake throughout the lactate/lactic acid infusion. Steady-level O2 uptake averaged 10.9 +/- 2.2 ml.kg-1.min-1 (0.49 +/- 0.10 mmol.kg-1.min-1) at rest, 39.3 +/- 2.1 (1.75 +/- 0.09) at 1 Hz, and 127.8 +/- 9.2 (5.70 +/- 0.41) at 4 Hz. Steady-level L increased with the metabolic rate from 0.113 +/- 0.058 mmol.kg-1.min-1 at rest to 0.329 +/- 0.026 at 1 Hz and 0.715 +/- 0.108 at 4 Hz. The increase in L from rest to 1 Hz was accomplished mainly by an increase in arteriovenous lactate difference, whereas the increase from 1 to 4 Hz was entirely due to a large increase in blood flow. These results support the idea that skeletal muscle is not simply a producer of lactate but can be a significant consumer of lactate even during contractions with a large elevation in metabolic rate.     

Distribution of blood flow in muscles of miniature swine during exercise

The purpose of this study was to determine how the distribution of blood flow within and among the skeletal muscles of miniature swine (22 +/- 1 kg body wt) varies as a function of treadmill speed. Radiolabeled microspheres were used to measure cardiac output (Q) and tissue blood flows in preexercise and at 3-5 min of treadmill exercise at 4.8, 8.0, 11.3, 14.5, and 17.7 km/h. All pigs (n = 8) attained maximal O2 consumption (VO2max) (60 +/- 4 ml X min-1 X kg-1) by the time they ran at 17.7 km/h. At VO2max, 87% of Q (9.9 +/- 0.5 l/min) was to skeletal muscle, which constituted 36 +/- 1% of body mass. Average total muscle blood flow at VO2max was 127 +/- 14 ml X min-1 X 100 g-1; average limb muscle flow was 135 +/- 17 ml X min-1 X 100 g-1. Within the limb muscles, blood flow was distributed so that the deep red parts of extensor muscles had flows about two times higher than the more superficial white portions of the same muscles; the highest muscle blood flows occurred in the elbow flexors (brachialis: 290 +/- 44 ml X min-1 X 100 g-1). Peak exercise blood flows in the limb muscles were proportional (P less than 0.05) to the succinate dehydrogenase activities (r = 0.84), capillary densities (r = 0.78), and populations of oxidative (slow-twitch oxidative + fast-twitch oxidative-glycolytic) fiber types (r = 0.93) in the muscles. Total muscle blood flow plotted as a function of exercise intensity did not peak until the pigs attained VO2max, although flows in some individual muscles showed a plateau in this relationship at submaximal exercise intensities. The data demonstrate that blood flow in skeletal muscles of miniature swine is distributed heterogeneously and varies in relation to fiber type composition and exercise intensity.     

Comparison of vasodilator potency of adrenomedulling and proadrenomedullin N-terminal 20 peptide in human

Adrenomedullin (ADM) and proadrenomedullin N-terminal peptide (PAMP), both of which are derived from preproadrenomedullin, are reported to have a potent hypotensive effect in animals. However, no data are available concerning the vasodilatory potency of PAMP or comparing this potency to that of ADM in human vasculature. We examined the effects of intra-arterial infusion of graded doses of ADM (1.25, 2.5, 5.0 and 7.5 pmol/min per 100 ml of tissue) and PAMP (125, 250, 500, 750 and 1000 pmol/min per 100 ml of tissue) on total forearm blood flow and forearm skin blood flow in 11 healthy subjects. ADM increased total forearm blood flow from 2.9 +/- 0.4 to 8.6 +/- 1.1 ml/min per 100 ml (p < 0.01), and skin blood flow from 0.07 +/- 0.02 to 0.14 +/- 0.03 volts (p < 0.01). In contrast to this potent vasodilatory effect, a significant rise in forearm skeletal blood flow was seen only in response to the maximum dose of PAMP (from 2.7 +/- 0.5 to 5.3 +/- 1.0 ml/min per 100 ml; p < 0.01). In addition, PAMP had no significant vasoactive effect on skin blood flow (from 0.06 +/- 0.02 to 0.09 +/- 0.03 volts; NS). In conclusion, the skeletal muscle vasodilator potency of PAMP is less than one hundredth of that of ADM in human forearm. Given its weak dilator potency, it seems unlikely that PAMP alone could significantly regulate resistance vessel tone as a circulating hormone in humans.     

Impaired endothelium-mediated vasodilation is not the principal cause of vasoconstriction in heart failure

The extent to which abnormal endothelium-dependent vasodilator mechanisms contribute to abnormal resting vasoconstriction and blunted reflex vasodilation seen in heart failure is unknown. The purpose of this study was to test the hypothesis that the resting and reflex abnormalities in vascular tone that characterize heart failure are mediated by abnormal endothelium-mediated mechanisms. Thirteen advanced heart-failure patients (New York Heart Association III-IV) and 13 age-matched normal controls were studied. Saline, acetylcholine (20 microg/min), or L-arginine (10 mg/min) was infused into the brachial artery, and forearm blood flow was measured by venous plethysmography at rest and during mental stress. At rest, acetylcholine decreased forearm vascular resistance in normal subjects, but this response was blunted in heart failure. During mental stress with intra-arterial acetylcholine or L-arginine, the decrease in forearm vascular resistance was not greater than during saline control in heart failure [saline control vs. acetylcholine (7 +/- 3 vs. 6 +/- 3, P = NS) or vs. L-arginine (9 +/- 2 units, P = NS)]. The increase in forearm blood flow was not greater than during saline control in heart failure [saline control vs. acetylcholine (1. 2 +/- 0.3 vs. 1.3 +/- 0.3, P = NS), or vs. L-arginine (1.2 +/- 0.2 ml x min(-1) x 100 ml(-1), P = NS)]. Furthermore, during mental stress with nitroprusside, the decrease in forearm vascular resistance was not greater than during saline control [saline control vs. nitroprusside (7 +/- 3 vs. 5 +/- 4 ml x min(-1) x 100 g(-1), P = NS)], and the increase in forearm blood flow was not greater than during saline control [saline control vs. nitroprusside (1.2 +/- 0.3 vs. 1.3 +/- 0.5 ml x min(-1) x 100 g(-1), P = NS)]. Because the endothelial-independent agent nitroprusside was unable to restore resting and reflex vasodilation to normal in heart failure, we conclude that impaired endothelium-mediated vasodilation with acetylholine-nitric oxide cannot be the principal cause of the attenuated resting- or reflex-mediated vasodilation in heart failure.