Plateau Tension Research Articles

Effects of boron waste as a reinforcement in the production of Al composite foams

Abstract In this study, the effect of using boron products in the production of aluminium composite foam on the formation of pores and mechanical properties of the material was investigated. Metallic foam was produced by powder metallurgy method by adding borogypsum to the Al matrix and using boric acid (H3BO3) as a pore former. Samples containing 5, 10 and 20 wt% borogypsum were prepared at initial pressing pressures of 420 MPa and 630 MPa and sintered at 550 °C for 3 h. As a result of microhardness measurements taken from the cell wall of foam samples with ∼40% porosity, it was determined that the hardness value increased as the borogypsum ratio increased. According to the results of the quasi-static compression test performed at a deformation rate of 10−3 s−1 at room temperature, the hardness and strength values of 20 wt% borogypsum reinforced Al composite foam prepared at 630 MPa pressure increased. In addition, mechanical properties such as plateau tension and elastic modulus for each sample were theoretically determined with the Gibson–Ashby approach.

Vacuum brazing and heat treatment of NiTi shape memory alloys

The pseudoelasticity of NiTi shape memory alloys is a unique material property which can be characterized by a complete recovery of a previously impressed component shape by a change of the thermal or mechanical load conditions after deforming. In contrast to the elastic deformation of ordinary materials like steels, twentyfold higher elastic strain rates up to 10 % are possible due to a temperature or a stress induced diffusion-free transformation of the crystal lattice between the austenite and martensite phases. Therefore, these superelastic alloys are frequently used as actuators, implants or stents so that there is an extraordinary high requirement of reliability and biocompatibility. In terms of joining, vacuum brazing might be a particularly suitable method to produce joined components which preserve a maximum of pseudoelasticity. Within the present research, it was shown that the vacuum brazing process at 1180 °C using pure niobium is well integrable into a solution annealing and a shape annealing heat treatment in a single furnace run. This led to a distinct tension plateau at around 285 MPa with an almost R-phase-free conversion of NiTi. Furthermore, it was proven that the share of the superelastic and proeutectic NiTiNb-phase was significantly increased with the dwell time.

Coil Interpenetration, Segment Aggregation and Adsorption of PEG at Water/Air Interface

AbstractPoly(ethylene glycol) (PEG), both as an oligomer and as a high molecular weight polymer, has been used as a surfactant active component in many applications, in its pure and unmodified structure, or as a block segment within more complex structures. Apart from assuming helical or coil conformations it may also undergo aggregation, depending on the solvent. In this work, we characterized the aggregation of PEG in water using surface tensiometry and turbidimetry. Aggregation was characterized by a non‐persistent surface tension plateau at higher PEG contents and confirmed by turbidimetry, which was correlated to the derivative of osmotic pressure in relation to concentration. One can divide a surface tension‐concentration curve into two regions, delimited by a characteristic concentration: one following a Langmuir adsorption isotherm (starting from a plateau equal to pure water surface tension) ending in a second plateau, and a second one (beginning at this plateau) following a Freundlich isotherm. The calculated surface excess for PEG segment adsorption was consistent with segment aggregation, indicating more two characteristic concentrations correlated to this occurrence. A total of three characteristic mer concentrations were found (6.2 × 10−7, 6 × 10−6, and 2 × 10−4mol L−1) and related to PEG segment aggregation states.

X-ray Diffraction Evidence for Low Force Actin-Attached and Rigor-Like Cross-Bridges in the Contractile Cycle.

Defining the structural changes involved in the myosin cross-bridge cycle on actin in active muscle by X-ray diffraction will involve recording of the whole two dimensional (2D) X-ray diffraction pattern from active muscle in a time-resolved manner. Bony fish muscle is the most highly ordered vertebrate striated muscle to study. With partial sarcomere length (SL) control we show that changes in the fish muscle equatorial A-band (10) and (11) reflections, along with (10)/(11) intensity ratio and the tension, are much more rapid than without such control. Times to 50% change with SL control were 19.5 (±2.0) ms, 17.0 (±1.1) ms, 13.9 (±0.4) ms and 22.5 (±0.8) ms, respectively, compared to 25.0 (±3.4) ms, 20.5 (±2.6) ms, 15.4 (±0.6) ms and 33.8 (±0.6) ms without control. The (11) intensity and the (10)/(11) intensity ratio both still change ahead of tension, supporting the likelihood of the presence of a head population close to or on actin, but producing little or no force, in the early stages of the contractile cycle. Higher order equatorials (e.g., (30), (31), and (32)), more sensitive to crossbridge conformation and distribution, also change very rapidly and overshoot their tension plateau values by a factor of around two, well before the tension plateau has been reached, once again indicating an early low-force cross-bridge state in the contractile cycle. Modelling of these intensity changes suggests the presence of probably two different actin-attached myosin head structural states (mainly low-force attached and rigor-like). No more than two main attached structural states are necessary and sufficient to explain the observations. We find that 48% of the heads are off actin giving a resting diffraction pattern, 20% of heads are in the weak binding conformation and 32% of the heads are in the strong (rigor-like) state. The strong states account for 96% of the tension at the tetanus plateau.

Exposure to a Low Lead Concentration Impairs Contractile Machinery in Rat Cardiac Muscle.

Lead exposure has been considered to be a risk factor for hypertension and cardiovascular disease. Our purpose was to evaluate the effects of low plasma lead concentration on cardiac contractility in isolated papillary muscles. Wistar rats were divided in control group or group treated with 100 ppm of lead acetate in the drinking water for 15 days. Blood pressure (BP) was measured weekly. At the end of the treatment period, the animals were anesthetized and euthanized, and parameters related to isolated papillary muscle contractility were recorded. The lead concentrations in the blood reached 12.3 ± 2 μg/dL. The BP was increased in the group treated with 100 ppm of lead acetate. Lead treatment did not alter force and time derivatives of the force of left ventricular papillary muscles. In addition, the inotropic response induced by an increase in the extracellular Ca(2+) concentration was reduced in the Pb(2+) group. However, the uptake of Ca(2+) by the sarcoplasmic reticulum and the protein expression of SERCA and phospholamban remained unchanged. Postrest contraction was similar in the both groups, and tetanic peak and plateau tension were reduced in lead group. These results demonstrated that the reduction in the inotropic response to calcium does not appear to be caused by changes in the trans-sarcolemmal calcium flux but suggest that an impairment of the contractile machinery might be taking place. Our results demonstrate that even at a concentration below the limit considered to be safe, lead exerts deleterious effects on the cardiac contractile machinery.

Is functional sympatholysis (FS) altered in chronically hypoxic (CH) rats?

Muscle sympathetic vasoconstriction is blunted in CH, however, there is no change in the modulatory role of NO. Exercise intolerance and CH are features of diseases such as COPD, this may reflect alterations in FS and fatigability. In accordance with UK legislation, experiments were performed on anaesthetised CH rats (3 weeks acclimation to 12% O2). Lumbar sympathetic chain stimulation was used to induce sympathetic vasoconstriction. Peroneal nerve stimulation evoked EDL contraction and tension was recorded. Femoral vascular resistance was calculated from recordings of arterial blood pressure and femoral blood flow. Sympathetic vasoconstriction was evoked during baseline and graded muscle contraction (10–30% max twitch) under control conditions, after NOS inhibition (L‐NAME) and again following restoration of background NO levels (SNP infusion). Compared to normal (N) rats, peak muscle tension was not different but CH rats fatigued more such that plateau tension was lower (even when O2 delivery was similar to N rats). FS was not different in CH and N rats and was not affected by either L‐NAME or SNP infusion. Muscle fatigue is greater in CH than N rats but it is not due to differences in O2 delivery. FS is unchanged despite sympathetic vasoconstriction being blunted in CH rats, and there appears to be no role for NO in mediating FS. Changes in FS are unlikely to explain exercise intolerance in diseases such as COPD.

Surface Behavior, Aggregation and Phase Separation of Aqueous Mixtures of Dodecyl Trimethylammonium Bromide and Sodium Oligoarene Sulfonates: the Transition to Polyelectrolyte/Surfactant Behavior

The properties and phase diagrams of aqueous mixtures of dodecyltrimethylammonium bromide (C(12)TAB) with the sodium oligoarene sulphonates (POSn), POS2, POS3, POS4, and POS6 have been studied using surface tension and neutron reflectometry to study the surface, and neutron small angle scattering and fluorescence to study the bulk solution. The behavior of POS2 and POS3 is reasonably consistent with mixed micelles of C(12)TAB and POSn-(C(12)TA)(n). These systems exhibit a single critical micelle concentration (CMC) at which the surface tension reaches the usual plateau. This is contrary to a recent report which suggests that the onset of the surface tension plateau does not coincide with the CMC. In the POS3 system, the micelles conform to the core-shell model, are slightly ellipsoidal, and have aggregation numbers in the range 70-100. In addition, the dissociation constant for ionization of the micelles is significantly lower than for free C(12)TAB micelles, indicating binding of the POS3 ion to the micelles. Estimation of the CMCs of the POSn-(C(12)TA)(n) from n = 1-3 assuming ideal mixing of the two component surfactants and the observed values of the mixed CMC gives values that are consistent with the nearest related gemini surfactant. The POS4 and POS6 systems are different. They both phase separate slowly to form a dilute and a concentrated (dense) phase. Fluorescence of POS4 has been used to show that the onset of aggregation of surfactant (critical aggregation concentration, CAC) occurs at the onset of the surface tension plateau and that, at the slightly higher concentration of the phase separation, the concentration of POS4 and C(12)TAB in the dilute phase is at or below its concentration at the CAC, that is, this is a clear case of complex coacervation. The surface layer of the C(12)TA ion in the surface tension plateau region, studied directly by neutron reflectometry, was found to be higher than a simple monolayer (observed for POS2 and POS3) for both the POS4 and POS6 systems. In POS6 this evolved after a few hours to a structure consisting of a monolayer with an attached subsurface bilayer, closely resembling that observed for one class of polyelectrolyte/surfactant mixtures. It is suggested that this structured layer, which must be present on the surface of the dilute phase of the coacervated system, is a thin wetting film of the dense phase. The close resemblance of the properties of the POS6 system to that of one large group of polyelectrolyte/surfactant mixtures shows that the surface behavior of oligoion/surfactant mixtures can quickly become representative of that of true polyelectrolyte/surfactant mixtures. In addition, the more precise characterization possible for the POS6 system identifies an unusual feature of the surface behavior of some polyelectrolyte/surfactant systems and that is that the surface tension can remain low and constant through a precipitation/coacervation region because of the characteristics of two phase wetting. The well-defined fixed charge distribution in POS6 also suggests that rigidity and charge separation are the factors that control whether a given system will exhibit a flat surface tension plateau or the alternative of a peak on the surface tension plateau.

Aggregation behavior of amphiphilic random copolymer of 2-(acrylamido)-2-methylpropanesulfonic acid and tris(trimethylsiloxy)silylpropylmethacrylate in aqueous solution

Aggregation behavior including dilute solution property and surface-activity of the amphiphilic random copolymer composed of 2-(acrylamido)-2-methylpropanesulfonic acid and tris(trimethylsiloxy)silylpropylmethacrylate (AMPS/TRIS copolymer) in aqueous solution were studied by static light scattering (SLS), dynamic light scattering (DLS), surface tension measurement, and transmission electron microscopy (TEM). The surface tension measurement made it clear that AMPS/TRIS copolymer exhibited weaker surface-activity than a typical low-molecular weight surfactant sodium dodecyl sulfate in water, that is, there were no plateau of surface tension γ versus concentration and no critical micelle concentration (CMC) in the whole concentration studied. SLS and DLS analyses, and TEM revealed that AMPS/TRIS copolymer self-associated into imperfect core-shell micelles having hydrophobic TRIS core surrounded by hydrophilic AMPS shell in water. AMPS shell was considered as a hard shell due to the stiffness of AMPS chain in water. TRIS chain could not densely aggregate in water due to the large steric hindrance between bulky trimethylsiloxy groups despite its hydrophobic nature, thereby providing TRIS core with less-dense structure. The balance between the spreading force of stiff AMPS chain and the cohesion force of bulky TRIS chain provides the driving force for forming the unique micelle having less-dense TRIS core and hard AMPS shell. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011

Association of muscle hardness with muscle tension dynamics: a physiological property

This study aimed to investigate the relationship between muscle hardness and muscle tension in terms of length-tension relationship. A frog gastrocnemius muscle sample was horizontally mounted on the base plate inside a chamber and was stretched from 100 to 150% of the pre-length, in 5% increments. After each step of muscle lengthening, electrical field stimulation for induction of tetanus was applied using platinum-plate electrodes positioned on either side of the muscle submerged in Ringer's solution. The measurement of muscle hardness, i.e., applying perpendicular distortion, was performed whilst maintaining the plateau of passive and tetanic tension. The relationship between normalised tension and normalised muscle hardness was evaluated. The length-hardness diagram could be created from the modification with the length-tension diagram. It is noteworthy that muscle hardness was proportional to passive and total tension. Regression analysis revealed a significant correlation between muscle hardness and passive and total tension, with a significant positive slope (passive tension: r = 0.986, P < 0.001; total tension: r = 0.856, P < 0.001). In conclusion, our results suggest that muscle hardness depends on muscle tension in most ranges of muscle length in the length-tension diagram.

Thick-Filament Strain and Interfilament Spacing in Passive Muscle: Effect of Titin-Based Passive Tension

We studied the effect of titin-based passive tension on sarcomere structure by simultaneously measuring passive tension and low-angle x-ray diffraction patterns on passive fiber bundles from rabbit skinned psoas muscle. We used a stretch-hold-release protocol with measurement of x-ray diffraction patterns at various passive tension levels during the hold phase before and after passive stress relaxation. Measurements were performed in relaxing solution without and with dextran T-500 to compress the lattice toward physiological levels. The myofilament lattice spacing was measured in the A-band ( d 1,0) and Z-disk ( d Z) regions of the sarcomere. The axial spacing of the thick-filament backbone was determined from the sixth myosin meridional reflection (M6) and the equilibrium positions of myosin heads from the fourth myosin layer line peak position and the I 1,1/I 1,0 intensity ratio. Total passive tension was measured during the x-ray experiments, and a differential extraction technique was used to determine the relations between collagen- and titin-based passive tension and sarcomere length. Within the employed range of sarcomere lengths (∼2.2–3.4 μm), titin accounted for >80% of passive tension. X-ray results indicate that titin compresses both the A-band and Z-disk lattice spacing with viscoelastic behavior when fibers are swollen after skinning, and elastic behavior when the lattice is reduced with dextran. Titin also increases the axial thick-filament spacing, M6, in an elastic manner in both the presence and absence of dextran. No changes were detected in either I 1,1/I 1,0 or the position of peaks on the fourth myosin layer line during passive stress relaxation. Passive tension and M6 measurements were converted to thick-filament compliance, yielding a value of ∼85 m/N, which is several-fold larger than the thick-filament compliance determined by others during the tetanic tension plateau of activated intact muscle. This difference can be explained by the fact that thick filaments are more compliant at low tension (passive muscle) than at high tension (tetanic tension). The implications of our findings are discussed.

Mechanical and contractile characteristics of rat thoracic duct and cervical lymphatics

Thoracic duct (TD) lymphatics exhibit an increase in tonic activity as transmural pressure increases; whereas cervical lymphatics show an increase in phasic contraction frequency. We hypothesize that mechanical and biochemical characteristics of these lymphatic myofilaments will be different. The length‐tension relationships were determined for cervical and TD lymphatics. Cervical and TD lymphatics exhibited a biphasic response to maximal activation with high K++PSS+Substance P. TD generated larger active peak and plateau tensions than did cervical lymphatics (in mN/mm; peak tension: 0.65±0.06 vs. 0.43±0.01 P&lt;0.03; plateau tension: 0.54±0.06 vs 0.25±0.04 p&lt;0.01). pCa‐tension relationships were also determined in a‐toxin permeabilized lymphatics. While maximal peak tension in a‐toxin permeabilized TD was greater than cervical lymphatics (0.76±0.08 vs. 0.50+0.07 p&lt;0.02), plateau tension was not significantly different (0.56±0.07 vs. 0.41±0.07 P&lt;0.15). The Ca2+sensitivity (pCa50) and cooperativity of TD and cervical lymphatics were not significantly different (5.93±0.02 vs 5.86±0.06 &amp; 1.73±0.11 vs. 1.49±0.09). Thus the differences in the mechanical characteristics between TD and cervical lymphatics are not attributable to variations in pCa50 and cooperativity; other Ca2+‐dependent or ‐independent mechanisms may play important roles in these lymphatics. Supported by NIH HL80526 &amp; LRF.

Crossbridge Properties During The Quick Force Recovery In Single Frog Muscle Fibers

Fast stretches (∼25 nmhs−1 amplitude and ∼400 μs duration) which induced the forced rupture of the crossbridge ensemble were applied to intact muscle fibers to investigate the actomyosin bond properties during the force recovery following a step length change (release or stretch of 2 or 4 nm amplitude). Force and sarcomere length were measured with a fast force transducer (∼50 kHz natural frequency) and a striation follower device. To reduce fiber damaging by the stretches and to reduce the influence of myofilament compliance on the measurements, experiments were made on the tetanus rise at tension of about 0.5 the maximum plateau tension. Fast stretches were applied before and at progressively increasing times (up to 20 ms) after the step length change. The rupture force of the crossbridge ensemble (Pc) and the sarcomere elongation at Pc (Lc) were measured. In contrast with the data obtained previously on the tetanus rise (Bagni et al. J. Physiol., 2005; 565). The results showed that: (1) Pc was almost independent of the tension developed by the fiber and (2), Lc was not constant but increased immediately after the release and decreased after the stretch. These changes were still present 2 ms later when the quick recovery was almost complete and disappeared completely within 15-20 ms. Data analysis suggests that: 1) crossbridge number remains almost constant during the quick force recovery; 2) crossbridge detachment by the fast stretch is preceded by the reversal of the myosin head power stroke and, 3) the extent of the power stroke can be measured by the changes in Lc occurring during the quick recovery.

Structural changes of the regulatory proteins bound to the thin filaments in skeletal muscle contraction by X-ray fiber diffraction

In order to clarify the structural changes related to the regulation mechanism in skeletal muscle contraction, the intensity changes of thin filament-based reflections were investigated by X-ray fiber diffraction. The time course and extent of intensity changes of the first to third order troponin (TN)-associated meridional reflections with a basic repeat of 38.4nm were different for each of these reflections. The intensity of the first and second thin filament layer lines changed in a reciprocal manner both during initial activation and during the force generation process. The axial spacings of the TN-meridional reflections decreased by ∼0.1% upon activation relative to the relaxing state and increased by ∼0.24% in the force generation state, in line with that of the 2.7-nm reflection. Ca2+-binding to TN triggered the shortening and a change in the helical symmetry of the thin filaments. Modeling of the structural changes using the intensities of the thin filament-based reflections suggested that the conformation of the globular core domain of TN altered upon activation, undergoing additional conformational changes at the tension plateau. The tail domain of TN moved together with tropomyosin during contraction. The results indicate that the structural changes of regulatory proteins bound to the actin filaments occur in two steps, the first in response to the Ca2+-binding and the second induced by actomyosin interaction.

Length-tension relationships of small arteries, veins, and lymphatics from the rat mesenteric microcirculation

The passive and active length-tension relationships of isolated rat mesenteric lymphatics ( approximately 150 microm ID), and adjacent small arteries ( approximately 240 microm) and veins ( approximately 275 microm) were compared under isometric conditions using a wire myograph. About 60% of the lymphatic vessels developed spontaneous contractions in physiological saline solution at nominal preload. To maximally activate smooth muscle, 145 mM K(+) + 5 x 10(-5) M norepinephrine was used for arteries, and 145 mM K(+) + 1 x 10(-6) M substance P was used for lymphatics and veins. In response, arteries exhibited monotonic force development to a plateau level, whereas lymphatics and veins showed biphasic force development, consisting of a transient force peak followed by partial relaxation to a plateau over approximately 5 min. The passive and the active length-tension curves were similar in shape among all three vessels. However, the maximal active tension of arteries (3.4 +/- 0.42 mN/mm) was significantly greater than peak active tension (0.59 +/- 0.04 mN/mm) or plateau tension (0.20 +/- 0.04 mN/mm) in small veins and greater than peak active tension (0.34 +/- 0.02 mN/mm) or plateau tension (0.21 +/- 0.02 mN/mm) in lymphatics. Maximal active medial wall stress was similar between lymphatics and veins but was approximately fivefold higher in small arteries. For lymphatics, the pressure calculated from the optimal preload was significantly higher than that found previously in isobaric studies of isolated lymphatics, suggesting the capacity to operate at higher than normal pressures for increased responsiveness. Our results represent the first mechanical comparisons of arterial, venous, and lymphatic vessels in the same vasculature.

C2-ceramide induces vasodilation in phenylephrine-induced pre-contracted rat thoracic aorta: role of RhoA/Rho-kinase and intracellular Ca2+ concentration

It is known that ceramide may play an important regulatory role in vascular tone although its effect on vascular tone and the mechanisms involved are controversial. The present study was designed to investigate the effects of ceramide and its key initial regulators, TNF-alpha and neutral sphingomyelinase (SMase), on vascular tone of isolated rat thoracic aortic rings and elucidate the mechanisms involved in the changes in vascular tone induced by ceramide. Contractile responses and Fura-2 Ca2+ signals were measured in rat thoracic aortic rings or strips. 10(-5) M C2-ceramide, 0.1 U/ml neutral sphingomyelinase (SMase), and 5x10(-7) g/ml TNF-alpha had no effect on resting tone in rat thoracic aortic rings. However, in phenylephrine-induced pre-contracted rings, treatment with ceramide, SMase, and TNF-alpha evoked a gradual but sustained vasodilation. Vasodilation effect in response to 10(-5) M C2-ceramide was not significantly changed by the absence or presence of endothelium, a cyclooxygenase pathway inhibitor (10(-6) M indomethacin), or PKC inhibitors (10(-5) M H-7 & 5x10(-7) M calphostin-C). Pretreatment with 1 microM Y-27632, a RhoA/Rho-kinase inhibitor, significantly inhibited the phenylephrine-induced contraction itself as well as the C2-ceramide-induced vasodilation. Pre-treatment with 10(-5) M C2-ceramide had no effect on phasic rise in [Ca2+]i and tension evoked by stimulation with 10(-8) M phenylephrine, but post-treatment of C2-ceramide significantly reduced the phenylephrine-induced secondary tonic [Ca2+]i and tension plateau. Our results indicate that C2-ceramide induces vasodilation in phenylephrine-induced pre-contracted rat thoracic aorta. Furthermore, inhibition of phenylephrine-induced activation of RhoA/Rho-kinase pathway as well as phenylephrine-induced elevations in [Ca2+]i are clearly a key factors in C2-ceramide-induced vasodilation.

Ultralight, sound and shock absorbing component set

The invention relates to an ultralight, sound and shock absorbing component set comprising at least one base layer (2), an intermediate layer (3) and a covering layer (6). The intermediate layer (3) consists of a plurality of hollow bodies (4) arranged next to each other, whose walls are perforated and which thus form a complex labyrinth of hollow spaces. The covering layer (6) preferably has a microporous stiffening layer (8) which generates an airflow resistance of 900 Ns/m3 < Rt < 2000 Ns/m3. The dimensions of the walls of the hollow bodies (4) are such that they allow for compression of at least 50 % at a maximum plateau tension of 0.5 MPa < σ < 1.2 MPa and inelastically and fully convert an impact energy of approximately 0.5 MJ/m3 into deformation work.

The Maturation of Airway Structure and Function

After completing this article, readers should be able to: 1. Explain the physiologic reasons that immature infants are at a greater risk of mechanical ventilation-induced airway damage than are more mature infants. 2. Describe the age-related differences in airway function during perinatal development. 3. Delineate how differential expression of smooth muscle myosin heavy-chain isoforms affects airway smooth muscle physiology and airway mechanics. 4. Explain how expression of smooth muscle proteins contributes to the functional maturation of airway smooth muscle. Airway formation is one of the earliest events in the development of the respiratory system. However, continued maturation of the airway continues well into postnatal life. With advances in the medical care of increasingly preterm infants, an understanding of developmental airway physiology becomes imperative. This article reviews maturation of the airway and its components as well as their contributions to airway function during development. Airway development in the human begins during the fourth week of gestation when the respiratory diverticulum, or lung bud, branches from the embryonic foregut. The esophagotracheal septum forms and separates the foregut into the esophagus dorsally and the trachea ventrally. Thus, the developing airway is of foregut endodermal origin. Elongation of the respiratory diverticulum forms the trachea; branching forms the main stem bronchi. Growth and elongation continues in the caudal direction under the influence of airway secretions and physical forces. By the end of the 16th week of gestation, branching of the conducting airways is complete. The airway matures initially in the trachea and proceeds distally. Tracheal cartilage begins to form during the seventh week of gestation. Maturation of the distal airway cartilage is not complete until after birth. Epithelial differentiation begins in the trachea during the 10th week of gestation. Together with phasic contractions of the fetal airway smooth muscle, which are present following the 23rd gestational week, lung fluid …

History-Dependent Mechanical Properties of Permeabilized Rat Soleus Muscle Fibers

Permeabilized rat soleus muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.03 l 0, ± 0.1 l 0 s −1 imposed under sarcomere length control) to investigate whether the rate of stiffness recovery after movement increased with the level of Ca 2+ activation. Actively contracting fibers exhibited a characteristic tension response to stretch: tension rose sharply during the initial phase of the movement before dropping slightly to a plateau, which was maintained during the remainder of the stretch. When the fibers were stretched twice, the initial phase of the response was reduced by an amount that depended on both the level of Ca 2+ activation and the elapsed time since the first movement. Detailed analysis revealed three new and important findings. 1) The rates of stiffness and tension recovery and 2) the relative height of the tension plateau each increased with the level of Ca 2+ activation. 3) The tension plateau developed more quickly during the second stretch at high free Ca 2+ concentrations than at low. These findings are consistent with a cross-bridge mechanism but suggest that the rate of the force-generating power-stroke increases with the intracellular Ca 2+ concentration and cross-bridge strain.

Axial Disposition of Myosin Heads in Isometrically Contracting Muscles

Meridional x-ray diffraction diagrams, recorded with high angular resolution, from muscles contracting at the plateau of isometric tension show that the myosin diffraction orders are clusters of peaks. These clusters are due to pronounced interference effects between the myosin diffracting units on either side of the M-line. A theoretical analysis based on the polarity of the myosin (and actin) filaments shows that it is possible to extract phase information from which the axial disposition of the myosin heads can be determined. The results show that each head in a crown pair has a distinct structural disposition. It appears that only one of the heads in the pair stereospecifically interacts with the thin filament at any one time.

Hypocapnic constriction in rabbit basilar artery in vitro: triggering by serotonin and dependence on endothelin-1 and alkalosis

This study tested whether hypocapnic constriction of the rabbit basilar artery in vitro can be triggered by serotonin, and whether the resulting constriction is (1) due to the alkaline pH associated with hypocapnia, and (2) endothelin-1 mediated. Hypocapnic alkaline solution (25 mM NaHCO 3; pH 7.76; pCO 2 14.2) or isocapnic alkaline solution (50 mM NaHCO 3; pH 7.73; pCO 2 35.0) rarely altered basal tension. Serotonin (3 μM) challenge in hypocapnic or isocapnic alkaline solution resulted in near maximal tension. Washout of the serotonin did not decrease tension in 54% of the tissues, as plateau tension was maintained for 2–2.5 h. The plateau tension of washed tissues was relaxed by 1–3 μM PD145065 (Ac- d-Bhg- l-Leu-Asp- l-Ile- l-Ile- l-Trp), BQ610 (homopiperidinyl-CO-Leu- d-Trp(CHO)- d-Trp), and BQ788 ( N- cis-2,6-dimethyl-piperidinocarbonyl- l-γ-MeLeu- d-Trp (COOCH 3)-Nle), endothelin ET A/ET B, endothelin ET A, and endothelin ET B receptor antagonists, respectively. In contrast, serotonin-induced tension in normal solution (25 mM NaHCO 3; pH 7.42; pCO 2 36.9) was maintained for only 40 min (mean). These results demonstrate that (1) constriction due to hypocapnia in vitro can be triggered by serotonin and is endothelin-1 mediated and (2) alkaline pH in the absence of decreased pCO 2 is sufficient to elicit the constriction triggered by serotonin.