Differences between VO2 maxima of twitch and tetanic contractions are related to blood flow

The purpose of this investigation was to compare oxygen uptake (VO2) and fatigue characteristics of isotonic tetanic contractions with those observed during isotonic twitches in dog gastrocnemius-plantaris muscle. Tetanic contractions (1/s, 200-ms trains of 50 impulses/s) elicited a peak VO2 of 9.01 +/- 0.42 mumol.g-1.min-1, which declined 29% in 30 min. The peak was significantly lower during 4/s twitches (6.23 +/- 0.36 mumol.g-1.min-1), but the rate of decline was similar. Peak blood flow (Q) was 37% higher and decreased more slowly during tetanic than twitch contractions. VO2/Q and VO2/venous PO2 were similar in both groups at peak VO2 and later declined or remained constant over time. Power was significantly greater with tetanic contractions with the relative decline between 3 and 30 min similar in both groups (32 and 37%). In conclusion, tetanic contractions result in significantly higher VO2 and power than do twitch contractions. This was derived primarily from increased Q because the arteriovenous O2 difference was similar. A significant determinant of the difference in Q between twitch and tetanic contractions is mechanical hindrance of Q. There is relatively more time for unhindered flow in the tetanic contractions. In electrically stimulated muscles, maximal VO2 is related to Q and reflects mainly Q through the muscle rather than the VO2 capacity of the muscle.     

Role of convective O(2) delivery in determining VO(2) on-kinetics in canine muscle contracting at peak VO(2).

A previous study (Grassi B, Gladden LB, Samaja M, Stary CM, and Hogan MC, J Appl Physiol 85: 1394-1403, 1998) showed that convective O(2) delivery to muscle did not limit O(2) uptake (VO(2)) on-kinetics during transitions from rest to contractions at approximately 60% of peak VO(2). The present study aimed to determine whether this finding is also true for transitions involving contractions of higher metabolic intensities. VO(2) on-kinetics were determined in isolated canine gastrocnemius muscles in situ (n = 5) during transitions from rest to 4 min of electrically stimulated isometric tetanic contractions corresponding to the muscle peak VO(2). Two conditions were compared: 1) spontaneous adjustment of muscle blood flow (Q) (Control) and 2) pump-perfused Q, adjusted approximately 15-30 s before contractions at a constant level corresponding to the steady-state value during contractions in Control (Fast O(2) Delivery). In Fast O(2) Delivery, adenosine was infused intra-arterially. Q was measured continuously in the popliteal vein; arterial and popliteal venous O(2) contents were measured at rest and at 5- to 7-s intervals during the transition. Muscle VO(2) was determined as Q times the arteriovenous blood O(2) content difference. The time to reach 63% of the VO(2) difference between resting baseline and steady-state values during contractions was 24.9 +/- 1.6 (SE) s in Control and 18.5 +/- 1.8 s in Fast O(2) Delivery (P < 0.05). Faster VO(2) on-kinetics in Fast O(2) Delivery was associated with an approximately 30% reduction in the calculated O(2) deficit and with less muscle fatigue. During transitions involving contractions at peak VO(2), convective O(2) delivery to muscle, together with an inertia of oxidative metabolism, contributes in determining the VO(2) on-kinetics.     

Effect of arterial oxygenation on quadriceps fatigability during isolated muscle exercise

The effect of various levels of oxygenation on quadriceps muscle fatigability during isolated muscle exercise was assessed in six male subjects. Twitch force (Q(tw)) was assessed using supramaximal magnetic femoral nerve stimulation. In experiment 1, maximal voluntary contraction (MVC) and Q(tw) of resting quadriceps muscle were measured in normoxia [inspired O(2) fraction (Fi(O(2))) = 0.21, percent arterial O(2) saturation (Sp(O(2))) = 98.4%, estimated arterial O(2) content (Ca(O(2))) = 20.8 ml/dl], acute hypoxia (Fi(O(2)) = 0.11, Sp(O(2)) = 74.6%, Ca(O(2)) = 15.7 ml/dl), and acute hyperoxia (Fi(O(2)) = 1.0, Sp(O(2)) = 100%, Ca(O(2)) = 22.6 ml/dl). No significant differences were found for MVC and Q(tw) among the three Fi(O(2)) levels. In experiment 2, the subjects performed three sets of nine, intermittent, isometric, unilateral, submaximal quadriceps contractions (62% MVC followed by 1 MVC in each set) while breathing each Fi(O(2)). Q(tw) was assessed before and after exercise, and myoelectrical activity of the vastus lateralis was obtained during exercise. The percent reduction of twitch force (potentiated Q(tw)) in hypoxia (-27.0%) was significantly (P < 0.05) greater than in normoxia (-21.4%) and hyperoxia (-19.9%), as were the changes in intratwitch measures of contractile properties. The increase in integrated electromyogram over the course of the nine contractions in hypoxia (15.4%) was higher (P < 0.05) than in normoxia (7.2%) or hyperoxia (6.7%). These results demonstrate that quadriceps muscle fatigability during isolated muscle exercise is exacerbated in acute hypoxia, and these effects are independent of the relative exercise intensity.     

Effects of ischemia on VO2, tension, and vascular resistance in contracting canine skeletal muscle

This study examined the changes in O2 consumption (VO2), vascular resistance, and tension development during skeletal muscle contractions at reduced flow. We tested the hypothesis that when VO2 is limited by O2 supply, the skeletal muscle vasculature is not maximally dilated because of the fall in contractile force that accompanies the decrease in O2 supply. During 30 min of ischemic contractions, tension fell by 45 +/- 4% and VO2 fell 54 +/- 1% from preischemic levels. The O2 cost per unit tension did not change compared with nonischemic muscles. After the initial flow reduction, flow fell an additional 16 +/- 3% over 30 min. Adenosine infusion after 30 min of ischemic contractions increased flow by 42 +/- 3% but increased VO2 by only 9.8 +/- 2.3% and had no effect on tension development. When perfusion pressure was returned to normal after 30 min of ischemic contractions, twitch tension did not begin to recover within 20 min but tetanic tension showed a small improvement. VO2, although increased, remained well below the preischemic level. These results suggest that because of the reduced tension during ischemic contractions, the O2 supply-to-consumption ratio is nearly normal, which could explain the presence of the vasodilator reserve. The defect in tension development is long lived, producing a "stunned" muscle in which excess O2 supply does not restore function or VO2 to normal.     

Effects of adrenergic agonists and antagonists on muscle O2 uptake and lactate metabolism

To investigate adrenergic receptor-mediated responses in dog gastrocnemius-plantaris muscle, several catecholamine agonists, isoproterenol, epinephrine, norepinephrine, and phenylephrine, and two antagonists, propranolol and phenoxybenzamine, were given during repetitive, isotonic, tetanic contractions. The response variables that were measured were muscle blood flow, shortening during constant load contractions, and arterial and venous O2 and lactate concentrations. The calculated variables were O2 uptake (VO2), net lactic acid output (L), and power output. In the control experiments, the contractions increased VO2 to approximately 50 times rest by 2 min. Thereafter, shortening, work, and VO2 declined together by 17% at 30 min, indicating muscle fatigue. L increased rapidly to nearly 0.8 mumol X g-1 X min-1 by 2 min, declined to 0.3-0.4 mumol X g-1 X min-1 by 7 min, and was like rest at 15, 22.5, and 30 min. The arterial lactate concentration rose steadily from rest to 30 min of contractions. Epinephrine infusion stopped the decline of VO2 during the contractions, but this effect was not observed with the other agonists. Propranolol decreased VO2 compared with controls at 22.5 and 30 min of contractions. Phenoxybenzamine decreased VO2 compared with controls at all times during contraction, and the decline with time was present. Coinfusion of epinephrine with propranolol reduced the decline in VO2 observed with propranolol alone. Both epinephrine and isoproterenol increased L compared with controls. This epinephrine response was antagonized by propranolol but enhanced by phenoxybenzamine. Both isoproterenol and epinephrine infusions increased arterial lactate concentration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of insulin preincubation in the contractile reactivity of rat aortic rings.

Preincubation with physiological concentrations of insulin affects contractile reactivity of isolated smooth muscle cells. We studied the effects of insulin on intact aortic rings of Wistar rats preincubated 1-2 h with 240 pM (I1) and 960 pM (I2) insulin with and without NO synthesis inhibition by N(omega)-nitro-L-arginine methyl ester (L-NAME). Resting force was tripled by 0.1 mM L-NAME in control (C) and I1 groups, but not in I2 groups. I1 treatment decreased the tachyphylaxis to two successive 1 microM arginine vasopressin (AVP) stimulations. Single contractions elicited by 1 microM AVP, 1 microM angiotensin II (AngII), or 0.01 microM endothelin (ET1) were not affected by insulin preincubation in either maximal force (Fmax) or relaxation times. L-NAME enhanced Fmax of AngII contractions by about 75% in C, 120% in I1, and 74% in I2 groups; accordingly, it augmented the final steady-state force in C and I1 but not in I2. Similarly, L-NAME increased Fmax (30-40%) of AVP and ET1 contractions in C and I1 groups but failed to do so in contractions of I2 group. Results obtained with 10 microM indomethacin suggest that this is due to insulin stimulation of prostacyclin effects.     

Interactions between angiotensin II and nitric oxide during exercise in normal and heart failure rats.

We hypothesized that nitric oxide (NO) opposes ANG II-induced increases in arterial pressure and reductions in renal, splanchnic, and skeletal muscle vascular conductance during dynamic exercise in normal and heart failure rats. Regional blood flow and vascular conductance were measured during treadmill running before (unblocked exercise) and after 1) ANG II AT(1)-receptor blockade (losartan, 20 mg/kg ia), 2) NO synthase (NOS) inhibition [N(G)-nitro-L-arginine methyl ester (L-NAME); 10 mg/kg ia], or 3) ANG II AT(1)-receptor blockade + NOS inhibition (combined blockade). Renal conductance during unblocked exercise (4.79 +/- 0.31 ml x 100 g(-1) x min(-1) x mmHg(-1)) was increased after ANG II AT(1)-receptor blockade (6.53 +/- 0.51 ml x 100 g(-1) x min(-1) x mmHg(-1)) and decreased by NOS inhibition (2.12 +/- 0.20 ml x 100 g(-1) x min(-1) x mmHg(-1)) and combined inhibition (3.96 +/- 0.57 ml x 100 g(-1) x min(-1) x mmHg(-1); all P < 0.05 vs. unblocked). In heart failure rats, renal conductance during unblocked exercise (5.50 +/- 0.66 ml x 100 g(-1) x min(-1) x mmHg(-1)) was increased by ANG II AT(1)-receptor blockade (8.48 +/- 0.83 ml x 100 g(-1) x min(-1) x mmHg(-1)) and decreased by NOS inhibition (2.68 +/- 0.22 ml x 100 g(-1) x min(-1) x mmHg(-1); both P < 0.05 vs. unblocked), but it was unaltered during combined inhibition (4.65 +/- 0.51 ml x 100 g(-1) x min(-1) x mmHg(-1)). Because our findings during combined blockade could be predicted from the independent actions of NO and ANG II, no interaction was apparent between these two substances in control or heart failure animals. In skeletal muscle, L-NAME-induced reductions in conductance, compared with unblocked exercise (P < 0.05), were abolished during combined inhibition in heart failure but not in control rats. These observations suggest that ANG II causes vasoconstriction in skeletal muscle that is masked by NO-evoked dilation in animals with heart failure. Because reductions in vascular conductance between unblocked exercise and combined inhibition were less than would be predicted from the independent actions of NO and ANG II, an interaction exists between these two substances in heart failure rats. L-NAME-induced increases in arterial pressure during treadmill running were attenuated (P < 0.05) similarly in both groups by combined inhibition. These findings indicate that NO opposes ANG II-induced increases in arterial pressure and in renal and skeletal muscle resistance during dynamic exercise.     

Kinetics of .VO2 and femoral artery blood flow during heavy-intensity, knee-extension exercise.

It has been suggested that, during heavy-intensity exercise, O(2) delivery may limit oxygen uptake (.VO2) kinetics; however, there are limited data regarding the relationship of blood flow and .VO2 kinetics for heavy-intensity exercise. The purpose was to determine the exercise on-transient time course of femoral artery blood flow (Q(leg)) in relation to .VO2 during heavy-intensity, single-leg, knee-extension exercise. Five young subjects performed five to eight repeats of heavy-intensity exercise with measures of breath-by-breath pulmonary .VO2 and Doppler ultrasound femoral artery mean blood velocity and vessel diameter. The phase 2 time frame for .VO2 and Q(leg) was isolated and fit with a monoexponent to characterize the amplitude and time course of the responses. Amplitude of the phase 3 response was also determined. The phase 2 time constant for .VO2 of 29.0 s and time constant for Q(leg) of 24.5 s were not different. The change (Delta) in .VO2 response to the end of phase 2 of 0.317 l/min was accompanied by a DeltaQ(leg) of 2.35 l/min, giving a DeltaQ(leg)-to-Delta.VO2 ratio of 7.4. A slow-component .VO2 of 0.098 l/min was accompanied by a further Q(leg) increase of 0.72 l/min (DeltaQ(leg)-to-Delta.VO2 ratio = 7.3). Thus the time course of Q(leg) was similar to that of muscle .VO2 (as measured by the phase 2 .VO2 kinetics), and throughout the on-transient the amplitude of the Q(leg) increase achieved (or exceeded) the Q(leg)-to-.VO2 ratio steady-state relationship (ratio approximately 4.9). Additionally, the .VO2 slow component was accompanied by a relatively large rise in Q(leg), with the increased O(2) delivery meeting the increased Vo(2). Thus, in heavy-intensity, single-leg, knee-extension exercise, the amplitude and kinetics of blood flow to the exercising limb appear to be closely linked to the .VO2 kinetics.     

Role of ET(A) receptors in the regulation of vascular reactivity in rats with congestive heart failure

Endothelium-derived nitric oxide (NO) and endothelin (ET)-1 interact to regulate vascular tone. In congestive heart failure (CHF), the release and/or the activity of both factors is affected. We hypothesized that the increased ET-1 production associated with CHF may result in a reduced smooth muscle sensitivity to NO. The aim of this study was to evaluate the effects of a chronic treatment with the ET(A)-receptor (ET receptor A) antagonist LU-135252 (LU) on cerebrovascular reactivity to sodium nitroprusside (SNP) in the rat infarct model of CHF. Rats were subjected to coronary artery ligation and were treated for 4 wk with placebo (n = 24) or LU (50 mg. kg(-1). day(-1), n = 29). CHF was associated with a decreased (P < 0.05) efficacy of SNP to induce relaxation of isolated middle cerebral arteries. Furthermore, neither NO synthase inhibition with N(omega)-nitro-L-arginine (L-NNA) nor endothelial denudation affected the efficacy of SNP. Thus the endothelium no longer influences smooth muscle sensitivity to SNP. LU treatment, however, normalized (P < 0.05) smooth muscle sensitivity to SNP. Sensitivity of ET-1-induced contraction was increased in CHF only in the presence of L-NNA, whereas contraction induced by ET(B) receptor (receptor B) stimulation was increased (P < 0.05) in endothelium-denuded vessels. LU treatment restored these changes in reactivity and revealed a significant endothelium-dependent ET(B)-mediated relaxation after NO synthase inhibition. In conclusion, CHF decreases and uncouples cerebrovascular smooth muscle sensitivity to SNP from endothelial regulation. The observation that chronic ET(A) blockade restored most of the changes associated with CHF suggests that activation of the ET-1 system importantly contributes to the alteration in vascular reactivity observed in experimental CHF.     

Membrane depolarization and NADPH oxidase activation in aortic endothelium during ischemia reflect altered mechanotransduction

Vascular endothelial growth factor (VEGF) is considered to be important in promotion of capillary growth in skeletal muscles exposed to increased activity. We studied its interactions with nitric oxide (NO) by examining the expression of endothelial NO synthase (NOS), VEGF, and VEGF receptor-2 (VEGFR-2) proteins in relation to capillary growth in rat extensor digitorum longus muscles electrically stimulated for 2, 4, or 7 days with and without NOS inhibition by N(omega)-nitro-L-arginine (L-NNA, 3 mg/day). Stimulation increased all proteins from 2 days onward, concomitantly with capillary proliferation (labeling for proliferating cell nuclear antigen). Capillary-to-fiber ratio was elevated by 25% after 7 days. Concurrent oral administration of L-NNA did not affect the increase in endothelial NOS but depressed its activity, as shown by increased blood pressure and decreased arteriolar diameters in 2-day-stimulated muscles. NOS inhibition eliminated the increased expression of VEGFR-2 and VEGF proteins in muscles stimulated for 2 and 4 days but not for 7 days. However, it depressed capillary proliferation and the increase in C/F at all time points. We conclude that, in stimulated muscles, NO, generated by activation of neuronal NOS by muscle activity or endothelial NOS by increased blood flow and capillary shear stress, may increase capillary proliferation in the early stages of stimulation through upregulation of VEGFR-2 and VEGF. With longer stimulation, capillary growth appears to require NO, and high levels of VEGF and VEGFR-2 may be contributing to maintenance of the increased capillary bed.     

Oxidation/reduction state of cytochrome oxidase during repetitive contractions

There is disagreement regarding whether inadequate O2 determines maximal O2 uptake (VO2max) and lactic acid output (L) during muscular activity. Direct assessment of mitochondrial cytochrome oxidase (cytochrome a-a3) oxidation/reduction (O/R) state should provide an unequivocal answer for this issue. A new near-infrared spectrophotometric method was used to measure the O/R state of cytochrome a-a3 of dog gastrocnemius-plantaris muscle in situ during repetitive isotonic twitch and tetanic contractions. Three contraction frequencies were used for each contraction type in alternating sequence to provide a wide range of VO2 up to VO2max. VO2 and L were measured after 3 and 9 min of a 10-min contraction period, and 15 min were allowed for recovery between contraction periods. VO2 increased with contraction frequency. L was variably increased with contraction frequency at 3 min and uptake usually occurred at 9 min, except at the highest tetanic frequency. The O/R span of cytochrome a-a3 was determined by respiring the animals with 100% N2 to determine the most reduced state. This was followed by respiration with 100% O2, which gave the most oxidized state transiently during recovery. Within this span in muscles at rest, cytochrome a-a3 was 50-80% oxidized. During contractions of both types at all frequencies, cytochrome a-a3 always became more oxidized by an additional 10-20%. These findings should put to rest any arguments that inadequate O2 is a determinant of VO2max or L under the conditions of these experiments: repetitive contractions with free flow in self-perfused muscles and normoxia.     

Influence of nitric oxide on vascular resistance and muscle mechanics during tetanic contractions in situ.

Studies of the effect of nitric oxide (NO) synthesis inhibition were performed in the isometrically contracting blood-perfused canine gastrocnemius-plantaris muscle group. Muscle blood flow (Q) was controlled with a pump during continuous NO blockade produced with either 1 mM L-argininosuccinic acid (L-ArgSA) or N(G)-nitro-L-arginine methyl ester (L-NAME) during repetitive tetanic contractions (50-Hz trains, 200-ms duration, 1/s). Pump Q was set to match maximal spontaneous Q (1.3-1.4 ml. min(-1). g(-1)) measured in prior, brief (3-5 min) control contraction trials in each muscle. Active tension and oxygen uptake were 500-600 g/g and 200-230 microl. min(-1). g(-1), respectively, under these conditions. Within 3 min of L-ArgSA infusion, vascular resistance across the muscle (R(v)) increased significantly (from approximately 100 to 300 peripheral resistance units; P < 0.05), whereas R(v) increased to a lesser extent with L-NAME (from approximately 100 to 175 peripheral resistance units; P < 0.05). The increase in R(v) with L-ArgSA was unchanged by simultaneous infusion of 0.5-10 mM L-arginine but was reduced with 1-3 microg/ml sodium nitroprusside (41-54%). The increase in R(v) with L-NAME was reversed with 1 mM of L-arginine. Increased fatigue occurred with infusion of L-ArgSA; active tension and intramuscular pressure decreased by 62 and 66%, whereas passive tension and baseline intramuscular pressure increased by 80 and 30%, respectively. These data indicate a possible role for NO in the control of R(v) and contractility within the canine gastrocnemius-plantaris muscle during repetitive tetanic contractions.     

Progressive recruitment of muscle fibers is not necessary for the slow component of VO2 kinetics.

The "slow component" of O2 uptake (VO2) kinetics during constant-load heavy-intensity exercise is traditionally thought to derive from a progressive recruitment of muscle fibers. In this study, which represents a reanalysis of data taken from a previous study by our group (Grassi B, Hogan MC, Greenhaff PL, Hamann JJ, Kelley KM, Aschenbach WG, Constantin-Teodosiu D, Gladden LB. J Physiol 538: 195-207, 2002) we evaluated the presence of a slow component-like response in the isolated dog gastrocnemius in situ (n=6) during 4 min of contractions at approximately 60-70% of peak VO2. In this preparation all muscle fibers are maximally activated by electrical stimulation from the beginning of the contraction period, and no progressive recruitment of fibers is possible. Muscle VO2 was calculated as blood flow multiplied by arteriovenous O2 content difference. The muscle fatigued (force decreased by approximately 20-25%) during contractions. Kinetics of adjustment were evaluated for 1) VO2, uncorrected for force development; 2) VO2 normalized for peak force; 3) VO2 normalized for force-time integral. A slow component-like response, described in only one muscle out of six when uncorrected VO2 was considered, was observed in all muscles when VO2/peak force and VO2/force-time were considered. The amplitude of the slow component-like response, expressed as a fraction of the total response, was higher for VO2/peak force (0.18+/-0.06, means+/-SE) and for VO2/force-time (0.22+/-0.05) compared with uncorrected VO2 (0.04+/-0.04). A progressive recruitment of muscle fibers may not be necessary for the development of the slow component of VO2 kinetics, which may be caused by the metabolic factors that induce muscle fatigue and, as a consequence, reduce the efficiency of muscle contractions.     

Ouabain-induced hypertension is accompanied by increases in endothelial vasodilator factors

The involvement of nitric oxide (NO), prostaglandins, and calcium-dependent potassium channel (K(Ca)) activators on the negative modulation of phenylephrine-induced contractions was evaluated on the isolated aorta and caudal (CAU) artery obtained from rats treated with ouabain for 5 wk to induce hypertension. In ouabain-treated rats, the reactivity to phenylephrine was reduced in the endothelium-intact aorta but not the CAU segments. Endothelial modulation of phenylephrine contraction, as demonstrated by endothelium removal, NO synthase (NOS) inhibition with N(omega)-nitro-L-arginine methyl ester and aminoguanidine, as well as K(Ca) inhibition with tetraethylammonium, was more pronounced in segments from ouabain-treated animals, and here greater effects were seen in the aorta than in CAU. An increased expression of endothelial NOS and neuronal NOS was seen in the aorta after ouabain treatment. In CAU, only endothelial NOS was detected and ouabain treatment did not alter its expression. These results suggest that ouabain-induced hypertension is accompanied by increased NO release derived from endothelial NOS and neuronal NOS and increased release of an endothelial hyperpolarizing factor that presumably opens K(Ca), all of which contribute to the increased negative modulation of the phenylephrine contraction.     

Acute effects of hydrogen peroxide on skeletal muscle microvascular oxygenation from rest to contractions

Reactive oxygen species, such as hydrogen peroxide (H(2)O(2)), exert a critical regulatory role on skeletal muscle function. Whether acute increases in H(2)O(2) modulate muscle microvascular O(2) delivery-utilization (Qo(2)/Vo(2)) matching [i.e., microvascular partial pressure of O(2) (Pmv(O(2)))] at rest and following the onset of contractions is unknown. The hypothesis was tested that H(2)O(2) treatment (exogenous H(2)O(2)) would enhance Pmv(O(2)) and slow Pmv(O(2)) kinetics during contractions compared with control. Anesthetized, healthy young Sprague-Dawley rats had their spinotrapezius muscles either exposed for measurement of blood flow (and therefore QO(2)), VO(2), and Pmv(O(2)), or exteriorized for measurement of force production. Electrically stimulated twitch contractions (1 Hz, ~7 V, 2-ms pulse duration, 3 min) were evoked following acute superfusion with Krebs-Henseleit (control) and H(2)O(2) (100 μM). Relative to control, H(2)O(2) treatment elicited disproportionate increases in QO(2) and VO(2) that elevated Pmv(O(2)) at rest and throughout contractions and slowed overall Pmv(O(2)) kinetics (i.e., ~85% slower mean response time; P < 0.05). Accordingly, H(2)O(2) resulted in ~33% greater overall Pmv(O(2)), as assessed by the area under the Pmv(O(2)) curve (P < 0.05). Muscle force production was not altered with H(2)O(2) treatment (P > 0.05), evidencing reduced economy during contractions (~40% decrease in the force/VO(2) relationship; P < 0.05). These findings indicate that, although increasing the driving force for blood-myocyte O(2) flux (i.e., Pmv(O(2))), transient elevations in H(2)O(2) impair skeletal muscle function (i.e., reduced economy during contractions), which mechanistically may underlie, in part, the reduced exercise tolerance in conditions associated with oxidative stress.     

Kinetics of muscle deoxygenation and microvascular PO(2) during contractions in rat: comparison of optical spectroscopy and phosphorescence-quenching techniques

The overarching presumption with near-infrared spectroscopy measurement of muscle deoxygenation is that the signal reflects predominantly the intramuscular microcirculatory compartment rather than intramyocyte myoglobin (Mb). To test this hypothesis, we compared the kinetics profile of muscle deoxygenation using visible light spectroscopy (suitable for the superficial fiber layers) with that for microvascular O(2) partial pressure (i.e., Pmv(O(2)), phosphorescence quenching) within the same muscle region (0.5～1 mm depth) during transitions from rest to electrically stimulated contractions in the gastrocnemius of male Wistar rats (n = 14). Both responses could be modeled by a time delay (TD), followed by a close-to-exponential change to the new steady level. However, the TD for the muscle deoxygenation profile was significantly longer compared with that for the phosphorescence-quenching Pmv(O(2)) [8.6 ± 1.4 and 2.7 ± 0.6 s (means ± SE) for the deoxygenation and Pmv(O(2)), respectively; P < 0.05]. The time constants (τ) of the responses were not different (8.8 ± 4.7 and 11.2 ± 1.8 s for the deoxygenation and Pmv(O(2)), respectively). These disparate (TD) responses suggest that the deoxygenation characteristics of Mb extend the TD, thereby increasing the duration (number of contractions) before the onset of muscle deoxygenation. However, this effect was insufficient to increase the mean response time. Somewhat differently, the muscle deoxygenation response measured using near-infrared spectroscopy in the deeper regions (～5 mm depth) (～50% type I Mb-rich, highly oxidative fibers) was slower (τ = 42.3 ± 6.6 s; P < 0.05) than the corresponding value for superficial muscle measured using visible light spectroscopy or Pmv(O(2)) and can be explained on the basis of known fiber-type differences in Pmv(O(2)) kinetics. These data suggest that, within the superficial and also deeper muscle regions, the τ of the deoxygenation signal may represent a useful index of local O(2) extraction kinetics during exercise transients.     

Activity of nitric oxide synthase in the ventilatory muscle vasculature

We evaluated in the in situ vascularly isolated canine diaphragm the role of nitric oxide (NO) in the regulation of basal vascular resistance and vascular responses to increased muscle activity (active hyperemia), brief occlusions of the phrenic artery (reactive hyperemia), and changes in arterial pressure. The vasculature of the left hemidiaphragm was either pump-perfused at a fixed flow rate or autoperfused with arterial blood from the femoral artery. Endothelial nitric oxide synthase (NOS) activity was inhibited by intraphrenic infusion of L-arginine analogues such as N(G)-nitro-L-arginine, N(G)-nitro-L-arginine methyl ester and argininosuccinic acid. Active hyperemia was produced by low (2 Hz) frequency stimulation of the left phrenic nerve. Reactive hyperemia was measured in response to 10, 20, 30, 60, and 120 sec duration occlusions of the left phrenic artery and was quantified in terms of postocclusive blood flow, vascular resistance, hyperemic duration, and hyperemic volume. Infusion of NOS inhibitors into the vasculature of the resting diaphragm increased phrenic vascular resistance significantly and to a similar extent. Reactive hyperemic volume and reactive hyperemic duration were also significantly attenuated after NOS inhibition, however, peak reactive hyperemic dilation was not influenced by NOS inhibition. It was also found that enhanced NO release contribute by about 41% to active dilation elicited by continuous 2 Hz stimulation. In addition, NOS inhibition had no effect on O2 consumption of the resting diaphragm, but significantly attenuated the rise in diaphragmatic O2 consumption during during 2 Hz stimulation. The decline in diaphragmatic O2 consumption was due to reduction in blood flow. These results indicate that NO release plays a significant role in the regulation of diaphragmatic vascular tone and O2 consumption.     

Chronic cold exposure increases skeletal muscle oxidative structure and function in Monodelphis domestica, a marsupial lacking brown adipose tissue

Monodelphis domestica (Marsupialia: Didelphidae) was used as a model animal to investigate and compare muscle adaptation to exercise training and cold exposure. The experimental treatment consisted of four groups of animals: either warm or cold acclimation temperature and with or without endurance exercise training. Maximal aerobic capacity during a running VO2max test in the warm-exercised or cold-exposed (with or without exercise) groups was about 130 mL O(2)/kg/min, significantly higher than the warm-acclimated controls at 113.5 mL O(2)/kg/min. Similarly, during an acute cold challenge (VO2summit), maximal aerobic capacity was higher in these three experimental groups at approximately 95 mL O(2)/kg/min compared with 80.4 mL O(2)/kg/min in warm-acclimated controls. Respiratory exchange ratio was significantly lower (0.89-0.68), whereas relative heart mass (0.52%-0.73%) and whole-body muscle mitochondrial volume density (2.59 to 3.04 cm(3)) were significantly higher following cold exposure. Chronic cold exposure was a stronger stimulus than endurance exercise training for tissue-specific adaptations. Although chronic cold exposure and endurance exercise are distinct challenges, physiological adaptations to each overlap such that the capacities for aerobic performance in response to both cold exposure and running are increased by either or both treatments.     

Effects of chronic N(omega)-nitro-L-arginine methyl ester administration on glucose tolerance and skeletal muscle glucose transport in the rat.

It has been suggested that nitric oxide (NO) is a key regulator of carbohydrate metabolism in skeletal muscle. The present study was undertaken to examine the effects of chronic in vivo competitive antagonism of NO synthase (NOS) by the administration of N(omega)-nitro-L-arginine methyl ester (L-NAME) in the drinking water (1 mg/ml) for 14 days on glucose tolerance and skeletal muscle glucose transport in rats. Oral glucose tolerance tests (OGTT) revealed an impaired glucose tolerance in the L-NAME-treated rats as reflected by the area under the glucose curve (4675 +/- 514 mg% x 120 min (control) vs 6653 +/- 571 mg% x 120 min (L-NAME treated); P < 0.03). While a large rise in plasma insulin concentration was present in the control rats (0.87 +/- 0.34 ng/ml, P < 0.001) during the first 15 min of the OGTT, rises in plasma insulin concentration were absent in the L-NAME-treated rats (0.18 +/- 0.13 ng/ml, P = NS). Intravenous glucose tolerance tests confirmed an impaired insulin secretion in the L-NAME-treated rats. In contrast, insulin-stimulated 2-deoxyglucose transport was enhanced (P < 0.03) by chronic NOS inhibition (5.29 +/- 0.83 nmol/g/min) compared to control rats (2.21 +/- 0.90 nmol/g/min). Plasma sodium concentrations were lower and plasma potassium concentrations were higher in the L-NAME-treated group, indicating an impaired electrolyte status. We conclude that chronic in vivo administration of a NOS inhibitor, while not impairing basal parameters of carbohydrate metabolism, may manifest different responses than acute exposure to the same agent in vitro.