Increasing temperature speeds intracellular PO2 kinetics during contractions in single Xenopus skeletal muscle fibers.

Precise determination of the effect of muscle temperature (T(m)) on mitochondrial oxygen consumption kinetics has proven difficult in humans, in part due to the complexities in controlling for T(m)-related variations in blood flow, fiber recruitment, muscle metabolism, and contractile properties. To address this issue, intracellular Po(2) (P(i)(O(2))) was measured continuously by phosphorescence quenching following the onset of contractions in single Xenopus myofibers (n = 24) while controlling extracellular temperature. Fibers were subjected to two identical contraction bouts, in random order, at 15°C (cold, C) and 20°C (normal, N; n = 12), or at N and 25°C (hot, H; n = 12). Contractile properties were determined for every contraction. The time delay of the P(i)(O(2)) response was significantly greater in C (59 ± 35 s) compared with N (35 ± 26 s, P = 0.01) and H (27 ± 14 s, P = 0.01). The time constant for the decline in P(i)(O(2)) was significantly greater in C (89 ± 34 s) compared with N (52 ± 15 s; P < 0.01) and H (37 ± 10 s; P < 0.01). There was a linear relationship between the rate constant for P(i)(O(2)) kinetics and T(m) (r = 0.322, P = 0.03). Estimated ATP turnover was significantly greater in H than in C (P < 0.01), but this increased energy requirement alone with increased T(m) could not account for the differences observed in P(i)(O(2)) kinetics among conditions. These results demonstrate that P(i)(O(2)) kinetics in single contracting myofibers are dependent on T(m), likely caused by temperature-induced differences in metabolic demand and by temperature-dependent processes underlying mitochondrial activation at the start of muscle contractions.

Measurement of activation energy and oxidative phosphorylation onset kinetics in isolated muscle fibers in the absence of cross-bridge cycling.

This study utilized N-benzyl-p-toluene sulfonamide (BTS), a potent inhibitor of cross-bridge cycling, to measure 1) the relative metabolic costs of cross-bridge cycling and activation energy during contraction, and 2) oxygen uptake kinetics in the presence and absence of myosin ATPase activity, in isolated Xenopus laevis muscle fibers. Isometric tension development and either cytosolic Ca2+ concentration ([Ca2+]c) or intracellular Po2 (PiO2) were measured during contractions at 20 degrees C in control conditions (Con) and after exposure to 12.5 microM BTS. BTS attenuated tension development to 5+/-0.4% of Con but did not affect either resting or peak [Ca2+]c during repeated isometric contractions. To determine the relative metabolic cost of cross-bridge cycling, we measured the fall in PiO2) (DeltaPiO2; a proxy for Vo2) during contractions in Con and BTS groups. BTS attenuated DeltaP(iO2) by 55+/-6%, reflecting the relative ATP cost of cross-bridge cycling. Thus, extrapolating DeltaPiO2 to a value that would occur at 0% tension suggests that actomyosin ATP requirement is approximately 58% of overall ATP consumption during isometric contractions in mixed fiber types. BTS also slowed the fall in PiO2) (time to 63% of overall DeltaPiO2) from 75+/-9 s (Con) to 101+/-9 s (BTS) (P<0.05), suggesting an important role of the products of ATP hydrolysis in determining the Vo2 onset kinetics. These results demonstrate in isolated skeletal muscle fibers that 1) activation energy accounts for a substantial proportion (approximately 42%) of total ATP cost during isometric contractions, and 2) despite unchanged [Ca2+]c transients, a reduced rate of ATP consumption results in slower Vo2 onset kinetics.

Inclined running increases pulmonary haemorrhage in the Thoroughbred horse.

REASONS FOR PERFORMING STUDY: Capillary stress failure-induced (exercise-induced) pulmonary haemorrhage (EIPH) during intense running in horses is thought to involve both intravascular (i.e. mean pulmonary arterial pressure [Ppa] > 100 mmHg) and extravascular (e.g. negative inspiratory pressure swings) mechanisms. HYPOTHESIS: That inclined running would reduce breathing frequency (coupled to stride frequency) and increase tidal volume thus increasing lung volume changes and intrapleural pressure swings resulting in more pronounced EIPH. METHODS: Six Thoroughbred horses were run to volitional fatigue (incremental step test) on a level (L) and inclined (I; 10%) treadmill in random order. Pulmonary minute ventilation, arterial blood gases and mean Ppa were obtained during each run while EIPH severity was quantified via bronchoalveolar lavage (BAL) 30 mins post run. RESULTS: Time to fatigue did not differ between trials (P > 0.05). At end-exercise, breathing frequency was reduced (L, 127.8 +/- 3.0; I, 122.6 +/- 2.1 breaths/min; P < 0.05) and tidal volume increased (L, 11.5 +/- 0.6; I, 13.1 +/- 0.5 L; P < 0.05) during inclined running. No differences existed in end-exercise plasma [lactate] between trials (L, 24.5 +/- 2.9; I, 26.2 +/- 3.4 mmol/l, P > 0.05); however, the mean peak Ppa was reduced during the inclined run (L, 105+5; I, 96 +/- 4 mmHg, P < 0.05). In the face of reduced Ppa, EIPH severity was increased significantly (P < 0.05) during the inclined vs. level run (L, 37.0 +/- 11.7; I, 49.6 +/- 17.0 x 10(6) red blood cells/ml BAL fluid). CONCLUSIONS: Although inclined running lowered peak Ppa, EIPH severity was increased. It is likely that this effect resulted, in part, from an altered ventilatory pattern (i.e. increased tidal volumes and associated intrapleural pressure changes). POTENTIAL RELEVANCE: This conclusion supports an important role for extravascular factors in the aetiology of EIPH.

Dynamics of microvascular oxygen partial pressure in contracting skeletal muscle of rats with chronic heart failure.

OBJECTIVE: This investigation tested the hypothesis that the dynamics of muscle microvascular O(2) pressure (PO(2)m, which reflects the ratio of O(2) utilization [V*O(2)] to O(2) delivery [Q*O(2)]) following the onset of contractions would be altered in chronic heart failure (CHF). METHODS: Female Sprague-Dawley rats were subjected to a myocardial infarction (MI) or a sham operation (Sham). Six to 10 weeks post Sham (n=6) or MI (n=17), phosphorescence quenching techniques were utilized to determine PO(2)m dynamics at the onset of spinotrapezius muscle contractions (1 Hz). RESULTS: MI rats were separated into groups with Moderate (n=10) and Severe (n=7) CHF based upon the degree of left ventricular (LV) dysfunction as indicated by structural abnormalities (increased right ventricle weight and lung weight normalized to body weight). LV end-diastolic pressure was elevated significantly in both CHF groups compared with Sham (Sham, 3+/-1; Moderate CHF, 9+/-2; Severe CHF, 27+/-4 mmHg, P<0.05). The PO(2)m response was modeled using time delay and exponential components to fit the PO(2)m response to the steady-state. Compared with Shams, the time constant (tau) of the primary PO(2)m response was significantly speeded in Moderate CHF (tau, Sham, 19.0+/-1.5; Moderate CHF, 13.2+/-1.9 s, P<0.05) and slowed in Severe CHF (tau, 28.2+/-3.4 s, P<0.05). Within the Severe CHF group, tau increased linearly with the product of right ventricular and lung weight (r=0.83, P<0.05). CONCLUSIONS: These results suggest that CHF alters the dynamic matching of muscle V*O(2)-to-Q*O(2) across the transition from rest to contractions and that the nature of that perturbation is dependent upon the severity of cardiac dysfunction.

Movements of thoracic and abdominal compartments during ventilation at rest and during exercise.

The present investigation utilised simultaneous measurements of chest (Ch) and abdominal (Ab) circumferences and respiratory airflow to test the hypothesis that Ch circumferential expansion contributes proportionally little to tidal volume in the running Thoroughbred. During exercise, there were only small changes in Ch and Ab circumference and no increase with increasing tidal volume. At rest, walk and trot, the flow, Ch and Ab signals were in phase. However, during canter and gallop, the Ch and Ab changes were 180 degrees out of phase with each other and both were out of phase with airflow. In contrast to exercise, increase in ventilation at rest achieved by administration of lobeline resulted in a 4-6-fold increase in tidal volume; large excursions of the chest were always in phase with airflow. Furthermore, 3 horses showed an increase in chest circumference, demonstrating that chest stiffness per se does not preclude chest circumferential expansion. In conclusion, in the absence of significant increases in either Ch or Ab expansion during running, elongation of the thoracoabdominal segment may be the main determinant of tidal volume.

Effect of body incline on cardiac performance.

Maximal cardiac performance is improved in man during upright compared to supine exercise. Whether cardiac performance in quadrupeds is dependent upon body position is unknown. Therefore, we undertook the present investigation to determine if peak cardiac output (Qpeak) would be influenced by body inclination in the Thoroughbred horse. To test the hypothesis, four Thoroughbred horses performed an incremental exercise protocol (speed increased by 1 m/s/min to fatigue) on both a level (L) and inclined (I: 6 degrees) treadmill. Specifically, we hypothesised that Qpeak would be increased on the incline, as this represents a progression towards upright exercise. Cardiac output was determined using the Fick relationship from continuous measurements of pulmonary VO2 and paired arterial (carotid artery or transverse facial) and mixed venous (pulmonary artery) samples. Qpeak was significantly increased on the incline (L: 279 +/- 20; I: 336 +/- 17 l/min; P<0.05), while CaO2 was not significantly different (L: 25.5 +/- 1.1; I: 25.4 +/- 1.9 ml/100 ml), and therefore, whole body O2 delivery (QO2) was significantly increased (L: 70.7 +/- 4.9; I: 84.4 +/- 3.1 l/min; P<0.05). In conclusion, within the scope of this investigation, these data suggest that cardiac performance, as judged by increased Qpeak and QO2, is enhanced in the inclined body position. Furthermore, these findings provide preliminary information that level and incline treadmill exercise tests may yield significantly different results in the Thoroughbred horse and consequently this factor should be considered when interpreting exercise testing and performance data.

NO inhalation reduces pulmonary arterial pressure but not hemorrhage in maximally exercising horses.

In horses, the exercise-induced elevation of pulmonary arterial pressure (Ppa) is thought to play a deterministic role in exercise-induced pulmonary hemorrhage (EIPH), and thus treatment designed to lower Ppa might reasonably be expected to reduce EIPH. Five Thoroughbred horses were run on a treadmill to volitional fatigue (incremental step test) under nitric oxide (NO; inhaled 80 ppm) and control (N(2), same flow rate as per NO run) conditions (2 wk between trials; order randomized) to test the hypothesis that NO inhalation would reduce maximal Ppa but that this reduction may not necessarily reduce EIPH. Before each investigation, a microtipped pressure transducer was placed in the pulmonary artery 8 cm past the pulmonic valve to monitor Ppa. EIPH severity was assessed via bronchoalveolar lavage (BAL) 30 min postrun. Exercise time did not differ between the two trials (P > 0.05). NO administration resulted in a small but consistent and significant reduction in peak Ppa (N(2), 102.3 +/- 4.4; NO, 98.6 +/- 4.3 mmHg, P < 0.05). In the face of lowered Ppa, EIPH severity was significantly higher in the NO trial (N(2), 22.4 +/- 6.8; NO, 42.6 +/- 15.4 x 10(6) red blood cells/ml BAL fluid, P < 0.05). These findings support the notion that extremely high Ppa may reflect, in part, an arteriolar vasoconstriction that serves to protect the capillary bed from the extraordinarily high Ppa evoked during maximal exercise in the Thoroughbred horse. Furthermore, these data suggest that exogenous NO treatment during exercise in horses may not only be poor prophylaxis but may actually exacerbate the severity of EIPH.

Efficacy of nasal strip and furosemide in mitigating EIPH in Thoroughbred horses.

The purpose of this investigation was to study the effects of an equine nasal strip (NS), furosemide (Fur), and a combination of both (NS + Fur) on exercise-induced pulmonary hemorrhage (EIPH) at speeds corresponding to near-maximal effort. Five Thoroughbreds (526 +/- 25 kg) were run on a flat treadmill from 7 to 14 m/s in 1 m x s(-1) x min(-)1 increments every 2 wk (treatment order randomized) under control (Con), Fur (1 mg/kg iv 4 h prior), NS, or NS + Fur conditions. During each run, pulmonary arterial (Ppa) and esophageal (Pes) pressures were measured. Severity of EIPH was quantified via bronchoalveolar lavage (BAL) 30 min postrun. Furosemide (Fur and NS + Fur trials) reduced peak Ppa approximately 7 mmHg compared with Con (P < 0.05) whereas NS had no effect (P > 0.05). Maximal Pes swings were not different among groups (P > 0.05). NS significantly diminished EIPH compared with the Con trial [Con, 55.0 +/- 36.2; NS, 30.8 +/- 21.8 x 10(6) red blood cells (RBC)/ml BAL fluid; P < 0.05]. Fur reduced EIPH to a greater extent than NS (5.2 +/- 3.0 x 10(6) RBC/ml BAL; P < 0.05 vs. Con and NS) with no additional benefit from NS + Fur (8.5 +/- 4.2 x 10(6) RBC/ml BAL; P > 0.05 vs. Fur, P < 0.05 vs. Con and NS). In conclusion, although both modalities (NS and Fur) were successful in mitigating EIPH, neither abolished EIPH fully as evaluated via BAL. Fur was more effective than NS in constraining the severity of EIPH. The simultaneous use of both interventions appears to offer no further gain with respect to reducing EIPH.

Effect of L-NAME on oxygen uptake kinetics during heavy-intensity exercise in the horse.

There is evidence that oxidative enzyme inertia plays a major role in limiting/setting the O(2) uptake (VO(2)) response at the transition to higher metabolic rates and also that nitric oxide (NO) competitively inhibits VO(2) within the electron transport chain. To investigate whether NO is important in setting the dynamic response of VO(2) at the onset of high-intensity (heavy-domain) running in horses, five geldings were run on a treadmill across speed transitions from 3 m/s to speeds corresponding to 80% of peak VO(2) with and without nitro-L-arginine methyl ester (L-NAME), an NO synthase inhibitor (20 mg/kg; order randomized). L-NAME did not alter (both P > 0.05) baseline (3 m/s, 15.4 +/- 0.3 and 16.2 +/- 0.5 l/min for control and L-NAME, respectively) or end-exercise VO(2) (56.9 +/- 5.1 and 55.2 +/- 5.8 l/min for control and L-NAME, respectively). However, in the L-NAME trial, the primary on-kinetic response was significantly (P < 0.05) faster (i.e., reduced time constant, 27.0 +/- 2.7 and 18.7 +/- 3.0 s for control and L-NAME, respectively), despite no change in the gain of VO(2) (P > 0.05). The faster on-kinetic response was confirmed independent of modeling by reduced time to 50, 63, and 75% of overall VO(2) response (all P < 0.05). In addition, onset of the VO(2) slow component occurred earlier (124.6 +/- 11.2 and 65.0 +/- 6.6 s for control and L-NAME, respectively), and the magnitude of the O(2) deficit was attenuated (both P < 0.05) in the L-NAME compared with the control trial. Acceleration of the VO(2) kinetics by L-NAME suggests that NO inhibition of mitochondrial VO(2) may contribute, in part, to the intrinsic metabolic inertia evidenced at the transition to higher metabolic rates in the horse.

Rat muscle microvascular PO2 kinetics during the exercise off-transient.

Dependent upon the relative speed of pulmonary oxygen consumption (VO2) and blood flow (Q) kinetics, the exercise off-transient may represent a condition of sub- or supra-optimal perfusion. To date, there are no direct measurements of the dynamics of the VO2/Q relationship within the muscle at the onset of the work/recovery transition. To address this issue, microvascular PO2 (PO2,m) dynamics were studied in the spinotrapezius muscles of 11 female Sprague-Dawley rats (weight approximately 220 g) during and following electrical stimulation (1 Hz) to assess the adequacy of Q. relative to VO2 post exercise. The exercise blood flow response (radioactive microspheres: muscle Q increased approximately 240 %), and post-exercise arterial blood pH (7.40 +/- 0.02) and blood lactate (1.3 +/- 0.4 mM x l(-1)) values were consistent with moderate-intensity exercise. Recovery PO2,m (i.e. off-transient) rose progressively until baseline values were achieved ((Delta)end-recovery exercise PO2,m, 14.0 +/- 1.9 Torr) and at no time fell below exercising PO2,m. The off-transient PO2,m was well fitted by a dual exponential model with both fast (tau = 25.4 +/- 5.1 s) and slow (tau = 71.2 +/- 34.2 s) components. Furthermore, there was a pronounced delay (54.9 +/- 10.7 s) before the onset of the slow component. These data, obtained at the muscle microvascular level, support the notion that muscle VO2 falls with faster kinetics than muscle Q during the off-transient, such that PO2,m increases systematically, though biphasically, during recovery.

Effects of emphysema on diaphragm microvascular oxygen pressure.

Pulmonary emphysema impairs lung and respiratory muscle function leading to restricted physical capacity and accelerated morbidity and mortality consequent to respiratory muscle failure. In the absence of direct evidence, an O2 supply-demand imbalance within the diaphragm and other respiratory muscles in emphysema has been considered the most likely explanation for this failure. To test this hypothesis, we utilized phosphorescence quenching techniques to measure mean microvascular PO2 (PO2m) within the medial costal diaphragm of control (C, n = 10) and emphysematous (E, elastase instilled, n = 7) hamsters. PO2m and mean arterial pressure (MAP) were measured in the spontaneously breathing anesthetized hamster at inspired O2 percentages of 10, 21, and 100, and across a range of mean MAPs from 40 to 115 mm Hg. At each inspired O2, diaphragm PO2m was significantly (p < 0.05) lower in E animals (10%: C, 19 +/- 3; E, 9 +/- 2; 21%: C, 32 +/- 2; E, 21 +/- 2; 100%: C, 60 +/- 8; E, 36 +/- 9 mm Hg). At 21% inspired O2, the PO2m decrease was correlated with reduced MAP in both C (r = 0.968) and E (r = 0.976) animals. We conclude that diaphragmatic PO2m (and therefore microvascular O2 content) is decreased in emphysematous hamsters reflecting a greater diaphragmatic O2 utilization at rest and a lower O2 extraction reserve. According to Fick's law, this lower PO2m will mandate an exaggerated fall in intramyocyte PO2, which is expected to accelerate muscle glycogen depletion and consequently fatigue. This provides empirical evidence in support of one possible mechanism for respiratory muscle failure in emphysema.

Dynamics of microvascular oxygen pressure across the rest-exercise transition in rat skeletal muscle.

There exists substantial controversy as to whether muscle oxygen (O2) delivery (QO2) or muscle mitochondrial O2 demand determines the profile of pulmonary VO2 kinetics in the rest-exercise transition. To address this issue, we adapted intravascular phosphorescence quenching techniques for measurement of rat spinotrapezius microvascular O2 pressure (PO2m). The spinotrapezius muscle intravital microscopy preparation is used extensively for investigation of muscle microcirculatory control. The phosphor palladium-meso-tetra(4-carboxyphenyl)porphyrin dendrimer (R2) at 15 mg/kg was bound to albumin within the blood of female Sprague-Dawley rats ( approximately 250 g). Spinotrapezius blood flow (radioactive microspheres) and PO2m profiles were determined in situ across the transition from rest to 1 Hz twitch contractions. Stimulation increased muscle blood flow by 240% from 16.6 +/- 3.0 to 56.2 +/- 8.3 (SE) ml/min per 100 g (P < 0.05). Muscle contractions reduced PO2m from a baseline of 31.4 +/- 1.6 to a steady-state value of 21.0 +/- 1.7 mmHg (n = 24, P < 0.01). The response profile of PO2m was well fit by a time delay of 19.2+/-2.8 sec (P < 0.05) followed by a monoexponential decline (time constant, 21.7 +/- 2.1 sec) to its steady state level. The absence of either an immediate and precipitous fall in microvascular PO2 at exercise onset or any PO2m undershoot prior to achievement of steady-state values, provides compelling evidence that O(2) delivery is not limiting under these conditions.

Sarcomere length-induced alterations of capillary hemodynamics in rat spinotrapezius muscle: vasoactive vs passive control.

Skeletal muscle blood flow is reduced as fibers are stretched longitudinally. Neither the underlying cause(s) of this decrement in blood flow nor the consequences in terms of capillary red blood cell (rbc) hemodynamics has been established clearly within the physiological range of muscle sarcomere length. Using intravital microscopy, this investigation determined arteriolar diameter and capillary rbc velocity (Vrbc), flux (Frbc), and hematocrit (Hct(t)) in the rat spinotrapezius muscle at shortened/resting (2.6 microm) and physiological extended (3.2 microm) sarcomere lengths under control (c) and local maximally vasodilated (v, phentolamine, 1 micromol/L; prazosin, 0.1 micromol/L; nitroprusside, 10 micromol/L) conditions. The hypothesis tested was that muscle stretch would reduce Vrbc and Frbc proportionally such that Hct(t) would remain unchanged and that these reductions in Vrbc and Frbc would be attenuated following maximal vasodilation. Vrbc and Frbc were increased significantly following maximal vasodilation at 2.6-microm (59 and 84%) and 3.2-microm (64 and 104%) sarcomere lengths, respectively. Irrespective of sarcomere length, Hct(t) was elevated significantly following vasodilation (c, 0.20 +/- 0.01; v, 0.27 +/- 0.01). At 3.2 microm compared with the 2.6-microm sarcomere length, Vrbc and Frbc were both reduced significantly under control and vasodilated conditions as expected. However, the percent reduction in either Vrbc (c, 27%, and v, 29%) or Frbc (c, 26%, and v, 33%) was not significantly different between the 2.6- and 3.2-microm sarcomere lengths. In addition, arteriolar diameter was not altered discernably as sarcomere length was increased from 2.7 microm (c, 29.0 +/- 4.5; v, 37.9 +/- 6.7 microm) to 3.2 microm (c, 29.4 +/- 4.5; v, 37.3 +/- 6.2 microm). These data suggest that increasing sarcomere length from resting to the upper extreme of the physiological range in the rat spinotrapezius muscle reduces Vrbc and Frbc (at constant hematocrit) by a mechanism that is independent of stretch-activated arteriolar vasoconstriction.

Spinotrapezius muscle microcirculatory function: effects of surgical exteriorization.

Intravital microscopy facilitates insights into muscle microcirculatory structural and functional control, provided that surgical exteriorization does not impact vascular function. We utilized a novel combination of phosphorescence quenching, microvascular oxygen pressure (microvascular PO(2)), and microsphere (blood flow) techniques to evaluate static and dynamic behavior within the exposed intact (I) and exteriorized (EX) rat spinotrapezius muscle. I and EX muscles were studied under control, metabolic blockade with 2,4-dinitrophenol (DNP), and electrically stimulated conditions with 1-Hz contractions, and across switches from 21 to 100% and 10% inspired O(2). Surgical preparation did not alter spinotrapezius muscle blood flow in either I or EX muscle. DNP elevated muscle blood flow approximately 120% (P < 0.05) in both I and EX muscles (P > 0.05 between I and EX). Contractions reduced microvascular PO(2) from 30.4 +/- 4.3 to 21.8 +/- 4.8 mmHg in I muscle and from 33.2 +/- 3.0 to 25.9 +/- 2.8 mmHg in EX muscles with no difference between I and EX. In each O(2) condition, there was no difference (each P > 0.05) in microvascular PO(2) between I and EX muscles (21% O(2): I = 37 +/- 1; EX = 36 +/- 1; 100%: I = 62 +/- 5; EX = 51 +/- 9; 10%: I = 20 +/- 1; EX = 17 +/- 2 mmHg). Similarly, the dynamic behavior of microvascular PO(2) to altered inspired O(2) was unaffected by the EX procedure [half-time (t(1/2)) to 100% O(2): I = 23 +/- 5; EX = 23 +/- 4; t(1/2) to 10%: I = 14 +/- 2; EX = 16 +/- 2 s, both P > 0.05]. These results demonstrate that the spinotrapezius muscle can be EX without significant alteration of microvascular integrity and responsiveness under the conditions assessed.

Cardiorespiratory impact of the nitric oxide synthase inhibitor L-NAME in the exercising horse.

To investigate the role of nitric oxide, NO, in facilitating cardiorespiratory function during exercise, five horses ran on a treadmill at speeds that yielded 50, 80 and 100% of peak pulmonary oxygen uptake (V(O(2)) peak) as determined on a maximal incremental test. Each horse underwent one control (C) and one (NO-synthase inhibitor; N(G)-L-nitro-arginine methyl ester (L-NAME), 20 mg/kg) trial in randomized order. Pulmonary gas exchange (open flow system), arterial and mixed-venous blood gases, cardiac output (Fick Principle), and pulmonary and systemic conductances were determined. L-NAME reduced exercise tolerance, as well as cardiac output (C, 291+/-34; L-NAME, 246+/-38 L/min), body O(2) delivery (C, 74.4+/-5. 5; L-NAME, 62.1+/-5.6 L/min), and both pulmonary (C, 3.07+/-0.26; L-NAME, 2.84+/-0.35 L/min per mmHg) and systemic (C, 1.55+/-0.24; L-NAME, 1.17+/-0.16 L/min per mmHg) effective vascular conductances at peak running speeds (all P<0.05). On the 50 and 80% trials, L-NAME increased O(2) extraction, which compensated for the reduced body O(2) delivery and prevented a fall in V(O(2)). However, at peak running speed in the L-NAME trial, an elevated O(2) extraction (P<0. 05) was not sufficient to prevent V(O(2)) from falling consequent to the reduced O(2) delivery. At the 50 and 80% running speeds (as for peak), L-NAME reduced pulmonary and systemic effective conductances. These data demonstrate that the NO synthase inhibitor, L-NAME, induces a profound hemodynamic impairment at submaximal and peak running speeds in the horse thereby unveiling a potentially crucial role for NO in mediating endothelial function during exercise.

Impaired capillary hemodynamics in skeletal muscle of rats in chronic heart failure.

Skeletal muscle blood flow is reduced and O(2) extraction is increased at rest in chronic heart failure (CHF). Knowledge of red blood cell (RBC) flow distribution within the capillary network is necessary for modeling O(2) delivery and exchange in this disease. Intravital microscopy techniques were used to study the in vivo spinotrapezius muscle microcirculation in rats with CHF 7 wk after myocardial infarction and in sham-operated controls (sham). A decrease in mean muscle fiber width from 51.3 +/- 1.9 microm in sham to 42.6 +/- 1.4 microm in CHF rats (P < 0.01) resulted in an increased lineal density of capillaries in CHF rats (P < 0.05). CHF reduced (P < 0.05) the percentage of capillaries supporting continuous RBC flow from 87 +/- 5 to 66 +/- 5%, such that the lineal density of capillaries supporting continuous RBC flow remained unchanged. The percentage of capillaries supporting intermittent RBC flow was increased in CHF rats (8 and 27% in sham and CHF, respectively, P < 0.01); however, these capillaries contributed only 2.3 and 3.3% of the total RBC flux in sham and CHF rats, respectively. In continuously RBC-perfused capillaries, RBC velocity (252 +/- 20 and 144 +/- 9 microm/s in sham and CHF, respectively, P < 0.001) and flux (21.4 +/- 2.4 and 9.4 +/- 1.1 cells/s in sham and CHF, respectively, P < 0.01) were markedly reduced in CHF compared with sham rats. Capillary "tube" hematocrit remained unchanged (0.22 +/- 0.02 and 0.19 +/- 0.02 in sham and CHF, respectively, P > 0.05). We conclude that CHF causes spinotrapezius fiber atrophy and reduces the number of capillaries supporting continuous RBC flow per fiber. Within these capillaries supporting continuous RBC flow, RBC velocity and flux are reduced 45-55%. This decreases the potential for O(2) delivery but enhances fractional O(2) extraction by elevating RBC capillary residence time. The unchanged capillary tube hematocrit suggests that any alterations in muscle O(2) diffusing properties in CHF are mediated distal to the RBC.

Effects of skeletal muscle sarcomere length on in vivo capillary distensibility.

The structural integrity of the capillary wall is such that capillary luminal distensibility is largely determined by support provided by the tissue in which it is located. Given that O2 flux density is greatest across the skeletal muscle capillary endothelium, any changes in capillary diameter (dc) would be expected to affect O2 diffusing capacity as well as hemodynamic resistance. We used intravital microscopy techniques to study the maximally vasodilated rat (n = 5) spinotrapezius muscle microcirculation in vivo within the physiological sarcomere length range, at high and low mean arterial pressures (MAP) systematically altered by blood withdrawal and infusion. We tested the hypothesis that in vivo capillary diameter alterations in response to changes of MAP would be reduced at extended sarcomere lengths. At 2.4-microm sarcomere length, mean dc (dc) within the spinotrapezius increased from 5.6 +/- 0.1 to 5.9 +/- 0.1 microm (P < 0.01) as MAP increased from 33 to 94 mm Hg. However, there was absolutely no change (i.e., 5.2 +/- 0.1 vs 5.2 +/- 0.1 microm) in dc in response to changes in MAP at 3.2-microm sarcomere length. Furthermore, at sarcomere lengths <2.8 microm there was a significant increase (P < 0.01) in dc (n = 40) as MAP increased, whereas dc (n = 49) remained unchanged with alterations of MAP when sarcomere length was >/=2.8 microm (P > 0.05). These data suggest that pressure-induced alterations in capillary luminal diameter and thus "in vivo capillary distensibility" are dependent upon the presiding sarcomere length. Furthermore, we conclude that the MAP-induced increases in capillary luminal diameter at the shorter sarcomere lengths are modest ( approximately 5%) and unlikely to affect O2 diffusing capacity and vascular resistance appreciably.

A comparison of the microcirculation in the rat spinotrapezius and diaphragm muscles.

Of all skeletal muscles examined in the rat, the spinotrapezius (S) and diaphragm (D) have the closest fiber-type composition. However, their oxidative capacities differ by two- to threefold. We have developed an intravital microscopy preparation to study diaphragm microcirculation in vivo. Using this preparation and the standard spinotrapezius model first described by S. D. Gray (1973, Microvasc. Res. 5, 395-400), we tested the hypothesis that pronounced microcirculatory differences would exist between these two muscles as a function of their disparate oxidative capacities. The lineal density of all capillaries in the spinotrapezius was 33.6 +/- 1.5 compared to 65.1 +/- 3.3 capillaries/mm in the diaphragm (P < 0.001). In the diaphragm compared with the spinotrapezius muscle, a significantly (P < 0.05) greater proportion of capillary countercurrent flow (D, 29 +/- 6% vs 8 +/- 6%) existed. Within both muscles, there was a similar proportion of capillaries supporting red blood cell (RBC) flow (S, 89 +/- 7% vs D, 92 +/- 2%). However, the diaphragm supported significantly (P < 0.001) greater intracapillary RBC velocities (D, 302 +/- 11 vs S, 226 +/- 9 micron/s) and fluxes (D, 33.4 +/- 1.1 vs S, 19.2 +/- 2.1 cells/s) compared with the spinotrapezius. Capillary "tube" hematocrit was greater (P = 0.01) in the diaphragm (0.32 +/- 0.02) than in the spinotrapezius (0.22 +/- 0.03) muscle. These data demonstrate that microcirculatory flow characteristics in resting muscle can be regulated independent of muscle fiber-type composition and may be related to muscle oxidative capacity.