Nonshivering thermogenesis in a marsupial (the tasmanian bettong Bettongia gaimardi) is not attributable to brown adipose tissue

The Tasmanian bettong (Bettongia gaimardi, a marsupial) is a rat-kangaroo that increases nonshivering thermogenesis (NST) in response to norepinephrine (NE). This study attempted to assess whether brown adipose tissue (BAT), a specialized thermogenic effector, is involved in NST in the bettong. Regulatory NST, indicated by resting oxygen consumption (Vo2) of the whole body, was measured under conscious conditions at 20 degrees C with various stimuli: cold (4 degrees -5 degrees C) or warm (25 degrees C) acclimation, NE injection, and the beta3-adrenoceptor agonist (BRL) 37344. In line with the functional studies in vivo, the presence of BAT was evaluated by examining the expression of the uncoupling protein 1 (UCP1) with both rat cDNA and oligonucleotide probes. Both NE and BRL 37344 significantly stimulated NST in the bettong. After cold acclimation of the animals (at 4 degrees -5 degrees C for 2 wk), the resting Vo2 was increased by 15% and the thermogenic effect of NE was enhanced; warm-acclimated animals showed a slightly depressed response. However, no expression of UCP1 was detected in bettongs either before or after cold exposure (2 wk). These data suggest that the observed NST in the marsupial bettong is not attributable to BAT.

Altered muscle metabolism associated with vasoconstriction in spontaneously hypertensive rats

In the rat muscle vascular bed, vasoconstrictors either increase or decrease oxygen consumption (VO2). The present study compared the effects of norepinephrine (NE), angiotensin II (ANG II), and 5-hydroxytryptamine (5-HT) on vasoconstriction-associated metabolism in the constant-flow perfused hindlimb of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) in the absence of insulin. Basal perfusion pressure, VO2, glucose uptake, and lactate production were increased by 21.4, 11.9, 46.4, and 44.9% (P < 0.05 for all), respectively, in SHR, which also had higher blood pressure and metabolic rate (P < 0.05) in vivo. Dose-response curves for NE-induced perfusion pressure, VO2, and lactate production in SHR were shifted to the left compared with WKY. Associated with the increased perfusion pressure, NE-induced VO2 and glucose uptake were both decreased (P < 0.01), particularly at high concentrations. These differences were unaffected by 10 microM propranolol but were all diminished by further addition of prazosin (2.5 nM). ANG II stimulated VO2, glucose uptake, and lactate production in both strains, but the increased lactate production was smaller in SHR (P < 0.05) with a proportional decrease (P < 0.05) in glucose uptake. Conversely, 5-HT decreased VO2 in both strains (P < 0.01), and this effect was greater in SHR (P < 0.01). These data suggest that SHR muscle thermogenesis and glucose uptake are impaired during vasoconstriction, especially in response to NE.

Acute and chronic effects of capsaicin in perfused rat muscle: the role of tachykinins and calcitonin gene-related peptide

In perfused rat skeletal muscle (hindlimb), capsaicin either stimulates (submicromolar concentrations) or inhibits (micromolar concentrations) oxygen consumption (VO2). Both VO2 effects are associated with vasoconstriction, evident as an increase in perfusion pressure (PP), under constant flow. We have proposed that these effects are mediated by two vanilloid receptor subtypes: VN1 (stimulation of VO2) and VN2 (inhibition of VO2) (; ). In the present study, the role of capsaicin-sensitive neurons and sensory neuropeptides in the VN1/VN2 receptor actions of capsaicin was investigated. The observed maximum stimulation of VO2 by capsaicin (0.4 microM; DeltaVO2, 1.35 +/- 0.14 micromol g-1 h-1) was accompanied by mild vasoconstriction (DeltaPP, 5.8 +/- 0.6 mm Hg). In contrast, 2 microM capsaicin produced strong inhibition of VO2 (DeltaVO2, -2.25 +/- 0.23 micromol g-1 h-1) with pronounced vasoconstriction (DeltaPP, 28.0 +/- 1.3 mm Hg). VO2 stimulation was significantly inhibited (P <.05) by the selective NK1 receptor antagonist CP-99994 (1 microM) and the NK2 receptor antagonist SR 48968 (1 microM) (by 42% and 51%, respectively), but PP was not altered. Infused SP and neurokinin A (NKA) stimulated VO2 (observed maximum DeltaVO2, 0.52 +/- 0.06 and 0.53 +/- 0.08 micromol g-1 h-1, respectively; EC50 values, 269 +/- 23 and 21.2 +/- 3.0 nM, respectively) and induced mild vasoconstriction (4.30 +/- 0.33 and 6. 75 +/- 1.18 mm Hg, respectively; EC50 values, 352 +/- 25.7 and 25.5 +/- 2.7 nM, respectively). Neurokinin B (NKB) also stimulated VO2 (maximum not determined) and vasoconstriction (maximum DeltaPP, 3.40 +/- 0.25 mm Hg; EC50, 34.4 +/- 5.2 nM). The rank order of potency for the tachykinins in this preparation was NKA > NKB > SP, which suggests stimulation primarily of NK2 receptors. Although infused calcitonin gene-related peptide (CGRP) did not alter hindlimb VO2 or PP, the selective CGRP antagonist CGRP(8-37) markedly potentiated the inhibition of VO2 produced by 1 microM capsaicin (84%) and the maximum capsaicin-induced vasoconstriction (57%), which indicates that endogenously released CGRP may act as a vasodilator. Hindlimbs perfused 1 day after capsaicin pretreatment showed attenuation of capsaicin-induced (0.4 microM) stimulation of VO2 (92%) (P <.05) and vasoconstriction (64%), but this returned to normal after 7 days. The inhibition of VO2 by 1 microM capsaicin was significantly (P <. 05) enhanced 7 and 14 days after pretreatment (66% and 140%, respectively), as was the maximum vasoconstriction (64% and 68%, respectively). These data suggest that capsaicin-sensitive neurons, presumably via release of SP and NKA, are involved in VN1 responses and that capsaicin pretreatment potentiates VN2 responses, either by depletion of CGRP reserves or by upregulation of putative VN2 receptors.

Changes in functional expression of alpha-1 adrenoceptors in hindlimb vascular bed of spontaneously hypertensive rats and their effects on oxygen consumption

Norepinephrine (NE) induces a sigmoidal dose-response curve for perfusion pressure and a bell-shaped curve for oxygen consumption (VO2) in the constant-flow perfused hindlimb of Wistar rats. These effects are now described in spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY). In SHR, the pressure curve was shifted left- and upward whereas the VO2 curve was shifted left- but downward, when compared with WKY. In the presence of 10 microM propranolol, prazosin (2.5 nM) shifted the pressure and VO2 curves much more than yohimbine (0.1 microM) to the right in both strains and its effects were greater in SHR, suggesting that these effects were mediated largely by alpha-1 receptors, particularly in SHR. In the presence of propranolol plus yohimbine, the pressure curve was markedly shifted to the right by both the selective alpha-1A-antagonist 5-methylurapidil (3.3 nM), and by the alpha-1D antagonist BMY 7378 (0.1 microM) or SK&F 105854 (2 microM) in SHR but not in WKY. With respect to the VO2 curve, 5-methylurapidil attenuated the descending limb without affecting the ascending limb. Similar effects were also obtained with another alpha-1A antagonist 1 nM KMD-3213 in both SHR and WKY. In contrast, BMY and SK&F markedly inhibited the ascending limb of the VO2 curve. These results indicate that both alpha-1A- and alpha-1D subtypes are functionally up-regulated in SHR muscle vascular bed where the ascending limb of VO2 is predominantly mediated by the alpha-1D at a much lower concentration for NE than the descending limb which is predominantly mediated by the alpha-1A subtype.

Development of endothermy in a Tasmanian marsupial, Bettongia gaimardi and its response to cold and noradrenaline

Marsupials at birth are ectothermic and gradually attain the ability to change their metabolic heat production during pouch life. How this process occurs in the bettong has been measured on 13 pouch young from week 1 until 3 weeks after pouch vacation (week 18). Oxygen consumption was measured at 35 degrees C (pouch temperature) and at 22 degrees C. The results at 35 degrees C showed an increase in metabolic rate from week 1 until week 12 when there was a decrease to near adult levels after pouch vacation. At 22 degrees C young bettongs had a lower metabolic rate (compared with measurements made at 35 degrees C) until week 9 after which there was an increase above measurements made at 35 degrees C. Noradrenaline had little effect until week 10 after which the metabolic rate (although measured at 35 degrees C) paralleled the levels measured at 22 degrees C. The free thyroxine level was low in early pouch life, increased to a peak at week 12 then decreased. Thermal conductance increased until week 10 after which it decreased, reaching values similar to those of adult bettongs by week 20. The results indicate that non-shivering thermogenesis occurs in this macropodoid marsupial. This phenomenon may be a phylogenetic difference between macropodid and non-macropodid marsupials as also suggested by Nicol et al. (1997).

Effects of noradrenaline and flow on lactate uptake in the perfused rat hindlimb

Skeletal muscle can release or take up lactate depending on the lactate concentration gradient across the cell membrane. In the perfused rat hindlimb without arterial lactate, both noradrenaline (NA) infusion and increased flow promote lactate release and oxygen consumption (VO2). However, it is unclear whether NA or increased flow rate have similar effects on lactate uptake. The present study compares these effects in the rat hindlimb perfused at a basal flow rate of 0.33 mL min-1 g-1 and 25 degrees C in the presence of added arterial lactate. When 10 mmol L-1 L-(+)-lactate was added to the arterial perfusate, lactate was taken up (16 +/- 1.0 mumol g-1 h-1, n = 13) by the hindlimb with a 35% higher VO2 than that without added lactate. Doubling perfusion flow rate enhanced lactate uptake and VO2 by 120% and 40%, respectively. Glucose uptake was also increased (by 253%) with increased flow. Infusion of NA increased perfusion pressure, VO2 and glucose uptake similarly to those induced by increased flow rate. However, lactate uptake was inhibited by NA. This inhibition was not altered by the beta-adrenergic antagonist propranolol. Vasopressin also showed similar effects to NA to decrease lactate uptake associated with increased VO2 and vasoconstriction. These data indicate that in the presence of a high arterial lactate concentration, NA has opposite effects from increased flow rate on skeletal muscle lactate uptake although both have similar effects on lactate release in the absence of arterial lactate. Inhibition of lactate uptake may relate to the vasoconstrictive action of NA.

Serotonin inhibition of 1-methylxanthine metabolism parallels its vasoconstrictor activity and inhibition of oxygen uptake in perfused rat hindlimb

The effect of serotonin (5-HT) on the metabolism of infused 1-methylxanthine (1-MX), a putative substrate of capillary endothelial xanthine oxidase (XO), and on the distribution of infused fluorescent microspheres (15 microns) by the artificially constant-flow perfused rat hindlimb preparation was investigated. 1-MX (5-100 microM) caused a slight inhibition of oxygen uptake (Vo2) but was not vasoactive, either alone or with 5-HT. 1-MX was converted to 1-methylurate (1-MU) and this conversion was inhibited by allopurinol and xanthine. 5-HT (0.35 microM), which caused vasoconstriction and decreased Vo2, also inhibited the conversion of 1-MX, indicated by a lowered venous perfusate steady-state 1-MU:1-MX ratio from 1.14 +/- 0.02 to 0.71 +/- 0.02 (P < 0.001), which is equivalent to the rate of conversion decreasing from 0.83 +/- 0.03 to 0.63 +/- 0.05 nmol min-1 g-1. This change closely followed the time course for changes in Vo2 and perfusion pressure and all three changes reversed in parallel when 5-HT was removed. Recoveries of 1-MU plus 1-MX at all times were high (100 +/- 5%). 5-HT did not act to inhibit XO. When compared with vehicle alone, 5-HT had either no effect (plantairs, gastrocnemius white, tibialis, extensor digitorum longus, vastus and thigh), or increased microsphere content (soleus and gastrocnemius red, P < 0.05) of muscles with only bone showing a significant decrease (P < 0.05). Since 5-HT did not inhibit XO or alter the net flow to individual muscles in this constant-flow model, the inhibition of conversion of 1-MX to 1-MU is concluded to be the result of a 5-HT-mediated decrease in the access of 1-MX to capillary XO within individual muscles. Possibilities include the redirection of flow to capillaries either in muscle or in connective tissue closely associated with muscle, where resistance is low and effective surface area is less. 1-MX has potential as a marker for muscle nutritive flow.

Potential for non-shivering thermogenesis in perfused chicken (Gallus domesticus) muscle

The humoral modulation of resting muscle heat production of chickens (Gallus domesticus) was investigated in vitro. The resting distal lower limb was perfused via the popliteal artery at 25 degrees C without erythrocytes at constant flow. The preparation was stable for at least 3 hr, showing a constant oxygen uptake (MO2) and perfusion pressure as well as adequately maintaining muscle energy charge and creatine phosphate: creatine ratio. Noradrenaline (NOR), adrenaline (ADR) and serotonin (5-HT) each caused a dose-dependent rise in perfusion pressure. NOR and ADR evoked increased MO2 at low doses eventually followed by decreased MO2 at higher agonist concentrations. 5-HT gave smaller but qualitatively similar MO2 effects. The actions of 50 nM NOR were blocked by prazosin (10 microM) and nitroprusside (0.5 mM), but not altered by propranolol (10 microM). NOR-induced stimulatory MO2 changes in the presence of pharmacological concentrations (1 microM) of glucagon were more pronounced and the thermogenic concentration range of NOR was increased. Taken together, these in vitro findings demonstrate a potential for vasoconstrictor-controlled muscle nonshivering thermogenesis in birds as in marsupials and mammals, suggesting that vascular control of muscle MO2 may be a widespread biological mechanism. The possible implications of these findings for avian nonshivering thermogenesis are discussed.

Sympathetic stimulation elicits increased or decreased VO2 in the perfused rat hindlimb via alpha 1-adrenoceptors

The effects of lumbar sympathetic nerve stimulation on oxygen uptake (VO2) in curarized muscle of the perfused rat hindlimb were investigated. Stimulation of sympathetic nerves elicited vasoconstriction at all frequencies. Importantly, this was associated with changes in VO2 that were generally stimulatory at low frequencies (0.5-2 Hz) and inhibitory at high frequencies (5-10 Hz). Both the pressor response and the changes in VO2 were almost completely blocked by the alpha 1/alpha 2-blocker phentolamine (1.0 microM) but were not affected by the beta 1/beta 2-blocker DL-propranolol (2.0 microM). The alpha 2-blocker yohimbine (0.1 microM) did not significantly affect either the pressor or VO2 response. The alpha 1-antagonist prazosin (0.1 microM) abolished the vasoconstriction with low-frequency stimulation and inhibited > 90% of the vasoconstriction with high-frequency stimulation. Intra-arterial infusion of phenylephrine (alpha 1-agonist), but not of UK-14304 (alpha 2-agonist), also elicited a similar biphasic response in VO2 during vasoconstriction. The changes in VO2 at both low- and high-frequency stimulation were fully reversed by prazosin. The vasodilator sodium nitroprusside also showed similar effects to prazosin in blocking both VO2 and vasoconstriction. Thus sympathetic control of VO2 in the perfused rat hindlimb appears to be initiated by activation of predominantly vascular alpha 1-adrenoceptors.

Alpha-adrenergic stimulation of thermogenesis in a rat kangaroo (Marsupialia, Bettongia gaimardi)

The Tasmanian bettong (Bettongia gaimardi) is a small rat kangaroo without detectable brown adipose tissue (BAT). In view of our previous findings of norepinephrine-mediated increase in O2 consumption (Vo2) in the perfused hindlimb of this species, the present study examined the effect of alpha-adrenoceptors on the thermogenesis of conscious bettongs at rest by infusing adrenergic agents via an indwelling catheter in the tail vein. The resting Vo2 was 22.9 +/- 1.9 mmol.kg-1.h-1. Norepinephrine (10-80 micrograms.kg-1.min-1) stimulated Vo2 in a dose-dependent manner with the maximal increment of 46.7%. Naphazoline (an alpha 1,alpha 2-adrenergic agonist) and phenylephrine (an alpha 1-adrenergic agonist) also elicited increases in Vo2 with maximal values of 29.6 and 34.8%, respectively. In contrast, the alpha 2-adrenergic agonist clonidine had no significant effects. Both alpha- and beta-adrenergic blockers were used to antagonize the submaximal increase in Vo2 elicited by norepinephrine. As a dose of 10 micrograms.kg-1.min-1, the alpha-adrenergic blocker phentolamine abolished the effects of naphazoline and phenylephrine and reduced norepinephrine-induced Vo2 by 45.5%. The beta-adrenergic blocker propranolol inhibited the norepinephrine-induced Vo2 by 58.8% at 20 micrograms.kg-1.min-1. A combination of the two antagonists blocked 82.5% of the norepinephrine-induced Vo2. Pretreatment of the animal with indomethacin (1 mg/kg), a known inhibitor of prostaglandin cyclooxygenase, had no effect on phenylephrine-elicited Vo2. Taken together, these results indicate that alpha 1-adrenoceptors are directly involved in norepinephrine-induced thermogenesis in non-BAT tissue(s).

Purine and pyrimidine nucleotide metabolism of vascular smooth muscle cells in culture

1. Cultures of vascular smooth muscle cells accumulate extracellular breakdown products of purine and pyrimidine nucleotides that, over 9 hr, represent 60 +/- 7 and 78 +/- 17%, respectively, of the intracellular nucleotide content. 2. The accumulation is stimulated during contracture with 20 mM KCl or 70 microM carbachol, consistent with the notion that both pyrimidine and purine nucleotides are involved in the energetics of smooth muscle contracture. 3. Because the intracellular levels of pyrimidine and purine nucleotides remain constant, it appears likely that rates of synthesis match the rates of release. 4. Ectonucleotidases are present that can degrade ATP, UTP, and CTP. High-energy nucleotides may be the primary products released.

Capsaicin-induced biphasic oxygen uptake in rat muscle: antagonism by capsazepine and ruthenium red provides further evidence for peripheral vanilloid receptor subtypes (VN1/VN2)

Previous studies with the vanilloid spice principle capsaicin have demonstrated a biphasic VO2 response, with vasoconstriction, in the perfused rat hindlimb that has led to suggestions of vanilloid receptor subtypes (VN1/VN2) in this preparation (1). In the present study, the known competitive vanilloid antagonist capsazepine inhibited the above capsaicin-mediated effects in a manner that was indicative of binding at specific vanilloid recognition sites. Low concentrations of capsazepine selectively inhibited the increased VO2 produced by the putative VN1 receptor at submicromolar concentrations of capsaicin, while the inhibition of VO2 produced by high concentrations of capsaicin (putative VN2) was enhanced. These observations, showing different susceptibilities to blockade by capsazepine, further support the presence of two vanilloid receptor subtypes in the rat hindlimb. Schild plots of the data yielded variable slopes that approach unity at greater responses to capsaicin (mean KB = 8.44 +/- 2.08 microM and 7.28 +/- 0.78 microM for VO2 and perfusion pressure curves, respectively). Low concentrations of the capsaicin antagonist ruthenium red selectively blocked the putative VN2 receptor-mediated effects produced by high concentrations of capsaicin. The noncompetitive nature of this inhibitor suggests an operation through separate receptor-coupled ion channel complexes at high and low concentrations of the vanilloid. Tetrodotoxin failed to attenuate any changes produced by capsaicin, suggesting that the mechanism of action of capsaicin in the rat hindlimb may differ from other tissues.

Norepinephrine and serotonin vasoconstriction in rat hindlimb control different vascular flow routes

The vasoconstrictors, norepinephrine at low dose ( < or = 0.1 microM; LDNE) and serotonin (5-HT), produce opposing metabolic effects in the constant-flow perfused rat hindlimb characterized by increased and decreased oxygen uptake, respectively. In the present study, the effects of each vasoconstrictor are compared in the red blood cell-free buffer-perfused hindlimb on postequilibration endogenous red blood cell efflux, vascular entrapment of fluorescein-labeled dextran (Fx), and vascular corrosion casting by use of 30-micron spheres of methyl methacrylate (MM). A marked transient washout of red blood cells occurred immediately in association with vasoconstriction induced by LDNE that was not apparent when a similar extent of vasoconstriction was induced by 5-HT. Fx perfusions indicated that LDNE recruited a new vascular space that was reaccessed by a second exposure to the vasoconstrictor. 5-HT closed off a previously perfused vascular space that was reaccessed when the vasoconstrictor was removed. Corrosion casting of the arterial tree with MM showed no increase in cast weight, but more vessels filled because of LDNE. Higher doses of NE (2.5 microM) or 5-HT caused a marked decrease in cast weight with fewer vessels filled. The data suggest that LDNE and 5-HT, in association with vasoconstriction at different sites, control different capillary flow routes in the hindlimb that in turn may influence metabolism by increasing or decreasing nutrient access, respectively.

Creatine phosphate as the preferred early indicator of ischemia in muscular tissues

Changes in creatine compounds, especially the creatine phosphate to creatine ratio (CrP/Cr), are more sensitive indicators than changes in other metabolites for early ischemia in the different muscular tissues of heart, small intestine, skeletal muscle, and aorta. Changes in adenine nucleotide ratios are buffered by CrP reserves and the absolute concentration of adenine nucleotides can vary greatly between different muscular tissues. Accumulation of lactate is indicative of ischemia, but is not as sensitive as the ratio of CrP/Cr, but may better indicate the duration of ischemia. Glycerol also accumulates in muscular tissues during prolonged ischemia, so that consideration of both lactate and glycerol levels together, might confer a better estimate of the duration of ischemia of different muscular tissues.

Constant-pressure perfusion of rat hindlimb shows alpha- and beta-adrenergic stimulation of oxygen consumption

Isolated rat hindlimbs were perfused at 37 degrees C and constant physiological pressure (80 +/- 0.5 mmHg) while the flow rate that was allowed to freely self-adjust was monitored. Under these conditions, evidence was obtained for both alpha- and beta-adrenergic stimulation of oxygen consumption (VO2) in contrast to constant-flow perfusion, which has only convincingly shown alpha-adrenergic stimulation of VO2 in response to adrenergic agents. Addition of norepinephrine (NE; 1-33 nM) led to an increase in VO2 with a maximum of 29% above the basal value at 3.3 nM, even though the flow rate decreased. Phenylephrine (3.3-33 nM) and vasopressin (10-100 pM) also showed similar, but lesser in magnitude, vasoconstriction-associated stimulatory effects on VO2. Prazosin (an alpha 1-antagonist) completely reversed the NE-mediated decrease in flow rate and significantly blocked the increased VO2. In contrast, isoproterenol (10-1,000 nM) increased both flow rate (30%) and VO2 (32%). The isoproterenol-stimulated VO2 was not blocked by the beta 1-, beta 2-antagonist propranolol (10 microM), although the increased flow was reversed. In the presence of propranolol (1 or 10 microM), BRL-35135A (a beta 3-agonist) also stimulated VO2 (18%) without significant change in flow rate. These results lend further support to the role of the alpha 1-adrenoceptor in muscle VO2. In addition there is evidence for the presence of a functional beta 3-adrenoceptor as an additional subtype responsible for NE-mediated thermogenesis in the rat hindlimb.

Functional and metabolic evidence for two different vanilloid (VN1 and VN2) receptors in perfused rat hindlimb

Vanilloid spice principles, including capsaicin, stimulate vasoconstriction in the rat hindlimb perfused at constant flow and, depending on dose, either stimulate or inhibit oxygen consumption by this vascular bed. We now present metabolic and functional evidence for two different vanilloid (VN1 and VN2) receptor types. These receptors can be distinguished on the basis of their differing agonist affinity for capsaicin, their different calcium and oxygen dependencies for inducing vasoconstriction, and whether they stimulate, or inhibit, oxygen consumption. The higher affinity vanilloid receptor, VN1 can be distinguished on the basis of initiating vasoconstriction at low doses of capsaicin and simultaneously stimulating oxygen consumption. Its apparent biological function is dependent on the presence of oxygen and external calcium. In contrast, the lower affinity receptor, VN2 induces vasoconstriction associated with inhibition of oxygen consumption. Its vasoconstriction action can occur independently of either external calcium ions, or the presence of oxygen in the perfusate.

Vascular and endocrine control of muscle metabolism

Important differences exist between perfused and incubated (or perifused) skeletal muscle preparations with regard to their metabolism and control. A growing body of evidence suggests that the differences may be due to the role played by the vascular system. In the constant-flow perfused rat hindlimb preparation, a group of vasoconstrictors has been identified that enhance muscle metabolism and aerobic contractility. Another group of vasoconstrictors decrease muscle metabolism and aerobic contractility even though perfusate flow remains constant. All effects of both groups of vasoconstrictors are opposed by vasodilators. Because none of the vasoconstrictor effects is evident when isolated muscles are incubated or perifused, involvement of an active vascular system is indicated. Although some hormones may act directly on muscle by purely endocrine effects, a vascular component of their actions is now emerging. Mechanisms to account for vascular control of perfused skeletal muscle metabolism may involve 1) functional vascular shunts where the proportion of flow processed by these is regulated by site-specific vasomodulators, 2) a direct response to a change in the rate of supply of nutrients and removal of products, and 3) a signal substance released by vascular tissue in association with vasoconstriction that interacts with surrounding skeletal muscle cells. Impaired control at the level of the vascular system may have implications for long-term access of nutrients and hormones and therefore the control of skeletal muscle metabolism and contractile performance.

Vasoconstrictors alter oxygen, lactate, and glycerol metabolism in the perfused hindlimb of a rat kangaroo

The Tasmanian bettong (Bettongia gaimardi) is a small marsupial rat kangaroo without detectable brown adipose tissue (BAT). The hindlimb was perfused with constant flow at 25 degrees C after cannulation under anesthesia of the femoral artery and vein to one hindlimb. Norepinephrine (NE, 25 nM-2.5 microM) and vasopressin (VP, 10 nM-0.1 microM) each increased perfusion pressure, oxygen consumption (VO2), and lactate and glycerol efflux of the perfused hindlimb. NE-mediated increases in VO2 and the efflux of lactate and glycerol were unaffected by propranolol (10 microM) but were completely blocked by the further addition of phentolamine (10 microM). In contrast, serotonin (5-HT; 0.1-2.5 microM) inhibited VO2 and inhibited lactate efflux. The changes induced by NE, VP, and 5-HT were all rapidly reversed by nitroprusside. These results suggest that resting thermogenesis in bettong hindlimb can be differentially controlled by the vasculature, which may also contribute to the induced VO2. This vascular control of skeletal muscle VO2 appears widespread in homeotherm evolution.

Treatment with the thiazolidinedione (BRL 49653) decreases insulin resistance in obese Zucker hindlimb

Hindlimbs of mature age obese fa/fa Zucker rats were perfused and found to be markedly insulin-resistant when compared to the hindlimbs of age-matched lean Fa/? animals. Hindlimb analysis also showed a greater content of fat and a lower content of muscle in the obese. Treatment of the obese animals for 7 days with the thiazolidinedione, BRL 49653 (3 mumol/kg/day) significantly decreased the insulin resistance of the hindlimb and significantly increased the rate of weight gain in the whole rat. However, the decreased insulin resistance due to BRL 49653 could not be accounted for by an increase in the proportion of hindlimb muscle to fat or by an increase in the hindlimb muscle mass perfused.

Inhibition of insulin-mediated glucose uptake in rat hindlimb by an alpha-adrenergic vascular effect

The vasoconstrictor norepinephrine, at high doses, inhibits oxygen uptake (VO2) in the perfused hindlimb, possibly by opening vascular shunts and reducing nutrient access. Thus, in the present study, the effect of norepinephrine on insulin-mediated glucose uptake (IMGU) was assessed. Rat hindlimbs were perfused at constant flow with medium containing 8.3 mM glucose and a tracer amount of 2-deoxy-D-[1-3H]glucose (2-DG) with and without 15 nM insulin, 10 microM norepinephrine (NE), and combinations of the adrenergic blockers propranolol (Prop) and prazosin. Perfusions were also conducted at a lower dose of 1 microM NE where VO2 is stimulated. NE (10 microM) inhibited IMGU > 80%, and this inhibition, when measured by 2-DG uptake, was most pronounced in muscles rich in white fibers. The inhibitory effect of NE on IMGU comprised a beta-adrenergic component also partly evident at lower concentrations of NE (i.e., 1 microM) and an alpha-adrenergic component only evident at 10 microM NE. In contrast to the results for the hindlimb, 10 microM NE plus Prop (alpha-adrenergic combination) had no significant effect on insulin-mediated 2-DG uptake by isolated incubated soleus or extensor digitorum longus muscles. It is concluded that NE, at doses likely to occur at sympathetic vasoconstrictor synapses in muscle, impairs IMGU by a vascular effect to cause shunting and reduce access.

Potential defect in the vascular control of nonshivering thermogenesis in the obese Zucker rat hind limb

Vascular control of nonshivering thermogenesis in the perfused hind limb of obese and lean Zucker rats were compared using two vasoconstrictors, norepinephrine and serotonin. For hind limbs of both phenotypes, norepinephrine infusions resulted in a dose-dependent uninterrupted increase in perfusion pressure and a biphasic change in oxygen uptake (VO2), characterized by a stimulation at low concentrations, and an increasing inhibition at higher concentrations that gradually overcame the stimulation in a dose-dependent manner. At concentrations of norepinephrine greater than 1 microM, the inhibitory effect predominated and gave rise to values for VO2 less than basal. The obese hind limb had a lower basal VO2 and a lower maximal VO2 mediated by norepinephrine than the lean rat, but these differences appeared to relate largely to the lower muscle mass and higher content of fat of the obese hind limb. Serotonin infusions resulted in dose-dependent increase in perfusion pressure and an accompanying decrease in VO2. Pressure changes were identical for the obese and lean hind limbs, but the decrease in VO2 due to serotonin was greater in the hind limbs from the lean rats, and this difference remained when the data were expressed in terms of muscle mass perfused. It is concluded that the relatively lower content of muscle of the obese hind limb accounts for its lower basal and lower maximal norepinephrine-mediated thermogenesis. In addition, an intrinsic defect in obese hind limb muscle response to serotonin is present, which may be indicative of a decrease in the potential for vasoconstrictor-regulated thermogenesis that could have implications for whole-body energy balance by the obese phenotype.

Vasoconstrictor-mediated release of purines and pyrimidines from perfused rat hindlimb, perfused mesenteric arcade and incubated de-endothelialized aorta

1. Reperfusion of hindlimbs that had previously been exposed to either 10 or 60 min global ischaemia resulted in transient washouts of uracil and uric acid in approximate proportion to the interval of ischaemia. However, changing the interval of sequential angiotensin II infusions from 10 to 60 min did not affect the magnitude of sustained uracil and uric acid release. 2. Perfused rat mesenteric artery arcade released uracil and uric acid and each was further increased approximately 2-fold by exposure to the vasoconstrictor, serotonin (6.7 microM). 3. Incubated de-endothelialized rat aorta also released purines and pyrimidines and this was increased further when subjected to increased work loads. 4. The increased rates of release of purines and pyrimidines from hindlimb, and the simpler vascular preparations of mesenteric arcade and aorta, were in proportion to the relative rates of increase in oxygen consumption under maximum vascular load. 5. It is concluded that the release of purine and pyrimidine nucleosides and their catabolites from perfused rat hindlimb occurring as a consequence of vasoconstriction is not the result of release from previously ischaemic tissue. In addition, release of purines and pyrimidines appears to be a general feature of vascular smooth muscle subjected to high workloads.

Aerobic muscle contraction impaired by serotonin-mediated vasoconstriction

Vasoconstriction mediated by serotonin (5-HT) inhibits muscle metabolism in resting constant-flow-perfused rat hindlimb and may do so by vascular shunting. In the present study, the effects of 5-HT on tension development and contraction-induced oxygen uptake by the sciatic nerve-stimulated gastrocnemius-plantaris-soleus muscle group of the perfused rat hindlimb and tension development by electrically stimulated isolated incubated soleus and extensor digitorum longus muscles were examined. In both erythrocyte and erythrocyte-free perfusions, 0.25 microM 5-HT increased perfusion pressure and markedly decreased contraction-induced tension, oxygen uptake, and lactate release. The release of metabolic vasodilators from exercising skeletal muscle did not appear to affect 5-HT-mediated vasoconstriction; rather, vascular resistance increased during the period of muscle contraction. In contrast, vasoconstriction during muscle contraction mediated by alpha-adrenoceptor stimulation did not impair tension and was partially overcome by metabolic vasodilators. In addition, contraction of isolated incubated soleus and extensor digitorum longus muscles was not affected by 5-HT addition to the incubation medium. We conclude that 5-HT impairs contractility of working muscle during the aerobic phase by limiting oxygen delivery through redistributing perfusate flow. The results are consistent with a vasoconstrictor action of 5-HT on larger vessels, perhaps at feed arteries external to the working muscle. When constricted by 5-HT, these vessels are apparently insensitive to metabolic vasodilatation.

Characterization of perfused periaortic brown adipose tissue from the rat

A technique was developed for the perfusion of periaortic brown adipose tissue (BAT) with a view to assessing vascular system involvement in BAT thermogenesis. The procedure involved cannulation of the thoracic aorta and ligation of the intercostal branches and the distal thoracic aorta. Perfusion was conducted in a buffer-filled chamber using constant flow at 37 degrees C. Lactate dehydrogenase leakage was less than 2%/h, and after 30 min of perfusion the energy charge was 0.72 +/- 0.05 (n = 4) and differed little from freshly sampled interscapular BAT (0.71 +/- 0.03 (n = 7)). Periaortic BAT was indistinguishable from interscapular BAT in enzyme content, mitochondrial size, mitochondrial cristae, lipid content, and cell size. Basal oxygen consumption (VO2) was 64.3 +/- 7.4 mumol.h-1.g-1 wet weight, and basal perfusion pressure was 65 +/- 3 mmHg (1 mmHg = 133.3 Pa). Norepinephrine and isoproterenol each increased VO2 of perfused periaortic BAT in a time-dependent and reversible manner. Half-maximal stimulation of VO2 occurred at 12 nM norepinephrine and 8 nM isoproterenol; maximally stimulated tissue had a VO2 of approximately 150 mumol.h-1.g-1 wet weight. Norepinephrine (50 nM) had no consistent effect on perfusion pressure, but the increase in VO2 by this agonist was completely blocked by 10 microM DL-propranolol and unaffected by phentolamine (1-20 microM) or nitroprusside (0.01-1 mM). Increasing the perfusion flow rate increased pressure and had no effect on basal VO2 but increased the VO2 response due to norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Resiniferatoxin and piperine: capsaicin-like stimulators of oxygen uptake in the perfused rat hindlimb

The naturally occurring capsaicin-like molecules, resiniferatoxin (RTX, Euphorbia spp.) and piperine (Piper nigrum), each stimulated oxygen uptake (VO2) in association with increased vascular resistance in a concentration-dependent manner when infused into the perfused rat hindlimb. 5 microM glyceryl trinitrate (GTN, a nitrovasodilator) significantly blocked the oxygen and pressure responses to both RTX and piperine, indicating a close relationship between changes in VO2 and the vasoconstriction. Concentrations greater than those required for maximal VO2 resulted in an inhibition of VO2, although perfusion pressure continued to increase. Time course studies showed that both RTX and piperine at high doses resulted in a tri-phasic response. An initial phase of transient VO2 stimulation was followed by a second phase of inhibition. A third phase involving an often larger but transient stimulation of VO2 followed removal of the agents and continued after the pressure returned to basal. The actions of RTX and piperine were similar to those of other active capsaicin-like molecules tested previously in this system, including capsaicinoids (Capsicum spp.), gingerols (Zingiber officinale), and shogoals (Zingiber officinale). RTX was the most potent, and piperine the least potent of this series. Although receptor involvement has yet to be unequivocally established, the data are consistent with the presence of a functional capsaicin-like (vanilloid) receptor in the vasculature of the rat hindlimb that mediates vasoconstriction and oxygen uptake. These findings may have implications for the future development of thermogenic agents.

[32P]phosphate autoradiography as an indicator of regional myocardial oxygen consumption?

The present study was undertaken to assess whether [32P]uptake and autoradiography could be used as an indicator of regional myocardial oxygen consumption. Rat hearts were perfused in the Langendorff mode with medium containing [32P]orthophosphate (32Pi) and the incorporation of label into ATP, perchloric acid-insoluble and soluble material and total tissue was determined. Oxygen uptake and perfusion pressure were continuously monitored. For control hearts the rate of incorporation of radioactivity into all four fractions was linear. Isoproterenol and pressure loading each increased total 32Pi uptake rate and the incorporation rate of 32Pi into ATP. Significant positive correlations were noted between the rates of total 32Pi uptake and oxygen uptake (r = 0.895; P < 0.001) and between the rates of 32P incorporation into ATP and oxygen uptake (r = 0.890; P < 0.001). Autoradiography of diffusible radioactive material (representing total 32Pi uptake) indicated that label correlated with oxygen uptake (r = 0.850; P < 0.001) and was distributed uniformly across the ventricle wall. Hearts subjected to 30 min of regional ischemia followed by reperfusion were indistinguishable from the control group in terms of either total 32Pi uptake or total oxygen uptake, but showed a marked necrotic area that was unlabeled surrounded by an area that was intensely labeled. It is concluded that autoradiography of diffusible radioactive material representing total 32Pi uptake may be applicable for assessing regional myocardial oxygen uptake. The technique has identified a region of tissue in reperfused ischemic hearts that surrounds necrotic tissue and which may have a compensatory increase in a oxidative metabolism.

Role of extracellular UTP in the release of uracil from vasoconstricted hindlimb

The source and function of elevated uracil release during vasoconstriction in the perfused rat hindlimb was investigated. The possibility that uracil release derived from the breakdown of released vasoactive uridine 5'-triphosphate (UTP) was examined. Exogenous UTP was found to be a potent vasodilator in the perfused rat hindlimb, opposing norepinephrine and angiotensin-induced increases in vasoconstriction and oxygen consumption. UTP was rapidly catabolized by the hindlimb to uridine 5'-monophosphate (UMP), uridine, and uracil, which were all devoid of vasoactivity. UTP was similarly catabolized by incubated rat aorta. Degradation of exogenous UTP by perfused hindlimb or aorta was inhibited by alpha, beta-methylene-adenosine 5'-diphosphate (AMP-CP), an inhibitor of ectonucleotidases. However, AMP-CP did not decrease uracil and uridine output by the hindlimb during angiotensin-mediated vasoconstriction and increased oxygen consumption. In particular, simultaneous infusion of AMP-CP with angiotensin did not increase efflux of UMP. Although exogenous UTP is a potent vasodilator in the perfused rat hindlimb, it appears not to be released intact during vasoconstriction. Hence, extracellular UTP is unlikely to be the precursor of the uracil release.

Serotonin-mediated acute insulin resistance in the perfused rat hindlimb but not in incubated muscle: a role for the vascular system

We have recently shown that the vasoconstrictor serotonin (5-HT) inhibits oxygen uptake in perfused hindlimb possibly due to vascular shunting. Thus in the present study the effect of 5-HT on insulin-mediated glucose uptake was assessed. Rat hindlimbs were perfused at constant flow with medium containing 8.3 mM glucose and a tracer amount of 2-deoxy-D-[1-3]glucose (2DG) with and without 10 microM 5-HT, 15 nM insulin and a combination of the two. 5-HT inhibited insulin-mediated stimulation of glucose uptake by 30.4% when added after insulin and 34.4% when added before insulin. In addition, 5-HT inhibited insulin-mediated 2DG uptake by perfused muscles with inhibition ranging from 32% (soleus) to 80% (extensor digitorum longus). The effects of 5-HT on insulin-mediated glucose uptake were partially reversed by vasodilation with carbachol. In contrast to the results for the hindlimb, 10 microM 5-HT had no significant effect on either basal glucose uptake or the stimulation of glucose uptake mediated by 15 nM insulin by isolated incubated soleus or extensor digitorum longus muscles. It is concluded that 5-HT impairs insulin-mediated glucose uptake in the perfused rat hindlimb that may derive from vascular shunting not apparent when muscles are incubated with 5-HT in vitro. These findings may have implications for the link between hypertension and diabetes.

Vasoconstrictor-mediated release of lactate from the perfused rat hindlimb

The effects of different vasomodulators on lactate release by the constant-flow-perfused rat hindlimb were examined and compared with that by perfused mesenteric artery, incubated preparations of aortas, soleus and epitrochlearis muscles, and perifused soleus muscles. Infusion of vasopressin (0.5 nM), angiotensin II (5 nM), norepinephrine (50 nM), and methoxamine (10 microM) into the hindlimbs of 180- to 200-g rats increased the perfusion pressure by 112-167% from 30.4 +/- 0.8 mmHg, O2 consumption by 26-68% from 6.4 +/- 0.2 mumol.g-1 x h-1, and lactate efflux by 148-380% from 5.41 +/- 0.25 mumol.g-1 x h-1. Hindlimbs of 100- to 120-g rats responded similarly to angiotensin II. Isoproterenol (1 microM) had no effect on O2 uptake or perfusion pressure but increased lactate release by 118%. Nitroprusside (0.5 mM) markedly inhibited the vasoconstrictor-mediated increases in lactate release, perfusion pressure, and O2 consumption by the hindlimb but had no effect on isoproterenol-mediated lactate efflux. Serotonin (6.7 microM) increased lactate release from the perfused mesenteric artery by 120% from 5.48 mol.g-1 x h-1. Lactate release by incubated aorta was increased by angiotensin II (50 nM), isoproterenol (1 microM), and mechanical stretch. The increase mediated by angiotensin II was blocked by glycerol trinitrate (2.2 microM), which had no effect on lactate release by isoproterenol. Neither angiotensin II (5 nM) nor vasopressin (0.5 nM) increased lactate release from incubated soleus and epitrochlearis muscles; however, lactate release was increased by isoproterenol, and this increase was unaffected by glycerol trinitrate (2.2 microM).(ABSTRACT TRUNCATED AT 250 WORDS)

Pungent principles of ginger (Zingiber officinale) are thermogenic in the perfused rat hindlimb

Crude extracts of both fresh and dry ginger induced the perfused rat hindlimb to consume oxygen in association with increases in perfusion pressure and lactate production. The principles responsible for these observations, the gingerols and shogaols, were isolated and tested for relative thermogenic activity. The gingerol homologues possessed greater molar potency than their shogaol counterparts. (6)-Gingerol was the most potent principle isolated, causing a mean maximal increase in oxygen consumption of 1.4 +/- 0.1 mumol/g/h (21%), an increase in lactate efflux of 4.7 +/- 0.6 mumol/g/h (87%) with a perfusion pressure increase of 7.7 +/- 0.7 mmHg (30%). Increases in alkyl chain length within each homologous series led to decreased molar potency. Specific nitro-vasodilation using glyceryl trinitrate demonstrated that thermogenesis was at least partly associated with vasoconstriction. Concurrent infusion of alpha or beta antagonists showed that neither adrenergic receptors nor secondary catecholamine release were responsible for the observed effects. Increasing doses of the ginger principles ultimately led to inhibition of steady state oxygen consumption, although perfusion pressure continued to increase. Removal of high ginger principle doses was followed by apparent increases in oxygen uptake unaccompanied by elevated perfusion pressure. As a consequence, the effective concentration ranges of the ginger principles were relatively narrow. The cause of high dose effects is as yet undetermined but may have been due in part to disruption of mitochondrial function.

Spicy meal disturbs sleep: an effect of thermoregulation?

Tabasco sauce and mustard taken with the evening meal markedly disturbed sleep of six, young, healthy male subjects; reducing slow wave and stage 2 sleep, increasing total time awake and tending to increase sleep onset latency. Whilst post meal effects on temperature and oxygen consumption were not significantly different from control meals the spicy food condition elevated body temperature during the first sleep cycle. The possibility that the spice principle capsaicin affects sleep via changes in body temperature is discussed.

Serotonin and norepinephrine vasoconstriction in rat hindlimb have different oxygen requirements

Serotonin (5-HT) and norepinephrine (NE) are potent vasoconstrictors in the non-erythrocyte constant-flow perfused hindlimb at 25 degrees C. Because 5-HT inhibits and NE stimulates oxygen uptake, these agonists may exert control at different sites in the vasculature that have different metabolic requirements. In this study, it was found that hypoxia, cyanide, or azide had no significant effect on 5-HT-mediated vasoconstriction; however, in the presence of cyanide and absence of glucose, vasoconstriction was transient. NE-mediated vasoconstriction displayed two prazosin-sensitive components. At low concentrations (less than 50 nM), NE-mediated vasoconstriction was totally blocked by hypoxia, cyanide, or azide. At concentrations of NE significantly greater than 50 nM, oxygen-independent vasoconstriction was observed. In conclusion, NE and 5-HT sites are biochemically distinguishable, with 5-HT sites probably located on larger vessels and supportable by anaerobic metabolism and oxygen-dependent NE sites probably located on smaller arteries or arterioles. Supraphysiological concentrations of NE may interact with a subgroup of vascular alpha 1-adrenoceptors on larger vessels giving results similar to 5-HT, thus highlighting problems with high levels of NE in the perfused rat hindlimb.

A close association between vasoconstrictor-mediated uracil and lactate release by the perfused rat hindlimb

1. Angiotensin II (5 nM) increased perfusion pressure, O2 uptake and the release of lactate, uracil and uric acid from the perfused rat hindlimb. The release of all three substances was greatest 5 min after commencement of angiotensin II infusion and then decreased over the next 20 min to reach a plateau value that was approx. 2.5-fold basal values. Following removal of angiotensin, pressure, O2 uptake as well as lactate, uracil and uric acid release each returned to pre-infusion (basal) values. 2. Cyanide (1 mM) when added during angiotensin II (5 nM) infusion blocked the pressor effect and completely inhibited all O2 uptake. Cyanide (1 mM) also inhibited the angiotensin-induced increase in uric acid, uracil and lactate release, but the effects differed. Whereas uric acid release remained inhibited throughout the cyanide infusion, uracil and lactate release were only temporarily interrupted and a secondary release of both ensued. 3. Nitroprusside (0.5 mM) when added during angiotensin II (5 nM) infusion blocked pressure and O2 uptake. Lactate and uracil release were partly blocked and returned to pre-infusion (basal) values. However uric acid release was totally blocked and no release occurred when nitroprusside was present with angiotensin II. 4. Combined data showed a significant correlation (r = 0.831; P less than 0.001) between effluent lactate and effluent uracil. 5. It is concluded that lactate and uracil release which increase markedly during vasoconstriction of the hindlimb reflect an association between glycolysis and uracil nucleotide turnover within the same tissue, possibly vascular smooth muscle.

Open Question: Has Thermogenesis in Muscle Been Overlooked and Misinterpreted?

Brown adipose tissue has been considered the major thermogenic tissue of the rat and, by implication, of other species. However, oxygen uptake by the perfused resting hindlimb is markedly accelerated by hormones, appears to be controlled by the vascular system, and may originate from vasoconstricting smooth muscle itself.

Norephedrine (phenylpropanolamine) stimulates oxygen consumption and lactate production in the perfused rat hindlimb

d,l-Norephedrine (phenylpropanolamine) which is both a demethylated analogue and a metabolite of d,l-ephedrine, is a reputed anorectic agent. In the present study the proposed most active isomer of this mixture, l-norephedrine has been assessed as a peripherally acting thermogenic agent in the isolated perfused rat hindlimb. l-Norephedrine produced a dose-dependent increase in oxygen uptake and perfusion pressure and increased lactate production. Whereas propranolol potentiated the increase in oxygen uptake and perfusion pressure produced by l-norephedrine, prazosin significantly and nitroprusside totally inhibited both of these changes. Nitroprusside also completely inhibited the increase in lactate production. We conclude that norephedrine has a hitherto unrecognized peripheral thermogenic activity in the perfused rat hindlimb resulting from its interaction with alpha 1 adrenergic receptors that control vasoconstriction in this tissue.

Hypertension in obesity may reflect a homeostatic thermogenic response

Systemic hypertension of mild to moderate degree is often associated with obesity. The hypothesis is that over-eating leads to increased sympathetic activity targeted at the peripheral vasculature as well as other tissues in an attempt (that in many cases may be futile) to stimulate facultative thermogenesis and burn-off the excess energy. This hypothesis represents an important modification of one proposed by Landsberg and is supported by: 1) recent observations that carbohydrate feeding to humans specifically increases muscle sympathetic vasoconstrictor activity in the peroneal nerve, and 2) studies with animal models in which active vasoconstriction in the limbs and elsewhere is associated with marked increases in oxygen consumption (energy expenditure).

The apparent absence of serotonin-mediated vascular thermogenesis in perfused rat hindlimb may result from vascular shunting

Vasoconstriction by norepinephrine, angiotensin II and vasopressin in the constant-flow perfused rat hindlimb is associated with increased oxygen uptake and has given rise to the concept of vascular thermogenesis. In the present study serotonin (5-hydroxytryptamine, 5HT) was found to inhibit oxygen uptake by up to 40% in a dose dependent manner whilst inducing vasoconstriction in this model, whereas norepinephrine increased oxygen consumption by up to 100% during vasoconstriction. This contrasted with the perfused isolated rat mesenteric artery arcade in which serotonin stimulated oxygen uptake by up to 130% in association with vasoconstriction in a dose dependent manner similar to the previously described norepinephrine induced vascular thermogenesis in this arterial preparation. In both perfusion systems, changes in pressure and oxygen uptake mediated by serotonin were completely blocked by ketanserin. These results and evidence from dye washout studies suggest that serotonin-mediated vascular thermogenesis, if it occurs in the constant-flow hindlimb, is masked by vascular shunting.

Release of purine and pyrimidine nucleosides and their catabolites from the perfused rat hindlimb in response to noradrenaline, vasopressin, angiotensin II and sciatic-nerve stimulation

Uric acid and uracil were released at constant rates (0.95 and 0.4 nmol/min per g respectively) by the perfused rat hindlimb. Noradrenaline, vasopressin or angiotensin II further increased the release of these substances 2-5-fold, coinciding with increases in both perfusion pressure (vasoconstriction) and O2 uptake. The hindlimb also released, but in lesser amounts, uridine, hypoxanthine, xanthine, inosine and guanosine, and all but hypoxanthine and guanosine were increased during intense vasoconstriction. Uric acid and uracil releases were increased by noradrenaline in a dose-dependent manner. However, the release of these substances did not fully correspond with the dose-dependent increase in O2 uptake and perfusion pressure, where changes in the latter occurred at lower doses of noradrenaline. Sciatic-nerve stimulation (skeletal-muscle contraction) did not increase the release of uracil, uric acid or uridine, but instead increased the release of inosine (7-fold) and hypoxanthine (2-fold). Since the UTP content as well as the UTP/ATP ratio are higher in smooth muscle than in skeletal muscle, it is proposed that release of uric acid and uracil arises from increased metabolism of the respective adenosine and uridine nucleotides during intense constriction of smooth muscle.

Capsaicin and dihydrocapsaicin stimulate oxygen consumption in the perfused rat hindlimb

The capsaicinoid spice principles capsaicin and dihydrocapsaicin were shown to be thermogenic in the isolated rat hindlimb perfused with constant flow. Both principles elicited similar maximal increases in oxygen consumption (VO2) and perfusion pressure. For capsaicin, the mean maximal increase in VO2 was 1.39 +/- 0.28 mumol/g h or 23 per cent with a perfusion pressure increase of 15.8 +/- 2.8 mmHg, or 54.5 per cent. Dihydrocapsaicin increased VO2 by 1.13 +/- 0.24 or 20 per cent and a perfusion pressure rise of 14.2 +/- 5.0 mmHg or 49 per cent. Above 0.8 microM of either capsaicinoid there was inhibition of oxygen consumption after transient stimulation. Concurrent infusion of the adrenergic antagonists prazosin (alpha 1) and propranolol (beta) had little or no effect on the actions of either capsaicin or dihydrocapsaicin, nor did division of the somatic nerves to the hindlimb. These results indicate a local site of action of these principles in the hindlimb not mediated by a secondary release of catecholamines. Increases in both VO2 and pressure were significantly blocked by the vasodilator nitroprusside. This is in agreement with our previous findings that nitroprusside can block the effects of angiotensin, vasopressin and flow-induced increases in VO2 and perfusion pressure in the perfused hindlimb. The present findings suggest that capsaicin and dihydrocapsaicin can be thermogenic in the rat and that the mechanism of action directly involves vasoconstriction in some manner. We have previously suggested that there might be significant direct smooth muscle vascular consumption of oxygen during sustained vasoconstriction. The findings with capsaicin and dihydrocapsaicin are consistent with this hypothesis.

Flow-induced oxygen uptake by the perfused rat hindlimb is inhibited by vasodilators and augmented by norepinephrine: a possible role for the microvasculature in hindlimb thermogenesis

Oxygen uptake in the perfused rat hindlimb was studied at 25 degrees C using an artificial perfusate, and the effects of perfusate flow rate, norepinephrine, and vasodilators were compared. Hindlimb oxygen uptake and perfusion pressure each increased as the flow rate was increased stepwise from 2 to 18.5 mL/min per hindlimb. At each flow rate, the rate of oxygen uptake was inhibited by the vasodilator nitroprusside (0.5 mM) and increased by norepinephrine (5 nM). A corresponding change in perfusion pressure also occurred, with norepinephrine leading to a marked increase and nitroprusside leading to a decrease; however, changes in oxygen uptake and pressure were not linearly related. The lactate/pyruvate ratio of the perfusate was used as an index of tissue perfusion and was determined at each flow rate. Lactate and pyruvate efflux increased as the flow rate was increased stepwise from 2 to 18.5 mL/min per hindlimb. At 2 mL/min per hindlimb, the lactate/pyruvate ratio was 15; at flow rates equal or greater than 4 mL/min per hindlimb, the ratio was constant at 9. Nitroprusside had no significant effect on the ratio at any flow rate even though a marked inhibitory effect on oxygen uptake was evident. Muscle content of high energy phosphates at 8 mL/min per hindlimb did not differ before and after treatment with vasodilators. In addition, the vasodilators had no apparent effect on skeletal muscle oxygen uptake or force development during electrical stimulation. The findings indicate that oxygen uptake by the hindlimb is not limited by inadequate perfusion and that oxygen uptake can be further increased by norepinephrine.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by vasodilators of noradrenaline and vasoconstrictor-mediated, but not skeletal muscle contraction-induced oxygen uptake in the perfused rat hindlimb; implications for non-shivering thermogenesis in muscle tissue

1. The effect of noradrenaline as well as of vasopressin and angiotensin II to increase oxygen uptake and perfusion pressure by the isolated perfused rat hindlimb were completely inhibited by the vasodilators, nitroprusside (0.5 mM), nifedipine (2.5 microM) and isoprenaline (50 nM). 2. Oxygen uptake due to sciatic nerve stimulation of skeletal muscle contraction was not inhibited by 0.5 mM nitroprusside but was found to increase further that produced by a maximum dose of either noradrenaline or angiotensin II. 3. Analysis of high energy phosphates in samples of freeze-clamped hindlimb muscle showed no difference before and after vasoconstrictor addition or with muscle sampled in vivo. 4. It is concluded that norepinephrine mediated increase in oxygen uptake by the perfused rat hindlimb results from its vasoconstrictor action.

A comparison of vasopressin and noradrenaline on oxygen uptake by perfused rat hindlimb, kidney, intestine and mesenteric arcade suggests that it is in part due to contractile work by blood vessels

1. The rat hindlimb, kidney and intestine were each perfused in a nonrecirculating mode at 25 degrees C using an artificial perfusate (initial pressure 85 +/- 5 mmHg) and the effects of vasopressin and noradrenaline on oxygen uptake and perfusion pressure determined. 2. Both vasopressin (K0.5 = 0.1 nM) and noradrenaline (K0.5 = 2 nM) increased oxygen uptake as well as perfusion pressure by the perfused hindlimb; changes in oxygen uptake were closely matched by changes in pressure. The maximum increase in oxygen uptake was approx. 9 mumol/hr per g wet wt of hindlimb. 3. The perfused kidney also responded to vasopressin and noradrenaline with parallel increases in oxygen uptake and perfusion pressure for each agent. The largest increase in oxygen uptake was approx. 30 mumol/hr per g wet wt but this was not maximal. 4. Vasopressin increased oxygen uptake and pressure by the perfused intestine over the range 0.01-2 nM, but the changes in pressure only became significant at doses greater than 0.1 nM. 5. Noradrenaline inhibited oxygen uptake and increased perfusion pressure in a dose-dependent manner at pharmacological concentrations (greater than 30 nM) when shunting of perfusate may have contributed to unperfused regions. 6. A network of mesenteric blood vessels estimated to contain approx. 6% vascular tissue by weight, with the remainder white fat cells, lymphatics and connective tissue, was also perfused. 7. Vasopressin (K0.5 = 0.3 nM) and noradrenaline (K0.5 = 30 nM) each increased oxygen uptake and perfusion pressure in a dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 1-adrenergic control of contractility and coronary flow in the perfused rat heart

Prazosin inhibition of phenylephrine plus propranolol (alpha 1-adrenergic agonist combination)-mediated inotropy, oxygen uptake, and coronary vasoconstriction of the perfused rat heart were compared with prazosin binding under identical conditions. Binding studies for the perfused heart indicated a population of high-affinity sites (Kd, 0.41 nM; Bmax, 13.2 pmol/g wet wt). Phenylephrine (50 microM) plus dl-propranolol (10 microM) did not significantly alter binding of 3.9 nM prazosin. The alpha 1-agonist combination mediated a dose-dependent increase in tension development and oxygen uptake and a decrease in coronary flow, each of which was inhibited by prazosin. The concentration of prazosin required for half-maximal inhibition of inotropy and oxygen uptake produced by 50-100 microM phenylephrine was 30-40 nM; no inhibition occurred at 2-3 nM prazosin concentration when binding sites were saturated. One nanomolar prazosin was required for half-maximal inhibition of the flow decrease produced by 10 microM phenylephrine. It is concluded that less than 1% of the alpha 1-binding sites are required for full development of inotropy and that this may result from an excess of binding sites or a small population of low-affinity receptors.

Characterization of alpha 1-adrenergic receptors in perfused rat heart

The characteristics of alpha 1-adrenergic receptors were investigated in perfused rat hearts at 37 degrees C. [3H]Prazosin was bound in a time-dependent manner and reached equilibrium at 15 min. Scatchard analysis of the specific binding isotherm for [3H]prazosin indicated a population of high affinity sites (Kd = 0.41 nM, Bmax = 13.2 pmol/g wet wt). Prazosin binding was displaced by epinephrine as well as by the adrenergic antagonists prazosin greater than phentolamine greater than yohimbine greater than propranolol. Specific prazosin binding was defined as that portion of the binding inhibited by 10 microM phentolamine; phentolamine and epinephrine displaced 3H-prazosin to the same level. [3H]Prazosin was not metabolized by the heart. When pre-labelled hearts were perfused at 37 degrees C with prazosin-free medium non-specific binding of [3H]prazosin decreased more rapidly (t0.5 = 4 min) than specific binding (t0.5 = 38 min). Perfusion of the heart at lower temperatures (less than 10 degrees C) decreased the rate of loss of nonspecific binding and prevented the loss of specific binding. Fractionation of [3H]prazosin perfused hearts at 0 degrees C, when dissociation was minimal, led to a loss of binding so that sarcolemma-enriched fractions contained approximately 2% of the binding sites present in the perfused heart. The binding characteristics of sarcolemma-enriched fractions (Kd 0.10 nM, Bmax 300 fmol/mg protein) differed significantly from those of the perfused heart. Exposure of the heart to 10 min of ischaemia prior to binding studies did not alter the characteristics of the [3H]prazosin binding sites. It is concluded that the perfused rat heart contains a population of alpha 1-adrenoceptors which differ from those of isolated sarcolemma preparations perhaps because of alterations that occur during sarcolemma isolation. The perfused heart should be an appropriate model system in which to study the relationship between receptor occupancy and biological response as well as the direct effects of perturbations such as ischaemia.

Vasopressin and angiotensin II stimulate oxygen uptake in the perfused rat hindlimb

Vasopressin and angiotensin II markedly stimulated oxygen uptake in the perfused rat hindlimb. The increase due to each agent approached 70% of the basal rate, and was greater than that produced by a maximal concentration of norepinephrine. Half-maximal stimulation occurred at 60 pM vasopressin, 0.5 nM angiotensin II and 10 nM norepinephrine. Angiotensins I and III were less potent than angiotensin II. For each agent, the dose-dependent increase in oxygen uptake coincided with a dose-dependent increase in perfusion pressure. The effects of both vasopressin and angiotensin to increase oxygen uptake and pressure were not inhibited by either phentolamine, propranolol or a combination of the two, but were completely inhibited by the vasodilator, nitroprusside. Nitroprusside also inhibited flow-induced increases in hindlimb oxygen uptake and perfusion pressure. The findings indicate a key role for the vascular system in the control of hindlimb oxygen uptake.

Alpha-adrenergic receptors in rat skeletal muscle

Sarcolemma-enriched preparations from muscles rich in slow oxidative red fibres contained specific binding sites for the alpha 1 antagonist, prazosin (e.g. soleus Kd 0.13 nM, Bmax 29 fmol/mg protein). Binding sites for prazosin were almost absent from white muscle. Displacement of prazosin binding from sarcolemma of soleus muscle (phentolamine greater than phenylephrine greater than idazoxan greater than yohimbine) suggested that the receptors were alpha 1. Binding sites for dihydroalprenolol (beta antagonist) were also more concentrated on red than white muscle and outnumbered prazosin sites by approx. 10:1. Binding sites for idazoxan (alpha 2 antagonist) were undetectable. Contamination of sarcolemma-enriched preparations by endothelial tissue indicated by the activity of angiotensin converting enzyme did not correlate with prazosin binding. It is concluded that post-synaptic alpha 1 adrenergic receptors are present on the sarcolemma of slow oxidative red fibres of rat skeletal muscle. The presence provides the mechanistic basis for apparent alpha-adrenergic effects to increase glucose and oxygen uptake in perfused rat hindquarter.

Lysosomal thyroid hormone 5'-deiodinase

Lysosomes prepared from rat liver and kidney after loading with the detergent Triton WR-1339 show membrane-bound 5'-deiodinase activity with marked specificity for 3,3',5'-triiodothyronine (reverse T3), lesser activity with respect to thyroxine (T4) and almost none towards 3,3',5'-triiodothyronine (T3). The enzyme is thiol dependent and shows maximal catalysis at pH 7.2. As many of the states known to alter thyroid hormone levels also affect lysosomal function, inhibition of the lysosomal 5'-deiodinase leading to an increase in intracellular reverse T3 may be an initiating mechanism for thyroid hormone change.