On simulating sustained isometric muscle fatigue: a phenomenological model considering different fiber metabolisms.

The present study shows a new computational FEM technique to simulate the evolution of the mechanical response of 3D muscle models subjected to fatigue. In an attempt to obtain very realistic models, parameters needed to adjust the mathematical formulation were obtained from in vivo experimental tests. The fatigue contractile properties of three different rat muscles (Tibialis Anterior, Extensor Digitorium Longus and Soleus) subjected to sustained maximal isometric contraction were determined. Experiments were conducted on three groups [Formula: see text] of male Wistar rats [Formula: see text] using a protocol previously developed by the authors for short tetanic contractions. The muscles were subjected to an electrical stimulus to achieve tetanic contraction during 10 s. The parameters obtained for each muscle were incorporated into a finite strain formulation for simulating active and passive behavior of muscles with different fiber metabolisms. The results show the potential of the model to predict muscle fatigue under high-frequency stimulation and the 3D distribution of mechanical variables such as stresses and strains.

A 3D active-passive numerical skeletal muscle model incorporating initial tissue strains. Validation with experimental results on rat tibialis anterior muscle.

This paper presents a three-dimensional finite element model of skeletal muscle and its validation incorporating inital tissue strains. A constitutive relation was determined by using a convex free strain energy function (SEF) where active and passive response contributions were obtained fitting experimental data from the rat tibialis anterior (TA) muscle. The passive and active finite strains response was modelled within the framework of continuum mechanics by a quasi-incompressible transversely isotropic material formulation. Magnetic resonance images (MRI) were obtained to reconstruct the external geometry of the TA. This geometry includes initial strains also taken into account in the numerical model. The numerical results show excellent agreement with the experimental results when comparing reaction force-extension curves both in passive and active tests. The proposed constitutive model for the muscle is implemented in a subroutine in the commercial finite element software package ABAQUS.

Active response of skeletal muscle: in vivo experimental results and model formulation.

The contractile properties of the tibial anterior (TA) of Wistar rats were measured by means of a multipurpose testing machine. The muscle was isolated from the connective tissues, preserving the proximal insertion. The distal tendon was transected and fixed to the machine actuator. The leg was inmobilised using a pin drilled through the femoral condyle. In this way the force response was studied in vivo at different constant lengths for some voltages and frequencies. Mathematical functions are proposed for adjusting the force-length, force-frequency and force-time relations. The model includes a novel formulation for the depression response during muscle tetanisation.

Passive nonlinear elastic behaviour of skeletal muscle: experimental results and model formulation.

The goal of this study was to characterize the passive elastic behaviour of muscle and tendon tissues of rat tibialis anterior. For that purpose, tissue samples from 3 month old female Wistar rats (210+/-11g) were mechanically tested in vitro. Moreover, an in vivo device was developed to measure the muscle-tendon unit response to increasing load. Mechanical tests, consisting of uniaxial loading along the longitudinal axis of tendon and muscle strips, revealed the nonlinear mechanical behaviour of these tissues. A material model was formulated and its parameters fit to the experimental data using the Levenberg-Marquardt optimization algorithm. The fit goodness was assessed and R(2) values close to 1 and very low epsilon values were obtained. The passive behaviour of a future finite element model of a muscle-tendon unit will be validated against the in vivo passive extension tests by comparing the reaction force-extension curves.

Superoxide anion and nitric oxide in high-grade esophagitis induced by acid and pepsin in rabbits.

It has been proposed that free radicals are involved in the pathogenesis of esophageal mucosal damage induced by acid and pepsin. Recent data have suggested that nitric oxide (NO) is involved in the mucosal defense of the esophagus and that superoxide anion plays a minor role in low-grade esophagitis. To study the role and potential interaction of NO and superoxide anion in an experimental model of high-grade esophagitis, acidified pepsin was perfused (45 min/12 hr) for five days in rabbits with different agents to modulate the generation of these radicals. Measurements included both macroscopic and microscopic mucosal damage, superoxide anion generation, NO synthase mucosal activity, and peroxynitrite formation. High-grade esophagitis was associated with mucosal superoxide anion generation. Treatment with exogenous superoxide dismutase completely prevented mucosal damage. The perfusion of acidified pepsin in the lumen of the esophagus was initially associated with increased NO synthase mucosal activity but decreased with the progression of damage. Generation of peroxynitrites was present in those cases with severe damage. Treatment with NO-modifying agents did not induce consistent modification of mucosal damage. It is concluded that superoxide anion is involved in the induction of high-grade esophagitis and that it interacts with nitric oxide to generate peroxynitrite radicals in this model. Superoxide dismutase but not NO-donor-modifying agents might have a therapeutic role in preventing severe esophageal mucosal damage induced by acid and pepsin.

Nitric oxide and superoxide anion in low-grade esophagitis induced by acid and pepsin in rabbits.

It has been suggested that free radicals are involved in esophagitis. To study the role and potential interaction of superoxide anion and nitric oxide (NO) in low-grade esophagitis, we perfused acidified pepsin (30 min every 12 hr) for seven days in rabbits treated with different agents to modulate the generation of these radicals. Measurements included macroscopic and microscopic damage, superoxide anion generation, mucosal nitric oxide synthase activity, and peroxynitrite formation. Low-grade esophagitis was associated with increased nitric oxide synthase mucosal activity and mucosal damage was dose-dependently increased by treatment with the NO synthase inhibitor NG-nitro-L-arginine. Superoxide anion was scarcely generated in the mucosa, but this was not accompanied by any change in the activity of mucosal superoxide dismutase. Treatment with superoxide dismutase did not improve mucosal damage. Generation of peroxynitrites was not detected. In conclusion, nitric oxide is involved in the mucosal defense of the esophagus against acid- and pepsin-induced damage. Superoxide anion generation seems irrelevant in the induction of low-grade esophagitis and not sufficient to interact with nitric oxide to generate measurable mucosal peroxynitrite radicals.

[The effect of thyroparathyroidectomy, parathyroid hormone and calcitonin on plasma strontium in rats]. / Efecto de la tiroparatiroidectomía, la parathormona y la calcitonina sobre el estroncio plasmático en rata.

The effect of thyroparathyroidectomy (TPTX) on the plasmatic Sr concentrations in rats previously supplemented with this element, has been studied, as well as its effect on the treatment of TPTX rats with hormonal combinations and, finally, the one presenting hormonal excess or defect of the phosphocalcium metabolism regulating hormones: parathormone (PTH) and calcitonin (CT). Twenty four hours after TPTX, the plasmatic Sr concentrations show a pattern similar to those of Ca and Mg and contrary to Pi. The subsequent evolution is different, as the plasmatic concentrations increase, probably due to the maintenance of Sr supplementation. The administration of this element to TPTX rats and the treatment with a hormonal combination with two of the following hormones: PTH, CT and T4 antagonize the hormonal effect on the restoration of the plasmatic concentrations of the elements analyzed. The PTH excess and defect (TPTX treated with CT + T4) show plasmatic increases in Sr; the CT excess provokes decreases while the defect (administration of PTH + T4) causes increases. The T4 administration reproduces the CT effects, but inconsistently. These results suggest that CT may be the hormone that plays a regulating role in the plasmatic Sr concentrations.

[Effects of lithium on serum calcium, magnesium and inorganic phosphorus after exercise in rats]. / Influencia del litio sobre calcio, magnesio y fósforo inorgánico séricos tras ejercicio en ratas.

Exercise, in patients on lithium (Li) treatment, causes alterations in serum concentration of this element, not well established yet. Besides, lithium treatment and exercise practice bring about variations in the serum levels of other mineral elements. This paper intends to analyze these variations in rats with and without lithium treatment after training and exhausting exercise. In normal rats, lithium treatment increases serum concentrations in all ions analyzed, larger than those found in the trained rats having received lithium treatment. In rats treated with Li, training (15 min/day during one week), significatively decreases serum concentration in this element, while exhausting exercise did not offer significant results. In normal rats, exhausting exercise causes serum increases of Ca, Mg and Pi; in rats treated with lithium, when compared with normal rats plus exhaustion, greater increases of serum Ca stand out, while Mg concentrations are smaller. Training arises serum Mg and Pi in normal rats; while it increases serum Ca in rats treated with Li.

[Evolution of the number of authors in clinical and basic science journals in the Spanish language]. / Evolución del número de autores en revistas clínicas y básicas de lengua española.

The number of signing authors in Revista Clínica Española. Revista Española de Fisiología and Revista Española de Oncología have been analyzed from their first to the last received issue. The results obtained show an increasing number of authors in all journals specially during the 70s. The results also point out a relative decrease in the number of authors in basic sciences in relation to clinical publications. The increase in the number of authors in The Revista Española de Oncología has started somewhat later than the others. The environmental and professional stress as well as the interrelations between different hospital members have been suggested, amongst others, as the possible cause of these events.