Rat hindlimb muscle blood flow during level and downhill locomotion

Mechanical, hormonal and metabolic influences on blood vessels, blood flow and bone

Bone tissue is highly vascularized due to the various roles bone blood vessels play in bone and bone marrow function. For example, the vascular system is critical for bone development, maintenance and repair and provides O₂, nutrients, waste elimination, systemic hormones and precursor cells for bone remodeling. Further, bone blood vessels serve as egress and ingress routes for blood and immune cells to and from the bone marrow. It is becoming increasingly clear that the vascular and skeletal systems are intimately linked in metabolic regulation and physiological and pathological processes. This review examines how agents such as mechanical loading, parathyroid hormone, estrogen, vitamin D and calcitonin, all considered anabolic for bone, have tremendous impacts on the bone vasculature. In fact, these agents influence bone blood vessels prior to influencing bone. Further, data reveal strong associations between vasodilator capacity of bone blood vessels and trabecular bone volume, and poor associations between estrogen status and uterine mass and trabecular bone volume. Additionally, this review highlights the importance of the bone microcirculation, particularly the vascular endothelium and NO-mediated signaling, in the regulation of bone blood flow, bone interstitial fluid flow and pressure and the paracrine signaling of bone cells. Finally, the vascular endothelium as a mediator of bone health and disease is considered.

Oxidative capacity and fatigability in run-trained malignant hyperthermia-susceptible mice

INTRODUCTION

The purpose of this study was to test the hypothesis that malignant hyperthermia model mice (RyR1Y522S/wt) are more vulnerable to exercise-induced muscle injury and fatigability and adapt less to run training.

METHODS

After 6 weeks of voluntary wheel running, we measured anterior crural muscle fatigability, muscle injury, and cytochrome oxidase (COX) and citrate synthase (CS).

RESULTS

Although RyR1Y522S/wt mice ran without undergoing MH episodes, they ran 42% less distance than wild-type (WT) mice. Muscles from WT mice exhibited increased fatigue resistance and COX content after training. Muscles from RyR1Y522S/wt mice demonstrated no significant change in fatigability or COX and CS after training. However, muscles from RyR1Y522S/wt mice displayed less intrinsic fatigability and greater COX/CS content and muscle damage than WT mice.

CONCLUSIONS

RyR1Y522S/wt mice can run without having rhabdomyolysis, and their inability to adapt to training appears to stem from intrinsic enhancement of mitochondrial enzymes and fatigue resistance.

Oxygen consumption (V̇O2) during trotting on a 10% decline

REASONS FOR PERFORMING STUDY

Although there have been reports of oxygen consumption measurements of horses running on the level and incline, there are no measurements during decline locomotion. This may be due, in part, to the potential for muscle damage produced by eccentric contractions. In man, running on a 10% decline, VO2 decreased by 35% and stride frequency (SF) decreased by 3% when compared to level locomotion.

HYPOTHESIS

The rate of O2 consumption and SF would be decreased in horses on a 10% decline when compared to the level.

METHODS

Six horses (average 467 +/- 68 kg) were acclimated to trotting on the level and decline prior to data collection. VO2 under moderate conditions was measured (using open flow respirometry) during trotting between 2.25 and 4.0 m/sec (at 0.25 m/sec increments) on a treadmill on the level and declined 10%. Stride frequencies were counted manually.

RESULTS

VO2 decreased (P<0.009) on the decline by an average of 45% (range 42-47%), and SF was 2.7% slower. The speed at which the minimum Cost of Transport occurs on the decline was faster than on the level. SF was reduced on the decline. No evidence of muscle soreness was noted in response to the downhill running.

CONCLUSIONS AND POTENTIAL RELEVANCE

Downhill trotting, eccentric exercise, can be done safely in the horse and requires almost half the energetic costs as trotting on the level. It is not known whether this is the optimum downhill gradient or if the horse adjusts its preferred speed to accommodate downhill trotting.

Mitochondrial superoxide production in skeletal muscle fibers of the rat and decreased fiber excitability

Mammalian skeletal muscles generate marked amounts of superoxide (O2·−) at 37°C, but it is not well understood which is the main source of O2·− production in the muscle fibers and how this interferes with muscle function. To answer these questions, O2·− production and twitch force responses were measured at 37°C in mechanically skinned muscle fibers of rat extensor digitorum longus (EDL) muscle. In mechanically skinned fibers, the sarcolemma is removed avoiding potential sources of O2·− production that are not intrinsically part of the muscle fibers, such as nerve terminals, blood cells, capillaries and other blood vessels in the whole muscle. O2·− production was also measured in split single EDL muscle fibers, where part of the sarcolemma remained attached, and small bundles of intact isolated EDL muscle fibers at rest, in the presence and absence of modifiers of mitochondrial function. The results lead to the conclusion that mitochondrial production of O2·− accounts for most of the O2·− measured intracellularly or extracellularly in skeletal muscle fibers at rest and at 37°C. Muscle fiber excitability at 37°C was greatly improved in the presence of a membrane permeant O2·− dismutase mimetic (Tempol), demonstrating a direct link between O2·− production in the mitochondria and muscle fiber performance. This implicates mitochondrial O2·− production in the down-regulation of skeletal muscle function, thus providing a feedback pathway for communication between mitochondria and plasma membranes that is not directly related to the main function of mitochondria as the power plant of the mammalian muscle cell.

Temperature-dependent skeletal muscle dysfunction in rats with congestive heart failure

Abnormalities in the excitation-contraction coupling of slow-twitch muscle seem to explain the slowing and increased fatigue observed in congestive heart failure (CHF). However, it is not known which elements of the excitation-contraction coupling might be affected. We hypothesize that the temperature sensitivity of contractile properties of the soleus muscle might be altered in CHF possibly because of alterations of the temperature sensitivity of intracellular Ca(2+) handling. We electrically stimulated the in situ soleus muscle of anesthetised rats that had 6-wk postinfarction CHF using 1 and 50 Hz and using a fatigue protocol (5-Hz stimulation for 30 min) at 35, 37, and 40 degrees C. Ca(2+) uptake and release were measured in sarcoplasmic reticulum vesicles at various temperatures. Contraction and relaxation rates of the soleus muscle were slower in CHF than in sham at 35 degrees C, but the difference was almost absent at 40 degrees C. The fatigue protocol revealed that force development was more temperature sensitive in CHF, whereas contraction and relaxation rates were less temperature sensitive in CHF than in sham. The Ca(2+) uptake and release rates did not correlate to the difference between CHF and sham regarding contractile properties or temperature sensitivity. In conclusion, the discrepant results regarding altered temperature sensitivity of contraction and relaxation rates in the soleus muscle of CHF rats compared with Ca(2+) release and uptake rates in vesicles indicate that the molecular cause of slow-twitch muscle dysfunction in CHF is not linked to the intracellular Ca(2+) cycling.

Eccentric Cycle Exercise: Training Application of Specific Circulatory Adjustments

PURPOSE

Despite identical oxygen uptake (VO2), enhanced heart rate (HR) and cardiac output (Q) responses have been reported in eccentric (ECC) versus concentric (CON) cycle exercise. The aim of this study was to describe the specific circulatory adjustments (HR and stroke volume (SV)) to incremental ECC cycle exercise in order to: 1) determine the HR values leading to identical VO2 in ECC and CON cycling; and 2) estimate the interindividual variability of this HR correspondence between the two exercise modes, with emphasis upon rehabilitation and training purposes.

METHODS

Eight healthy male subjects (age, 28 +/- 2 yr) participated in this study. They performed CON and ECC cycle incremental exercises (power output increases of 50 W every 3 min). Breath-by-breath gas exchange analysis and beat-by-beat thoracic impedancemetry were used to determine VO2 and Q, respectively.

RESULTS

At the same metabolic power (VO2 of 1.08 +/- 0.05 L x min(-1) in CON vs 1.04 +/- 0.06 L x min in ECC), SV was not different, but HR was 17% higher in ECC (P < 0.01), leading to a 27% enhanced Q (P < 0.01). Q and HR net adjustments (exercise minus resting values) in ECC versus CON muscle involvement demonstrated important interindividual variability with coefficients of variation amounting to 32% and 30%, respectively.

CONCLUSION

In practice, if a given level of VO2 is to be reached, ECC HR has to be set above the CON one. Taking into account the interindividual variability of the circulatory adjustments in ECC versus CON muscle involvement, a precise HR correspondence can be established individually from the VO2/HR relationship obtained using ECC incremental testing, allowing prescription of accurate target HR for rehabilitation or training purposes.

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° 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-μm 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° incline; 331 ± 5 g body wt, n = 7) and level (i.e., 0° 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).

Temperature dependency of force loss and Ca(2+) homeostasis in mouse EDL muscle after eccentric contractions

The goals of this study were first to determine the effect of temperature on the force loss that results from eccentric contractions in mouse extensor digitorum longus (EDL) muscles and then to evaluate a potential role for altered Ca(2+) homeostasis explaining the greater isometric force loss observed at the higher temperatures. Isolated muscles performed five eccentric or five isometric contractions at either 15, 20, 25, 30, 33.5, or 37 degrees C. Isometric force loss, caffeine-induced force, lactate dehydrogenase (LDH) release, muscle accumulation of (45)Ca(2+) from the bathing medium, sarcoplasmic reticulum (SR) Ca(2+) uptake, and resting muscle fiber free cytosolic Ca(2+) concentration ([Ca(2+)](i)) were measured. The isometric force loss after eccentric contractions increased progressively as temperature rose; at 15 degrees C, there was no significant loss of force, but at 37 degrees C, there was a 30-39% loss of force. After eccentric contractions, caffeine-induced force was not affected by temperature nor was it different from that of control muscles at any temperature. Loss of cell membrane integrity and subsequent influx of extracellular Ca(2+) as indicated by LDH release and muscle (45)Ca(2+) accumulation, respectively, were minimal over the 15-25 degrees C range, but both increased as an exponential function of temperature between 30 and 37 degrees C. SR Ca(2+) uptake showed no impairment as temperature increased, and the eccentric contraction-induced rise in resting fiber [Ca(2+)](i) was unaffected by temperature over the 15-25 degrees C range. In conclusion, the isometric force loss after eccentric contractions is temperature dependent, but the temperature dependency does not appear to be readily explainable by alterations in Ca(2+) homeostasis.

Effect of concentric and eccentric muscle actions on muscle sympathetic nerve activity

The purpose of this study was to determine the effects of concentric (Con) and eccentric (Ecc) muscle actions on leg muscle sympathetic nerve activity (MSNA). Two protocols were utilized. In protocol 1, eight subjects performed Con and Ecc arm curls for 2 min, with a resistance representing 50% of one-repetition maximum for Con curls. Heart rate (HR) and mean arterial pressure (MAP) were greater (P < 0. 05) during Con than during Ecc curls. Similarly, the MSNA was greater (P < 0.05) during Con than during Ecc curls. In protocol 2, eight different subjects performed Con and Ecc arm curls to fatigue, followed by postexercise muscle ischemia, by using the same resistance as in protocol 1. Endurance time was significantly greater for Ecc than for Con curls. The increase in HR, MAP, and MSNA was greater (P < 0.05) during Con than during Ecc curls. However, when the data were normalized as a function of endurance time, the differences in HR, MAP, and MSNA between Con and Ecc curls were no longer present. HR, MAP, and MSNA responses during postexercise muscle ischemia were similar for Con and Ecc curls. Con curls elicited greater increase (P < 0.05) in blood lactate concentration than did Ecc curls. In summary, Con actions contribute significantly more to the increase in cardiovascular and MSNA responses during brief, submaximal exercise than do Ecc actions. However, when performed to a similar level of effort (i.e., fatigue), Con and Ecc muscle actions elicit similar cardiovascular and MSNA responses. These results indicate that the increase in MSNA during a typical bout of submaximal dynamic exercise is primarily mediated by the muscle metaboreflex, which is stimulated by metabolites produced predominantly during Con muscle action.