Interaction of force transmission and sarcomere assembly at the muscle-tendon junctions of carp (Cyprinus carpio): ultrastructure and distribution of titin (connectin) and α-actinin

At muscle-tendon junctions of red and of white axial muscle fibres of carp, new sarcomeres are found adjacent to existing sarcomeres along the bundles of actin filaments that connect the myofibrils with the junctional sarcolemma. As the filament bundles that transmit force to the junction originate proximal to new sarcomeres, they probably relieve these new sarcomeres from premature loading. In red fibres, these filament bundles are long (up to 20 m) and dense, permitting light-microscopical immunohistochemistry (double reactions: anti-titin or anti--actinin and phalloidin). New sarcomeres have clear I bands; their A band lengths are similar to those of older sarcomeres and the thick filaments lie in register. T tubules are found at the distal side of new sarcomeres but terminal Z lines are absent. The late addition of -actinin suggests that -actinin mainly has a stabilizing role in sarcomere formation. The presence of titin in the terminal fibre protrusions is in agreement with its supposed role in sarcomere formation, viz. the integration of thin and thick filaments. The absence of a terminal Z line from sarcomeres with well-registered A bands suggests that this structure is not essential for the anchorage of connective (titin) filaments.     

Influence of central command and ergoreceptors on the splanchnic circulation during isometric exercise

The splanchnic circulation can make a major contribution to blood flow changes. However, the role of the splanchnic circulation in the reflex adjustments to the blood pressure increase during isometric exercise is not well documented. The central command and the muscle chemoreflex are the two major mechanisms involved in the blood pressure response to isometric exercise. This study aimed to examine the behaviour of the superior mesenteric artery during isometric handgrip (IHG) at 30% maximal voluntary contraction (MVC). The pulsatility index (PI) of the blood velocity waveform of the superior mesenteric artery was taken as the study parameter. A total of ten healthy subjects [mean age, 21.1 (SEM 0.3) years] performed an IHG at 30% MVC for 90 s. At 5 s prior to the end of the exercise, muscle circulation was arrested for 90 s to study the effect of the muscle chemoreflex (post exercise arterial occlusion, PEAO). The IHG at 30% MVC caused a decrease in superior mesenteric artery PI, from 4.84 (SEM 1.57) at control level to 3.90 (SEM 1.07) (P = 0.015). The PI further decreased to 3.17 (SEM 0.70) (P = 0.01) during PEAO. Our results indicated that ergoreceptors may be involved in the superior mesenteric artery vasodilatation during isometric exercise.     

Motor unit recruitment strategy of knee antagonist muscles in a step-wise, increasing isometric contraction

The purpose of this study was to determine if differences exist between the control strategies of two antagonist thigh muscles during knee flexion and extension muscular coactivation. Surface myoelectric signal (MES) of the quadriceps (rectus femoris) and the hamstrings (semitendinosus) were obtained from both muscles while performing step-wise increasing contractions during flexion and extension with the knee at 1.57 rad of flexion (90 degrees). The median frequency of the power density spectrum, which is related to the average muscle fiber action potential conduction velocity and therefore to motor unit recruitment, was calculated from each MES. The results suggest that, in all the subjects tested, when the muscle acts as antagonist most motor units are recruited up to 50% of the maximal voluntary force, whereas when the muscle acts as antagonist motor units are recruited up to 40% of the maximal voluntary force. The force range past 40–50% of the maximal force is also characterized by differences between the agonist/antagonist.     

Role of agonist and antagonist muscle strength in performance of rapid movements

Six subjects performed rapid self-terminated elbow movements under different mechanical conditions prior to, and 5 weeks after an elbow extensor strengthening programme. Despite the large difference in the strengths of elbow flexors and extensors, the pretest did not demonstrate significant differences between the movement time of flexion and extension movements performed under the same mechanical conditions. The results obtained in the posttest demonstrated a decrease in movement time (i.e. an increase in movement speed) in both elbow flexion and extension movements under some mechanical conditions. In addition, flexion movements demonstrated a relative increase in the acceleration time (acceleration time as a proportion of the movement time). It was concluded that the strength of both the agonist and antagonist muscles was important for the performance of rapid movements. Stronger agonists could increase the acceleration of the limb being moved, while stronger antagonists could facilitate the arrest of the limb movement in a shorter time, providing a longer time for acceleration.     

Control of the rate of phosphocreatine resynthesis after exercise in trained and untrained human quadriceps muscles

We examined the effect of differences in exercise intensity on the time constant (t _c) of phosphocreatine (PCr) resynthesis after exercise and the relationships betweent _c and maximal oxygen uptake (VO_2max) in endurance-trained runners (n = 5) and untrained controls (n = 7) (average VO_2max = 66.2 and 52.0 ml · min^–1 · kg^–1, respectively). To measure the metabolism of the quadriceps muscle using phosphorus nuclear magnetic resonance spectroscopy, we developed a device which allowed knee extension exercise inside a magnet. All the subjects performed four types of exercise: light, moderate, severe and exhausting. The end-exercise PCr: [PCr + inorganic phosphate (P_i)] ratio decreased significantly with the increase in the exercise intensity (P < 0.01). Although there was little difference in the end-exercise pH, adenosine diphosphate concentration ([ADP]) and the lowest intracellular pH during recovery between light and moderate exercise, significant changes were found at the two higher intensities (P < 0.01). These changes for runners were smaller than those for the controls (P < 0.05). The _c remained constant after light and moderate exercise and then lengthened in proportion to the increase in intensity (P < 0.05). The runners had a lowert _c at the same PCr and pH than the controls, particularly at the higher intensity (P < 0.05). There was a significant correlation betweent _c and [ADP] in light exercise and betweent _c and both end-exercise PCr and pH in severe and exhausting exercise (P < 0.05). The threshold of changes in pH andt _c was a PCr: (PCr + P_i) ratio of 0.5. There was a significant negative correlation between the VO_2max andt _c after all levels of exercise (P<0.05).However, in the controls a significant correlation was found in only light and moderate exercise (P < 0.05). These findings suggest the validity of the use oft _c at an end-exercise PCr:(PCr + P_i) ratio of more than 0.5 as a stable index of muscle oxidative capacity and the correlation between local and general aerobic capacity. Moreover, endurance-trained runners are characterized by the faster PCr resynthesis at the same PCr and intracellular pH.     

The larval development of lateral musculature in gilthead sea bream Sparus aurata and sea bass Dicentrarchus labrax

Fibre-type differentiation of the lateral musculature has been studied in Sparus aurata (L.) and Dicentrarchus labrax (L.) during larval development. Histochemical and ultrastructural techniques show two presumptive muscle layers and two germinative zones of presumptive myoblasts. At hatching, myotomal muscle consists of a monolayer of thin undifferentiated cells near the skin (first germinative zone) overlying another mono-layer of small diameter fibres extending hypaxially and epaxially away from the transverse septum. Below this, there is a much thicker, deep layer of fibres, generally large in diameter and polygonal in shape. The presumptive myoblasts are located between these two layers of fibres in the second germinative zone. Initially, the superficial and deep muscle fibres show high and low myosin ATPase activity, respectively. Both layers grow by generating new fibres from the two mentioned germinative zones. At the end of larval life, the superficial layer changes its histochemical profile from high to low myosin ATPase activity and, at the same time, intermediate or pink muscle fibres can be observed by oxidative activity (the NADH-TR reaction). Morphometric analysis shows a significant increase in mean fibre diameter during successive ages, as shown by the Student's t-test (hypertrophic growth). Skewness and kurtosis values of fibre diameters point to the generation of a new fibre population from the germinative zones (hyperplastic growth).     

A mechanism for early branching in lung morphogenesis

The lung is a highly branched fluid-filled structure, that develops by repeated dichotomous branching of a single bud off the foregut, of epithelium invaginating into mesenchyme. Incorporating the known stress response of developing lung tissues, we model the developing embryonic lung in fluid mechanical terms. We suggest that the repeated branching of the early embryonic lung can be understood as the natural physical consequence of the interactions of two or more plastic substances with surface tension between them. The model makes qualitative and quantitative predictions, as well as suggesting an explanation for such observed phenomena as the asymmetric second branching of the embryonic bronchi.     

Muscle determinants in the ascidian egg are inactivated by UV irradiation and the inactivation is partially rescued by injection of maternal mRNAs

The ascidian egg contains cytoplasmic determinants that specify the fate of larval muscle cells. In a previous study, we developed an experimental system to identify the molecular nature of muscle determinants, in which unfertilized Ciona savignyi eggs were fragmented into four pieces by centrifugation. When inseminated, only nucleated fragments (red fragments) develop into partial embryos that only show differentiation of epidermal cells. One type of enucleated fragment (black fragment) has the remarkable ability to promote muscle differentiation when fused with red fragments. In the present study, using this experimental system, we investigated the molecular nature of muscle determinants. UV irradiation of black fragments suppressed the ability to promote expression of the muscle-specific protein, myosin heavy chain. The wavelength of UV light responsible for the inactivation (250–275 nm) suggested that UV-sensitive targets are nucleic acids. Injection of poly(A)⁺ RNA isolated from an un-irradiated black-fragment-rich fraction into UV-irradiated black fragments partially recovered the ability to promote the expression of myosin heavy chain protein. Poly(A)⁺ RNA from a red-fragment-rich fraction did not rescue the suppression of UV-irradiated black fragments. These results suggest that maternal mRNAs enriched in black fragments are closely associated with muscle determinants in the ascidian egg.     

Active muscle migration during insect metamorphosis

The R1 abdominal retractor muscles of the insect Tenebrio molitor change position during the course of metamorphosis. These muscles detach from the epidermal tendon cells at their anterior ends, and migrate in a posterior direction, parallel to the body axis, to form completely new attachments shortly before adult emergence. Movement is preceded by the loss of sarcomere structure, and the muscles migrate in a partially dedifferentiated condition, closely accompanied by satellite cells and haemocytes. Movement appears to result from the extension of muscle processes towards the epidermis posterior to the larval attachment sites, which contact reciprocal processes extended from the epidermis. Contacts at the new posterior sites are then reinforced, and relinquished at the anterior. This cycle is subsequently repeated. It is envisaged that migration ceases when the muscles encounter a contour in the epidermal gradient known to specify the position of the adult muscle attachment sites. This positional information may be encoded in the epidermal basal lamina. The muscles then redifferentiate, with concurrent differentiation of new epidermal tendon cells. Development of adult muscle attachments appears to require reciprocal morphogenetic interactions between muscle and epidermis.