X-Ray Study of the Cat Hindlimb During Treadmill Locomotion

With pulsed X-ray cinematography we have analysed the angular excursions of the distal hindlimb joints (proximal interphalangeal, PIP; metatarsophalangeal, MTP; ankle) in cats walking on a treadmill. These distal joints transmit the body weight and the dynamic forces onto the ground. We have included the knee and hip joints in the analysis to relate the angular excursions of the proximal and distal joints and to verify the data previously obtained with external markers on the kinematics of the proximal joints. At the beginning of the stance phase the PIP joints flexed rapidly, the MTP joints extended slowly and the ankle and knee yielded under body weight. Whereas the PIP joints maintained a rather constant angular position of −75° throughout the stance phase, extension continued in the MTP joints from −230° at touch-down to −270° at the end of the stance phase. Around 50 ms before lift-off the MTP joints flexed rapidly. Early (−30 ms) after lift-off this flexion changed into a slow extension. The PIP joints extended swiftly at the stance-swing transition and moderately at the end of the swing phase. During the middle part of the swing phase they flexed slowly. Small rotatory movements around the long axis of the foot took place in the last 100 ms of the swing phase. The results of this study on the distal joints are discussed in relation to the placing of the paw, to the translation of forward propulsion into a MTP movement and to the lifting of the paw (conventionally described as toe curling). They show a differentiated mechanical interaction between the different distal limb joints during these different phases, which must be known in detail to interpret the corresponding electromyographic data and to understand how the hip is moved forward over the MTP joints which serve as the final pivot during stance.     

Physiological pharmacokinetics of solutes in the isolated perfused rat hindlimb: Characterization of the physiology with changing perfusate flow,protein content,and temperature using statistical moment analysis

Distribution of Evans Blue (EB), sucrose, and water into the isolated perfused rat hindlimb was studied under various conditions using the multiple indicator dilution (MID) technique. Statistical moment analyses of the outflow profiles for the EB, sucrose, and water were used to define the vascular, extravascular, and total water spaces, respectively. The varied perfusion conditions included albumin content (2, 4.7, and 7%), temperature (25, 37, and 42 C), perfusate flow rate (2, 4, 8, and 12 ml/min) and the presence/absence of red blood cells. The range of studies undertaken were chosen to represent the variety of conditions used in the preparation of both isolated animal and human limbs, the latter being particularly important in cytotoxic therapy for recurrent malignant melanoma. The distribution volumes of EB, sucrose, and water were dependent on the flow rate and the albumin content of perfusate. The normalized variances (CV ² ) of the markers were of the following order: sucrose (2.18) > water (1.58) > EB (0.68), indicating that some disequilibrium occurs during the capillary exchange of water and sucrose. It is suggested that a Krebs-Henseleit buffer containing 2% BSA is a suitable perfusate for most studies of the isolated rat hindlimb perfusion. The effect of albumin concentration manifests itself only at higher flows.We acknowledge the support of the National Heart Foundation (Queensland) and the Mayne Bequest Foundation. This study was conducted while the investigator (Z.Y. Wu) was in receipt of a WHO Research Training Grant. Professor M. S. Roberts also acknowledges the support of the Queensland and Northern New South Wales Lions Kidney & Medical Research Foundation.     

Possible causal relationships between cerebellar patterns of foliation and hindlimb coordination in laboratory mice: a quantitative trait locus analysis

The cerebellum is involved in a large set of integrative functions including memory, affect, and motricity. The cerebellar patterns of foliation and their causal relationships with motricity were investigated via a wide genome scan approach and quantitative trait locus (QTL) strategy. QTLs were mapped in an F₂ population derived from NZB/B1NJ and C57BL/6By inbred strains of mice for cerebellar fissures in the four vermal lobules (intraculminate, uvula, declival, and intracentral) and for hindpaw slips in a bar crossing test. No linkage was detected for uvula and intracentral fissures. We found five QTLs linked to declival fissure: Cpfd-1q and Cpfd-2q (chromosome 1), Cpfd-3q (chromosome 5), Cpfd-4q (chromosome 9), and Cpfd-5q (chromosome 13). Two QTLs were also mapped for intraculminate fissure Cpfi-1q (chromosome 4) and Cpfi-2q (chromosome 1). Most of the confidence intervals of these QTLs included genes that were previously identified for their implication in the physiological mechanisms underlying cerebellar patterns of foliation. Only one significant QTL was found for the measure of hindpaw coordination (Tne-1q). It was linked with Cpfd-1q and Cpfd-2q on the telomeric part of chromosome 1.     

Early postnatal motor experience shapes the motor properties of C57BL/6J adult mice

This study aimed to evaluate the long‐term consequences of early motor training on the muscle phenotype and motor output of middle‐aged C57BL/6J mice. Neonatal mice were subjected to a variety of motor training procedures, for 3 weeks during the period of acquisition of locomotion. These procedures are widely used for motor training in adults; they include enriched environment, forced treadmill, chronic centrifugation, and hindlimb suspension. At 9 months, the mice reared in the enriched environment showed a slower type of fibre in slow muscles and a faster type in fast muscles, improved performance in motor tests, and a modified gait and body posture while walking. The proportion of fibres in the postural muscles of centrifuged mice did not change, but these mice showed improved resistance to fatigue. The suspended mice showed increased persistence of immature hybrid fibres in the tibialis, with a slower shift in the load‐bearing soleus, without any behavioural changes. The forced treadmill was very stressful for the mice, but had limited effects on motor output, although a slower profile was observed in the tibialis. These results support the hypothesis that motor experience during a critical period of motor development shapes muscle phenotype and motor output. The different impacts of the various training procedures suggest that motor performance in adults can be optimized by appropriate training during a defined period of motor development.     

Is normalized hindlimb length measurement in assessment of thyroid disruption in the amphibian metamorphosis assay relevant?

The amphibian metamorphosis assay represents an OECD Level 3 and EDSP Tier 1 ecotoxicity test assessing thyroid activity of chemicals in African clawed frog (Xenopus laevis). To evaluate the effectiveness of snout‐vent length (SVL) normalization of hindlimb length (HLL), correlation between the HLL and SVL or body weight was evaluated in the control groups of 10 individual studies from three laboratories. Two studies required separate analysis of the Nieuwkoop‐Faber (NF) stage ≤60 and >60 animals creating a total of 12 data sets. On study day 7, significant positive correlation between HLL and SVL or body weight was observed in eight and seven of the 10 data sets, respectively (r = 0.608‐0.843 and 0.583‐0.876). On study day 21, significant positive correlation between HLL and SVL or body weight was found in three and four of the 12 data sets, respectively (r = 0.452, 0.480 and 0.553 and r = 0.621, 0.546, 0.564 and 0.378). Significant positive correlation between HLL and SVL was found in three of five studies, including ≤NF stage 60 data (r = 0.564, 0.546 and 0.621). In one of eight studies, including >NF stage 60 data, the positive correlation between HLL and body weight was determined (r = 0.378). Negative or no correlation between HLL and SVL or body weight was found in the other late stage data sets. Therefore, use of SVL‐normalized HLL to assess thyroid‐mediated effects in X. laevis tadpoles is not warranted. HL stage relative to body stage should be considered.     

Plasticity of skeletal muscle phenotype: mechanical consequences

Muscles are complex biological machines that perform a wide variety of mechanical activities. Over the past 30 to 40 years, a large amount of effort has been devoted to understanding cellular/molecular responses of skeletal muscle to various altered physiological states (e.g., altered loading state induced via immobilization/spaceflight, resistance training). Many cellular/molecular adaptations brought about by such interventions act on underlying processes that regulate activation, force and velocity of shortening/lengthening, and relaxation. In this context, measurements of mechanical properties (e.g., force-velocity relationship) are important, because they can provide insight into the physiological consequences of such adaptations. During the course of the past 10 to 15 years, a number of investigators have employed the work-loop technique to provide a more realistic approach toward understanding muscle function. Additionally, the work-loop technique provides a unique conceptual perspective that integrates: (1) the length-tension relationship, (2) activation kinetics, (3) the force-velocity relationship in the shortening domain, (4) relaxation kinetics, (5) the force-velocity relationship in the lengthening domain, and (6) the compliance of the passive elastic elements. A discussion of those factors (i.e., factors 2-5) that appear to be highly malleable forms the basis of this paper.     

The role of adrenoceptors in norepinephrine-stimulated VO2 in muscle.

The purpose of this study was to characterize the norepinephrine (NE)-evoked calorigenic response of an isolated perfused rat muscle preparation into alpha-and/or beta-adrenergic components. Epinephrine was as effective as NE in evoking the increase in oxygen consumption. The alpha-agonist, phenylephrine, was also as effective as NE in eliciting an increase in oxygen consumption by the muscle. Isoproterenol (beta-agonist) had no effect on the VO2 of the preparation. Phentolamine, an alpha-blocker, completely blocked the NE-stimulated VO2. Propranolol (beta-antagonist) could also completely block the NE induced VO2 but this drug was found to be less potent. It was concluded that the NE-induced calorigenic response of muscle cannot be classified either strictly as an alpha-or beta-response, but unlike brown adipose tissue, the alpha-response seems to predominate in skeletal muscle. Also, it appears as though different mechanisms of action may be involved in the NE-stimulated VO2 in skeletal muscle and brown fat.     

Functional alterations of mesenteric vascular bed, vas deferens and intestinal tracts in a rat hindlimb unloading model of microgravity

1. Prolonged bed rest or exposure to microgravity may cause several alterations in autonomic nervous system response (ANSR). 2. Hindlimb unloading (HU) rats were used as an animal model of simulated microgravity to investigate ANSR changes. The experiments were carried out to investigate the effects of simulated microgravity on the autonomic nervous response of the perfused mesenteric vascular bed (MVB), vas deferens and the colon and duodenum from 2-week HU rats. 3. In MVB preparations of HU rats, the frequency-dependent increases in perfusion pressure with perivascular nerve stimulation (PNS; 8-40 Hz) were inhibited, whereas the noradrenaline (NA) concentration-dependent (1-100 microM) perfusion pressure increases were potentiated. The latter most probably reflected up-regulation of alpha-adrenergic receptor function. Relaxant responses of NA-precontracted MVB to PNS (4-30 Hz) or isoprenaline were not different between control and HU preparations, while vasodilation induced by the endothelial agonist ACh was reduced. 4. Transmural stimulation (2-40 Hz) induced frequency-dependent twitches of the vas deferens which were reduced in vas deferens of HU rats, while the sensitivity to NA-induced contraction was significantly increased. 5. In the gastroenteric system of HU rat, direct contractile responses to carbachol or tachykinin as well as relaxant or contractile responses to nervous stimulation appeared unchanged both in the proximal colon rings and in duodenal longitudinal strips. 6. In conclusion, HU treatment affects peripheral tissues in which the main contractile mediators are the adrenergic ones such as resistance vessels and vas deferens, probably by reducing the release of neuromediator. This study validates NA signalling impairment as a widespread process in microgravity, which may most dramatically result in the clinical phenotype of orthostatic intolerance.     

In vivo assessment of absolute perfusion in the murine skeletal muscle with spin labeling MRI. Magnetic resonance imaging

PURPOSE: To assess absolute perfusion in the skeletal muscle of mice in vivo with spin labeling magnetic resonance imaging (MRI) under normal and stress conditions. MATERIALS AND METHODS: Absolute perfusion in the skeletal muscle of 27 C57BL/6 mice was assessed in vivo non-invasively by spin labeling MRI at 7.05 T. This technique was based on the acquisition of T1 maps with global and slice-selective spin inversion in separate acquisitions. T1 mapping was performed by inversion recovery snapshot fast low angle shot imaging. To guarantee proper spin inversion within the whole mouse, a dedicated radiofrequency (RF) coil combination was constructed. A birdcage resonator was used for transmission, while detection of the MRI signal was achieved by a surface coil. RESULTS: Basal perfusion in the hindlimbs was determined to be 94 +/- 10 mL (100 g x minute)(-1) (mean +/- standard error of the mean [SEM], N = 27). This value is in good agreement with perfusion values determined by invasive techniques such as microspheres. A subgroup of six animals received a constant dose of 4 mg (kg x minute)(-1) of the vasodilator adenosine by an intraperitoneal catheter. In this case, perfusion was significantly increased to 179 +/- 56 mL (100 g x minute)(-1) (mean +/- SEM, N = 6, P < 0.02). Mean basal perfusion in this subgroup was 96 +/- 26 mL (100 g x minute)(-1). CONCLUSION: Spin labeling MRI is a well-suited technique for the in vivo assessment of absolute perfusion in the murine skeletal muscle.