Viscoelastic properties of microvessels in rat spinotrapezius muscle

In order to establish a quantitative model of blood flow in skeletal muscle, the mechanical properties of the blood vessels need to be measured. We present measurements of the viscoelastic properties of arterioles, venules, and capillaries in exteriorized rat spinotrapezius muscle. Muscles were perfused with an inert silicone polymer and a uniform static pressure was established by occlusion of the venous outflow. Vessel diameters were then measured as a function of the static pressure. This study provides the first measurements of the viscoelastic properties of microvessels in skeletal muscle in situ. Over a pressure range of 20-200 mmHg, the transverse arterioles are the most distensible vessels, while the arcade venules are the stiffest. In response to a step change in pressure, all vessels show an initial elastic deformation, followed by a nonlinear creep. Based on the experimental results for different pressure histories a constitutive equation relating vessel diameter to the local transmural pressure is proposed. Diameter changes are expressed in the form of a diameter strain, analogous to a Green's strain, and are related to the local transmural pressure using a standard linear solid model. This model has only three empirical coefficients and could be fitted to all experimental results for all vessels within error of measurement.     

Downhill treadmill running trains the rat spinotrapezius muscle.

There are currently no models of exercise that recruit and train muscles, such as the rat spinotrapezius, that are suitable for transmission intravital microscopic investigation of the microcirculation. Recent experimental evidence supports the concept that running downhill on a motorized treadmill recruits the spinotrapezius muscle of the rat. Based on these results, we tested the hypothesis that 6 wk of downhill running (-14 degrees grade) for 1 h/day, 5 days/wk, at a speed of up to 35 m/min, would 1) increase whole body peak oxygen uptake (Vo(2 peak)), 2) increase spinotrapezius citrate synthase activity, and 3) reduce the fatigability of the spinotrapezius during electrically induced 1-Hz submaximal tetanic contractions. Trained rats (n = 6) elicited a 24% higher Vo(2 peak) (in ml.min(-1).kg(-1): sedentary 58.5 +/- 2.0, trained 72.7 +/- 2.0; P < 0.001) and a 41% greater spinotrapezius citrate synthase activity (in mumol.min(-1).g(-1): sedentary 14.1 +/- 0.7, trained 19.9 +/- 0.9; P < 0.001) compared with sedentary controls (n = 6). In addition, at the end of 15 min of electrical stimulation, trained rats sustained a greater percentage of the initial tension than their sedentary counterparts (control 34.3 +/- 3.1%, trained 59.0 +/- 7.2%; P < 0.05). These results demonstrate that downhill running is successful in promoting training adaptations in the spinotrapezius muscle, including increased oxidative capacity and resistance to fatigue. Since the spinotrapezius muscle is commonly used in studies using intravital microscopy to examine microcirculatory function at rest and during contractions, our results suggest that downhill running is an effective training paradigm that can be used to investigate the mechanisms for improved microcirculatory function following exercise training in health and disease.     

Effects of aging on capillary geometry and hemodynamics in rat spinotrapezius muscle

The effects of aging on muscle microvascular structure and function may play a key role in performance deficits and impairment of O2 exchange within skeletal muscle of senescent individuals. To determine the effects of aging on capillary geometry, red blood cell (RBC) hemodynamics, and hematocrit in a muscle of mixed fiber type, spinotrapezius muscles from Fischer 344 x Brown Norway hybrid rats aged 6-8 mo [young (Y); body mass 421 +/- 10 g, n = 6] and 26-28 mo [old (O); 561 +/- 12 g, n = 6] were observed by high-resolution transmission light microscopy under resting conditions. The percentage of RBC-perfused capillaries (Y: 78 +/- 3%; O: 75 +/- 2%) and degree of tortuosity and branching (Y: 13 +/- 2%; O: 13 +/- 2%, additional capillary length) were not different in O vs. Y muscles. Lineal density of RBC-perfused capillaries in O was significantly reduced (Y: 30.7 +/- 1.8, O: 22.8 +/- 3.1 capillaries/mm; P < 0.05). However, RBC-perfused capillaries from O rats (n = 78) exhibited increased RBC velocity (VRBC) (Y: 219 +/- 12, O: 310 +/- 14 microm/s; P < 0.05) and RBC flux (FRBC) (Y: 27 +/- 2, O: 41 +/- 2 RBC/s; P < 0.05) vs. Y rats (n = 66). Thus O2 delivery per unit of muscle was not different between groups (Y: 894 +/- 111, O: 887 +/- 118 RBC. s-1. mm muscle-1). Capillary hematocrit was not different in Y vs. O rats (Y: 26 +/- 1%, O: 28 +/- 1%: P > 0.05). These data indicate that in resting spinotrapezius muscle, aging decreases the lineal density of RBC-perfused capillaries while increasing mean VRBC and FRBC within those capillaries. Whereas muscle conductive O2 delivery and capillary hematocrit were unchanged, elevated VRBC reduces capillary RBC transit time and may impair the diffusive transport of O2 from blood to myocyte particularly under exercise conditions.     

Exercise training enhances adrenergic constriction and dilation in the rat spinotrapezius muscle

Treadmill training increases functionalvasodilation in the rat spinotrapezius muscle, although there is noacute increase in blood flow and no increase in oxidative capacity. Toassess concurrent changes in vascular reactivity, we measured arterial diameters in the spinotrapezius muscle of sedentary (Sed) and treadmill-trained (Tr; 9-10 wk; terminal intensity 30 m/min,1.5° incline, for 90 min) rats during iontophoretic application of norepinephrine, epinephrine (Epi), andH⁺ (HCl) and during superfusionwith adenosine. Terminal-feed arteries and first-order arterioles in Trrats constricted more than those in Sed rats at the higher currentdoses of norepinephrine and Epi. In contrast, at low-current doses ofEpi, first- and second-order arterioles dilated in Tr but not in Sedrats. The vascular responses to HCl were highly variable, butsecond-order arterioles of Tr rats constricted more than those of Sedrats at intermediate-current doses. There were no significantdifferences between Sed and Tr rats in the vascular responses toadenosine. Both adrenergic vasodilation and vasoconstriction wereenhanced in the spinotrapezius muscle of Tr rats, and enhancedadrenergic vasodilation may contribute to increased functionalvasodilation. These observations further demonstrate vascularadaptations in "nontrained" skeletal muscle tissues.

     

Downhill running: a model of exercise hyperemia in the rat spinotrapezius muscle.

To utilize the rat spinotrapezius muscle as a model to investigate the microcirculatory consequences of exercise training, it is necessary to design an exercise protocol that recruits this muscle. There is evidence that the spinotrapezius is derecruited during standard treadmill exercise protocols performed on the uphill treadmill (i.e., 6 degrees incline). This investigation tested the hypothesis that downhill running would effectively recruit the spinotrapezius muscle as assessed by the presence of an exercise hyperemia response. We used radioactive 15-microm microspheres to determine blood flows in the spinotrapezius and selected hindlimb muscles of female Sprague-Dawley rats at rest and during downhill (i.e., -14 degrees incline; 331 +/- 5 g body wt, n = 7) and level (i.e., 0 degrees incline; 320 +/- 11 g body wt, n = 5) running at 30 m/min. Both level and downhill exercise increased blood flow to all hindlimb muscles (P < 0.01). However, in marked contrast to the absence of a hyperemic response to level running, blood flow to the spinotrapezius muscle increased from 26 +/- 6 ml.min(-1).100 g(-1) at rest to 69 +/- 8 ml.min(-1).100 g(-1) during downhill running (P < 0.01). These findings indicate that downhill running represents an exercise paradigm that recruits the spinotrapezius muscle and thereby constitutes a tenable physiological model for investigating the adaptations induced by exercise training (i.e., the mechanisms of altered microcirculatory control by transmission light microscopy).     

Effect of erythrocyte aggregation at normal human levels on functional capillary density in rat spinotrapezius muscle

Previous studies have shown that functional capillary density (FCD) is substantially reduced by erythrocyte aggregation. However, only supranormal levels of aggregability were studied. To investigate the effect of erythrocyte aggregability at the level seen in healthy humans, the FCD of selected capillary fields in rat spinotrapezius muscle was determined with high-speed video microscopy under normal (nonaggregating) conditions and after induction of erythrocyte aggregation with Dextran 500 (200 mg/kg). To examine shear rate dependence, the effect was studied both at normal and reduced arterial pressures (50 and 25 mmHg), the latter achieved by short periods of hemorrhage. In a separate study, volume flow was determined in arterioles (52.1 +/- 3.7 microm) under the same conditions. Before Dextran 500 infusion, FCD fell to 91% and 76% of control values, respectively, when arterial pressure was reduced to 50 and 25 mmHg. After Dextran 500 infusion, FCD was 96% at normal arterial pressure and fell to 79% and 37% of normal control values at 50 and 25 mmHg. All FCD values were significantly lower after dextran infusion. FCD reduction after lowering arterial pressure or dextran infusion appeared to be due to plasma skimming rather than capillary plugging. Reduction of FCD by dextran at reduced pressure was compensated by increased red blood cell flux in capillaries with red blood cell flow. We conclude that the level of aggregability seen in healthy humans is an important determinant of FCD only at reduced arterial pressure.     

Effects of eccentric exercise on microcirculation and microvascular oxygen pressures in rat spinotrapezius muscle.

A single bout of eccentric exercise results in muscle damage, but it is not known whether this is correlated with microcirculatory dysfunction. We tested the following hypotheses in the spinotrapezius muscle of rats either 1 (DH-1; n = 6) or 3 (DH-3; n = 6) days after a downhill run to exhaustion (90-120 min; -14 degrees grade): 1) in resting muscle, capillary hemodynamics would be impaired, and 2) at the onset of subsequent acute concentric contractions, the decrease of microvascular O(2) pressure (Pmv(o(2))), which reflects the dynamic balance between O(2) delivery and O(2) utilization, would be accelerated compared with control (Con, n = 6) rats. In contrast to Con muscles, intravital microscopy observations revealed the presence of sarcomere disruptions in DH-1 and DH-3 and increased capillary diameter in DH-3 (Con: 5.2 +/- 0.1; DH-1: 5.1 +/- 0.1; DH-3: 5.6 +/- 0.1 mum; both P < 0.05 vs. DH-3). At rest, there was a significant reduction in the percentage of capillaries that sustained continuous red blood cell (RBC) flux in both DH running groups (Con: 90.0 +/- 2.1; DH-1: 66.4 +/- 5.2; DH-3: 72.9 +/- 4.1%, both P < 0.05 vs. Con). Capillary tube hematocrit was elevated in DH-1 but reduced in DH-3 (Con: 22 +/- 2; DH-1: 28 +/- 1; DH-3: 16 +/- 1%; all P < 0.05). Although capillary RBC flux did not differ between groups (P > 0.05), RBC velocity was lower in DH-1 compared with Con (Con: 324 +/- 43; DH-1: 212 +/- 30; DH-3: 266 +/- 45 mum/s; P < 0.05 DH-1 vs. Con). Baseline Pmv(O(2)) before contractions was not different between groups (P > 0.05), but the time constant of the exponential fall to contracting Pmv(O(2)) values was accelerated in the DH running groups (Con: 14.7 +/- 1.4; DH-1: 8.9 +/- 1.4; DH-3: 8.7 +/- 1.4 s, both P < 0.05 vs. Con). These findings are consistent with the presence of substantial microvascular dysfunction after downhill eccentric running, which slows the exercise hyperemic response at the onset of contractions and reduces the Pmv(O(2)) available to drive blood-muscle O(2) delivery.     

Manganese depresses rat heart muscle respiration

It has previously been reported that moderately high dietary manganese (Mn) in combination with marginal magnesium (Mg) resulted in ultrastructural damage to heart mitochondria. Manganese may replace Mg in biological functions, including the role of enzyme cofactor. Manganese may accumulate and substitute for Mg during the condition of Mg-deficiency. The objective of the current study was to determine whether high Mn alters heart muscle respiration and Mg-enzyme activity as well as whole body Mn retention under marginal Mg. An additional objective was to determine whether high Mn results in increased oxidative stress. In experiment 1: forty-eight rats were fed a 2 x 3 factorial arrangement of Mn (10, 100, or 1000 mg/kg) and Mg (200 or 500 mg/kg). In experiment 2: thirty-two rats were fed one of four diets in a 2 x 2 factorial arrangement of Mn (10 or 250 mg/kg) and Mg (200 or 500 mg/kg). In experiment 3: thirty-two rats were fed one of four diets in a 2 x 2 factorial arrangement of Mn (10 or 650 mg/kg) and Mg (200 or 500 mg/kg). In experiment 2, high Mn and marginal Mg reduced (P<0.05) oxygen consumption of left ventricle muscle. Marginal Mg, but not Mn, reduced (P<0.05) activity of sarcoplasmic reticulum calcium-ATPase enzyme. Dietary Mg had no affect on (54)Mn kinetics, but high dietary Mn decreased (P<0.01) absorption, retention, and rate of excretion of (54)Mn. Neither cellular stress, measured by Comet assay, nor antioxidant activities were increased by high Mn. A strong interaction (P<0.001) between increasing Mn and adequate Mg on hematology was observed. These results confirm previous research in swine that high Mn alters myocardial integrity as well as function, but not as a result of altered calcium transport or oxidative stress.     

Erythrocyte-associated transients in capillary PO2: an isovolemic hemodilution study in the rat spinotrapezius muscle

Mathematical simulations of oxygen delivery to tissue from capillaries that take into account the particulate nature of blood flow predict the existence of oxygen tension (Po(2)) gradients between erythrocytes (RBCs). As RBCs and plasma alternately pass an observation point, these gradients are manifested as rapid fluctuations in Po(2), also known as erythrocyte-associated transients (EATs). The impact of hemodilution on EATs and oxygen delivery at the capillary level of the microcirculation has yet to be elucidated. Therefore, in the present study, phosphorescence quenching microscopy was used to measure EATs and Po(2) in capillaries of the rat spinotrapezius muscle at the following systemic hematocrits (Hct(sys)): normal (39%) and after moderate (HES1; 27%) or severe (HES2; 15%) isovolemic hemodilution using a 6% hetastarch solution. A 532-nm laser, generating 10-micros pulses concentrated onto a 0.9-microm spot, was used to obtain plasma Po(2) values 100 times/s at points along surface capillaries of the muscle. Mean capillary Po(2) (Pc(O(2)); means +/- SE) significantly decreased between conditions (normal: 56 +/- 2 mmHg, n = 45; HES1: 47 +/- 2 mmHg, n = 62; HES2: 27 +/- 2 mmHg, n = 52, where n = capillary number). In addition, the magnitude of Po(2) transients (DeltaPo(2)) significantly decreased with hemodilution (normal: 19 +/- 1 mmHg, n = 45; HES1: 11 +/- 1 mmHg, n = 62; HES2: 6 +/- 1 mmHg, n = 52). Results suggest that the decrease in Pc(O(2)) and DeltaPo(2) with hemodilution is primarily dependent on Hct(sys) and subsequent microvascular compensations.     

Oxygen regulation and limitation to cellular respiration in mouse skeletal muscle in vivo

In skeletal muscle, intracellular Po2 can fall to as low as 2-3 mmHg. This study tested whether oxygen regulates cellular respiration in this range of oxygen tensions through direct coupling between phosphorylation potential and intracellular Po2. Oxygen may also behave as a simple substrate in cellular respiration that is near saturating levels over most of the physiological range. A novel optical spectroscopic method was used to measure tissue oxygen consumption (Mo2) and intracellular Po2 using the decline in hemoglobin and myoglobin saturation in the ischemic hindlimb muscle of Swiss-Webster mice. 31P magnetic resonance spectroscopic determinations yielded phosphocreatine concentration ([PCr]) and pH in the same muscle volume. Intracellular Po2 fell to <2 mmHg during the ischemic period without a change in the muscle [PCr] or pH. The constant phosphorylation state despite the decline in intracellular Po2 rejects the hypothesis that direct coupling between these two variables results in a regulatory role for oxygen in cellular respiration. A second set of experiments tested the relationship between intracellular Po2 and Mo2. In vivo Mo2 in mouse skeletal muscle was increased by systemic treatment with 2 and 4 mg/kg body wt 2,4-dinitrophenol to partially uncouple mitochondria. Mo2 was not dependent on intracellular Po2 above 3 mmHg in the three groups despite a threefold increase in Mo2. These results indicate that Mo2 and the phosphorylation state of the cell are independent of intracellular Po2 throughout the physiological range of oxygen tensions. Therefore, we reject a regulatory role for oxygen in cellular respiration and conclude that oxygen acts as a simple substrate for respiration under physiological conditions.     

Pressure dependence of baroreceptor-mediated vasoconstriction in rat skeletal muscle

To determine whether microvessels in resting or contracting skeletal muscle constrict during baroreceptor activation, vascular diameters were measured in the spinotrapezius muscle of adult rats (n = 12) during occlusion of the common carotid arteries. Neural and myogenic components were distinguished using two types of occlusion: 1) "normal" (arterial pressure was allowed to increase with baroreceptor activation) and 2) "isobaric" (arterial pressure was maintained constant by decreasing blood volume). During normal occlusions, intermediate and small arteriolar diameters decreased in resting and contracting muscle (10-15% and 25-30%, respectively). Large arterioles and all-sized venules distended slightly (approximately 5%) in resting muscle, but diameters were maintained or decreased in contracting muscle. When arterial pressure was maintained constant (isobaric), the microvascular responses to baroreceptor activation in both resting and contracting muscle were essentially eliminated. We conclude that nearly all the arteriolar constriction observed in the spinotrapezius muscle during normal carotid artery occlusion is myogenic in origin, secondary to increased arterial pressure. This pressure-dependent constriction is augmented during skeletal muscle contraction and functional vasodilation.     

Effects of Pseudomonas aeruginosa endotoxin on vasodilation in the intact spinotrapezius muscle.

The purpose of this study was to determine whether short-term exposure to clinically relevant concentrations of Pseudomonas aeruginosa lipopolysaccharide (LPS) impairs vasoreactivity of resistance arterioles in the intact spinotrapezius muscle microcirculation and, if so, to determine the mechanisms mediating this response. Using intravital microscopy, we found that 60-min suffusion of P. aeruginosa LPS (0.03-3.0 microg/ml) on the in situ hamster spinotrapezius muscle elicited an immediate, profound, and prolonged concentration-dependent vasodilation (P < 0.05). This response was reversible once suffusion of P. aeruginosa LPS was stopped. Pretreatment with N(G)-nitro-L-arginine methyl ester (10.0 microM), a nonselective nitric oxide (NO) synthase inhibitor, but not N(G)-nitro-D-arginine methyl ester, abrogated P. aeruginosa LPS-induced vasodilation and elicited a small, albeit significant, vasoconstriction. Indomethacin had no significant effects on P. aeruginosa LPS-induced responses. P. aeruginosa LPS had no significant effects on acetylcholine- and nitroglycerin-induced vasodilation in the spinotrapezius muscle. Collectively, these data indicate that short-term exposure to clinically relevant concentrations of P. aeruginosa LPS evokes an immediate, potent, prolonged, and reversible NO-dependent, prostaglandin-independent vasodilation in skeletal muscles in vivo. We suggest this response could play an important role in the pathophysiology of the profound vasomotor dysfunction observed in the peripheral circulation of patients with P. aeruginosa sepsis syndrome.     

Oxygen supply and oxidative phosphorylation limitation in rat myocardium in situ

The 1H-NMR signal of the proximal histidyl-N(delta)H of deoxymyoglobin is detectable in the in situ rat myocardium and can reflect the intracellular PO2. Under basal normoxic conditions, the cellular PO2 is sufficient to saturate myoglobin (Mb). No proximal histidyl signal of Mb is detectable. On ligation of the left anterior descending coronary artery, the Mb signal at 78 parts/million (ppm) appears, along with a peak shoulder assigned to the corresponding signal of Hb. During dopamine infusion up to 80 microg. kg(-1) x min(-1), both the heart rate-pressure product (RPP) and myocardial oxygen consumption (MVO2) increase by about a factor of 2. Coronary flow increases by 84%, and O2 extraction (arteriovenous O2 difference) rises by 31%. Despite the increased respiration and work, no deoxymyoglobin signal is detected, implying that the intracellular O2 level still saturates MbO2, well above the PO2 at 50% saturation of Mb. The phosphocreatine (PCr) level decreases, however, during dopamine stimulation, and the ratio of the change in P(i) over PCr (DeltaP(i)/PCr) increases by 0.19. Infusion of either pyruvate, as the primary substrate, or dichloroacetate, a pyruvate dehydrogenase activator, abolishes the change in DeltaP(i)/PCr. Intracellular O2 supply does not limit MVO2, and the role of ADP in regulating respiration in rat myocardium in vivo remains an open question.     

Role of O2 in regulating tissue respiration in dog muscle working in situ.

This study was designed to investigate the role of tissue oxygenation in some of the factors that are thought to regulate muscle respiration and metabolism. Tissue oxygenation was altered by reductions in O2 delivery (muscle blood flow x arterial O2 content), induced by decreases in arterial PO2 (PaO2). O2 uptake (VO2) was measured in isolated in situ canine gastrocnemius at rest and while working at two stimulation intensities (isometric tetanic contractions at 0.5 and 1 contractions/s) on three separate occasions, with only the level of PaO2 (78, 30, and 21 Torr) being different for each occasion. Muscle blood flow was held constant (pump perfusion) at each work intensity for the three different levels of PaO2. Muscle biopsies were obtained at the end of each rest and work period. Muscle VO2 was significantly less (P less than 0.05) at both stimulation intensities for the hypoxemic conditions, whereas [ATP] was reduced only during the highest work intensity during both hypoxemic conditions (31% reduction at 21 Torr PaO2 and 17% at 30 Torr). For each level of PaO2, the relationships between the changes that occurred in VO2 and levels of phosphocreatine, ADP, and ATP/ADP.P(i) as the stimulation intensity was increased were significantly correlated; however, the slopes and intercepts of these lines were significantly different for each PaO2. Thus a greater change in any of the proposed regulators of tissue respiration (e.g., phosphocreatine, ADP) was required to achieve a given VO2 as PaO2 was decreased.(ABSTRACT TRUNCATED AT 250 WORDS)     

Procedures for rat in situ skeletal muscle contractile properties

There are many circumstances where it is desirable to obtain the contractile response of skeletal muscle under physiological circumstances: normal circulation, intact whole muscle, at body temperature. This includes the study of contractile responses like posttetanic potentiation, staircase and fatigue. Furthermore, the consequences of disease, disuse, injury, training and drug treatment can be of interest. This video demonstrates appropriate procedures to set up and use this valuable muscle preparation. To set up this preparation, the animal must be anesthetized, and the medial gastrocnemius muscle is surgically isolated, with the origin intact. Care must be taken to maintain the blood and nerve supplies. A long section of the sciatic nerve is cleared of connective tissue, and severed proximally. All branches of the distal stump that do not innervate the medial gastrocnemius muscle are severed. The distal nerve stump is inserted into a cuff lined with stainless steel stimulating wires. The calcaneus is severed, leaving a small piece of bone still attached to the Achilles tendon. Sonometric crystals and/or electrodes for electromyography can be inserted. Immobilization by metal probes in the femur and tibia prevents movement of the muscle origin. The Achilles tendon is attached to the force transducer and the loosened skin is pulled up at the sides to form a container that is filled with warmed paraffin oil. The oil distributes heat evenly and minimizes evaporative heat loss. A heat lamp is directed on the muscle, and the muscle and rat are allowed to warm up to 37°C. While it is warming, maximal voltage and optimal length can be determined. These are important initial conditions for any experiment on intact whole muscle. The experiment may include determination of standard contractile properties, like the force-frequency relationship, force-length relationship, and force-velocity relationship. With care in surgical isolation, immobilization of the origin of the muscle and alignment of the muscle-tendon unit with the force transducer, and proper data analysis, high quality measurements can be obtained with this muscle preparation.     

Prolonged tissue PO2 reduction after contraction in spinotrapezius muscle of spontaneously hypertensive rats

We tested the hypothesis that a deficit in oxygen extraction or an increase in oxygen demand after skeletal muscle contraction leads to delayed recovery of tissue oxygen tension (Po(2)) in the skeletal muscle of hypertensive rats compared with normotensive rats. Blood flow and Po(2) recovery at various sites in the spinotrapezius muscle of spontaneously hypertensive rats (SHRs) were evaluated after a 3-min period of muscle contraction and were compared with corresponding values in Wistar-Kyoto rats (WKYs). The recovery of tissue Po(2) [75 +/- 7 (SHRs) vs. 99 +/- 12% (WKYs) of resting values] and venular Po(2) [72 +/- 13 (SHRs) vs. 104 +/- 10% (WKYs) of resting values] were significantly depressed in the SHRs 30 s postcontraction. The delayed recovery persisted for 120 s postcontraction for both tissue [86 +/- 11 (SHRs) vs. 119 +/- 13% (WKYs) of resting values] and venular [74 +/- 2 (SHRs) vs. 100 +/- 9% (WKYs) of resting values] Po(2) levels. There was no significant difference in the recovery of arteriolar Po(2) between the two groups 30 s postcontraction [95 +/- 7 (SHRs) vs. 84 +/- 8% (WKYs) of resting values]. Values for resting diameter of arcade arterioles in the two groups were not different [52 +/- 3 (SHRs) vs. 51 +/- 3 microm (WKYs)], but the arteriolar diameter after the 3-min contraction period was greater in the SHRs (71 +/- 4 microm) than the WKYs (66 +/- 4). Likewise, red blood cell (RBC) velocity [5.8 +/- 0.3 (SHRs) vs. 4.7 +/- 0.2 mm/s (WKYs)] and blood flow [23.0 +/- 0.8 (SHRs) vs. 16.0 +/- 1.0 nl/s (WKYs)] measurements were significantly greater in the SHRs at 30 s postcontraction. The delayed recovery of tissue Po(2) in the SHRs compared with the WKYs can be explained by a decrease in oxygen diffusion from the rarefied microvascular network due to the increased RBC velocity and the shorter residence time in the microcirculation and the consequent disequilibrium for oxygen between plasma and RBCs. The delayed recovery of venular Po(2) in the SHRs is consistent with this explanation, as venular Po(2) is slowly restored to baseline by release of oxygen from the RBCs. This leaves the arterioles in the primary role as oxygen suppliers to restore Po(2) in the tissue after muscle contraction.     

Temperature dependence of rat diaphragm muscle contractility and fatigue

The diaphragm is a skeletal muscle of mixed fiber type that is unique in its requirement to maintain contractile function and fatigue resistance across a wide range of temperatures to sustain alveolar ventilation under conditions of hypo- or hyperthermia. The direct effect of temperature (15-41 degrees C) on rat diaphragm isometric contractility and fatigue was determined in vitro. As temperature decreased from 37 to 15 degrees C, contraction and relaxation times increased, and there was a left shift of the diaphragm's force-frequency curve, with decreased contractility at 41 and 15 degrees C. Fatigue was induced by 10 min of stimulation with 30 trains/min of 5 Hz at a train duration of 900 ms. Compared with 37 degrees C, fatigue resistance was enhanced at 25 degrees C, but no difference in fatigue indexes was evident at extreme hypothermia (15 degrees C) or hyperthermia (41 degrees C). Only when the fatigue program was adjusted to account for hypothermia-induced increases in tension-time indexes was fatigue resistance evident at 15 degrees C. These findings indicate that despite the diaphragm's unique location as a core structure, necessitating exposure to in vivo temperatures higher than found in limb muscle, the temperature dependence of rat diaphragm muscle contractility and fatigue is similar to that reported for limb muscle of mixed fiber type.