Static Pressurization Research Articles

Study on thermodynamic response in liquefied natural gas storage tanks under static pressurization and sloshing conditions

AbstractLiquefied natural gas (LNG) is becoming a potential power fuel in ocean transport and will be widely utilized in the near future. However, severe thermodynamic imbalance issues, caused by environmental heat leakage and external sloshing disturbances, must to be efficiently addressed to improve the operation reliability and safety storage of LNG fuel tanks. In this paper, a comprehensive theoretical model is developed to investigate the thermal response in a type C LNG storage tank, with consideration of composition migration, heat penetration, liquid evaporation, fluid sloshing, and vapor pressure rise. The prediction accuracy of the theoretical model is validated by comparing with selected tank pressurization experiments, with deviation limited within 5.0%. Based on the theoretical model, the aging process of LNG is first involved. The variations of composition migration in vapor and liquid regions are specially considered and discussed. During static pressurization, the thermal physical performance, including tank pressure rise, vapor temperature change, and boil‐off gas (BOG) generation, is detailedly researched under heat penetration. Finally, the effect of external sloshing excitation on thermal behavior in LNG fuel tanks is explored. Compared to static pressurization, external sloshing excitation causes obvious influences on thermodynamic performance of LNG tanks, including promotion on tank pressure and enhancement of heat and mass transfer. With some valuable conclusions achieved, this work is significant to comprehensive understanding on the thermal response of LNG storage tanks under different operation conditions.

Viscoelasticity of human descending thoracic aorta in a mock circulatory loop

Healthy human descending thoracic aortas, obtained during organ donation for transplant and research, were tested in a mock circulatory loop to measure the mechanical response to physiological pulsatile pressure and flow. The viscoelastic properties of the aortic segments were investigated at three different pulse rates. The same aortic segments were also subjected to quasi-static pressure tests in order to identify the aortic dynamic stiffness ratio, which is defined as the ratio between the stiffness in case of pulsatile pressure and the stiffness measured for static pressurization, both at the same value of pressure. The loss factor was also identified. The shape of the deformed aorta under static and dynamic pressure was measured by image processing to verify the compatibility of the end supports with the natural deformation of the aorta in the human body. In addition, layer-specific experiments on 10 human descending thoracic aortas allowed to precisely identify the mass density of the aortic tissue, which is an important parameter in cardiovascular dynamic models.

Biomechanical analysis of neochordal repair error from diastolic phase inversion of static left ventricular pressurization.

ObjectiveNeochordal implantation is a common form of surgical mitral valve (MV) repair. However, neochord length is assessed using static left ventricular pressurization, leading surgeons to evaluate leaflet coaptation and valve competency when the left ventricle is dilating instead of contracting physiologically, referred to as diastolic phase inversion (DPI). We hypothesize that the difference in papillary muscle (PM) positioning between DPI and physiologic systole results in miscalculated neochord lengths, which might affect repair performance.MethodsPorcine MVs (n = 6) were mounted in an ex vivo heart simulator and PMs were affixed to robots that accurately simulate PM motion. Baseline hemodynamic and chordal strain data were collected, after which P2 chordae were severed to simulate posterior leaflet prolapse from chordal rupture and subsequent mitral regurgitation. Neochord implantation was performed in the physiologic and DPI static configurations.ResultsAlthough both repairs successfully reduced mitral regurgitation, the DPI repair resulted in longer neochordae (2.19 ± 0.4 mm; P < .01). Furthermore, the hemodynamic performance was reduced for the DPI repair resulting in higher leakage volume (P = .01) and regurgitant fraction (P < .01). Peak chordal forces were reduced in the physiologic repair (0.57 ± 0.11 N) versus the DPI repair (0.68 ± 0.12 N; P < .01).ConclusionsBy leveraging advanced ex vivo technologies, we were able to quantify the effects of static pressurization on neochordal length determination. Our findings suggest that this post-repair assessment might slightly overestimate the neochordal length and that additional marginal shortening of neochordae might positively affect MV repair performance and durability by reducing load on surrounding native chordae.

Abstract 12588: Biomechanical Analysis of Neochordal Mitral Valve Repair Geometry to Quantitate the Impact of Diastolic Phase Inversion During Static Left Ventricular Pressurization

Introduction: Artificial neochord implantation is a common form of surgical mitral valve (MV) repair, and neochord length is commonly determined by static left ventricular (LV) pressurization in diastole. However, this causes a geometric phase inversion as the subvalvular apparatus will dilate when the LV would be contracting during systole in a beating heart. We hypothesize that static LV pressurization and the subsequent papillary muscle (PM) geometric shift affects the implanted neochord length, which may alter hemodynamic performance and chordal stresses. Methods: Porcine MV (n=6) were explanted and mounted in an ex vivo heart simulator and PM were affixed to image-guided robots that accurately simulate PM motion (Fig A, B). Baseline valve hemodynamic and chordal strain data were collected, after which P2 chordae were severed to simulate posterior leaflet prolapse from chordal rupture and subsequent mitral regurgitation (MR) (Fig C). Neochord implantation was performed in both systolic and diastolic stationary PM configurations. Data is presented as mean ± standard error, and statistics were performed using two-tailed Student’s T-Tests. Results: While both repairs adequately reduced MR, we found that the systolic vs diastolic configuration repair resulted in marginally reduced MR and neochord lengths, 2.25±1.384% and 2.19±0.4 mm or 16.0±3.2%, respectively ( p=0.011 , p=0.004 ). Moreover, peak primary and secondary chordal forces were reduced by 14.4±7.0% ( p=0.07 ) and 18.8±6.4% ( p=0.03 ), respectively. Conclusions: By leveraging advanced ex vivo technologies and biomechanics analyses, we were able to quantify the impact of diastolic phase inversion on neochordal MV repair geometry. Our findings suggest that this technique may slightly overestimate the neochordal length and that marginal additional shortening of neochordae may positively affect MV repair performance and durability by reducing load on surrounding native chordae.

Nonlinear analysis of cylindrical sandwich shells with porous core and CNT reinforced face-sheets by higher-order thickness and shear deformation theory

This paper presents the nonlinear static analysis of circular cylindrical sandwich shells using a higher-order thickness and shear deformation theory with nine kinematic parameters. The sandwich shell consists of a functionally graded porous core perfectly bonded with two carbon nanotubes (CNT) reinforced composite face sheets. The gradation is done in the thickness direction using three different types of distributions. An equivalent single layer theory is adopted to develop the model by assuming continuous displacements and strain functions across the thickness of the shell. The in-plane displacements and transverse deflections are expanded as third and fourth-order functions of thickness coordinate, respectively. The governing differential equations for the circular cylindrical shell are derived using the principle of minimum total potential energy and further discretized into algebraic equations by employing the Galerkin's method. These equations of motion are solved by adopting the arc-length continuation method, and the shell is analyzed for both the cases of displacement independent (radial distributed load) and displacement dependent (actual pressure) static pressurization. A parametric study is conducted to see the effect of different porosity coefficients, core-to-face sheet thickness ratios, porosity distributions, and boundary conditions.

Effects of static pressure, pressurization, and depressurization on n‐heptane pool fires in an airplane cargo compartment

SummaryThe cargo compartment of an airplane in flight is a complex environment with dynamic pressure (pressurization and depressurization), nonconservative oxygen, and unidirectional ventilation. In this study, n‐heptane pool fires were performed under static pressure, pressurization, and depressurization in a full‐scale airplane cargo compartment. The static pressure included 30 and 90 kPa, the pressurization was from 30 to 90 kPa at rates of 6, 12, 19, and 25 kPa/min, while the depressurization was from 90 to 30 kPa at rates of 6, 12, 17, and 20 kPa/min. The effects of pressure, oxygen concentration, and ventilation on pool fire characteristics including fuel mass loss rate (MLR), flame centerline temperature, and flame shape under each condition were concluded. The results show that the predominant factor of MLR was different in three conditions. The flame is divided into four regimes, in which the fuel vapor regime is used to emphasize the influence of fuel vapor on flame temperature above the fuel surface. The concept of average flame shape is put forward to reflect the flame occurrence probability. And its bottom, which named average flame root, presents the negative correlation with compartment pressure.

Restoration of Impact Damage in Polymers via a Hybrid Microcapsule–Microvascular Self‐Healing System

AbstractA hybrid microcapsule–microvascular system is introduced to regenerate the multiscale damage that results from impact puncture of vascularized polymeric sheets. Microvascular delivery of a two‐stage healing agent restores lost damage volume (puncture) to recover impact energy absorption, while embedded microcapsules heal microcracks to facilitate sealing. Modulation of the mechanical properties (1.4 GPa to 1.1 MPa stiffness) of the healing agent after curing is achieved by selection of compatible reactive acrylate monomers. Specimens are punctured and the impacted hole and surrounding damaged volume is restored by delivering the two‐stage healing agents to the site of damage via a microvascular network. Rapid gelling of two‐stage healing agents enables their retention in the damage region, while subsequent polymerization recovers structural performance. Impact recovery efficiency is assessed in terms of energy absorption, comparing reimpacted specimens to the initial impact. Recovery of impact energy absorption as high as 100% is observed for the optimal specimen design. Specimens are tested for sealing under static pressurization to monitoring leakage through the restored damage. A hybrid system incorporating both microvascular delivery of the two‐stage healing agents and microcapsules containing solvated epoxy enables sealing of 100% of specimens.

Bacterial survival following shock compression in the GigaPascal range

The possibility that life can exist within previously unconsidered habitats is causing us to expand our understanding of potential planetary biospheres. Significant populations of living organisms have been identified at depths extending up to several km below the Earth's surface; whereas laboratory experiments have shown that microbial species can survive following exposure to GigaPascal (GPa) pressures. Understanding the degree to which simple organisms such as microbes survive such extreme pressurization under static compression conditions is being actively investigated. The survival of bacteria under dynamic shock compression is also of interest. Such studies are being partly driven to test the hypothesis of potential transport of biological organisms between planetary systems. Shock compression is also of interest for the potential modification and sterilization of foodstuffs and agricultural products. Here we report the survival of Shewanella oneidensis bacteria exposed to dynamic (shock) compression. The samples examined included: (a) a “wild type” (WT) strain and (b) a “pressure adapted” (PA) population obtained by culturing survivors from static compression experiments to 750MPa. Following exposure to peak shock pressures of 1.5 and 2.5GPa the proportion of survivors was established as the number of colony forming units (CFU) present after recovery to ambient conditions. The data were compared with previous results in which the same bacterial samples were exposed to static pressurization to the same pressures, for 15 minutes each. The results indicate that shock compression leads to survival of a significantly greater proportion of both WT and PA organisms. The significantly shorter duration of the pressure pulse during the shock experiments (2–3 µs) likely contributes to the increased survival of the microbial species. One reason for this can involve the crossover from deformable to rigid solid-like mechanical relaxational behavior that occurs for bacterial cell walls on the order of seconds in the time-dependent strain rate.

Inactivation of polyphenol oxidase and peroxidase enzymes during pulsed, static and cyclic pressurization of litchi (Litchi chinensis) juice

Effects of three pressurization modes, namely pulsed pressurization (PP), static pressurization (SP) and cyclic pressurization (CP), were studied on enzymes polyphenol oxidase (PPO) and peroxidase (POD) in litchi juice. Process parameters considered were pressure (300–600MPa), number of pressure pulses or cycles (1–3) and isobaric pressure-hold time (1s to 45min). PP resulted in increased activity of both the enzymes, and its effect was more pronounced on POD than PPO. Effect of SP on enzymes was assumed as a two-stage inactivation: initial impact of pressurization–depressurization step calculated as pulse effect (PE), and effect of isobaric stage described using various kinetic models, among which the biphasic first-order reaction model fitted best. Further, an empirical model was proposed to explain the combined effect of pressurization–depressurization and isobaric phases on the enzymes inactivation as a function of pressure and hold-time during SP of litchi juice. During CP, the effect of number of cycles was found to be dissimilar on both the enzymes in litchi juice, and it was relatively more effective than SP for their inactivation. Within the studied domain of process parameters, the baroresistance of POD was found to be more than PPO in litchi juice, irrespective of the pressurization mode.

A membrane-type acoustic metamaterial with adjustable acoustic properties

A new realization of a membrane-type acoustic metamaterial (MAM) with adjustable sound transmission properties is presented. The proposed design distinguishes itself from other realizations by a stacked arrangement of two MAMs which is inflated using pressurized air. The static pressurization leads to large nonlinear deformations and, consequently, geometrical stiffening of the MAMs which is exploited to adjust the eigenmodes and sound transmission loss of the structure. A theoretical analysis of the proposed inflatable MAM design using numerical and analytical models is performed in order to identify two important mechanisms, namely the shifting of the eigenfrequencies and modal residuals due to the pressurization, responsible for the transmission loss adjustment. Analytical formulas are provided for predicting the eigenmode shifting and normal incidence sound transmission loss of inflated single and double MAMs using the concept of effective mass. The investigations are concluded with results from a test sample measurement inside an impedance tube, which confirm the theoretical predictions.

Cavitation nuclei in water exposed to transient pressures

A model of skin-stabilized interfacial cavitation nuclei and their response to tensile and compressive stressing is presented. The model is evaluated in relation to experimental tensile strength results for water at rest at the bottom of an open water-filled container at atmospheric pressure and room temperature. These results are obtained by recording the initial growth of cavities generated by a short tensile pulse applied to the bottom of the container. It is found that the cavitation nuclei shift their tensile strength depending on their pressure history. Static pressurization for an extended period of time prior to testing is known to increase the tensile strength of water, but little information is available on how it is affected by compression pulses of short duration. This is addressed by imposing compression pulses of approximately 1 ms duration and a peak intensity of a few bar prior to the tension pulse. The observations are interpreted on the basis of the new model.

Vibroacoustic Tailoring of a Rod-Stiffened Composite Fuselage Panel with Multidisciplinary Considerations

An efficient multi-objective design tailoring procedure seeking to improve the vibroacoustic performance of a fuselage panel while maintaining or reducing weight is presented. The structure considered is the pultruded rod stitched efficient unitized structure, a highly integrated composite structure concept designed for a noncylindrical, next-generation flight vehicle fuselage. Modifications to a baseline design are evaluated within a six-parameter design space including spacing, flange width, and web height for both frame and stringer substructure components. The change in sound power radiation attributed to a design change is predicted using finite-element models sized and meshed for analyses in the 500 Hz, 1 kHz, and 2 kHz octave bands. Three design studies are carried out in parallel while considering a diffuse acoustic field excitation and two types of turbulent boundary-layer excitation. Kriging surrogate models are used to reduce the computational costs of resolving the vibroacoustic and weight objective Pareto fronts. The resulting Pareto optimal designs are then evaluated under a static pressurization ultimate load to assess structural strength and stability. Results suggest that choosing alternative configurations within the considered design space can reduce weight and improve vibroacoustic performance without compromising strength and stability of the structure under the static load condition considered, but the tradeoffs are significantly influenced by the spatial characteristics of the assumed excitation field.

Structural analysis of the natural aortic valve in dynamics: From unpressurized to physiologically loaded

A novel finite element model of the natural aortic valve was developed implementing anisotropic hyperelastic material properties for the leaflets and aortic tissues, and starting from the unpressurized geometry. Static pressurization of the aortic root, silicone rubber moulds and published data helped to establish the model parameters, while high-speed video recording of the leaflet motion in a left-heart simulator allowed for comparisons with simulations. The model was discretized with brick elements and loaded with time-varying pressure using an explicit commercial solver. The aortic valve model produced a competent valve whose dynamic behavior (geometric orifice area vs. time) closely matched that observed in the experiment. In both cases, the aortic valve took approximately 30 ms to open to an 800 mm 2 orifice and remained completely or more than half open for almost 200 ms, after which it closed within 30–50 ms. The highest values of stress were along the leaflet attachment line and near the commissure during diastole. Von Mises stress in the leaflet belly reached 600–750 kPa from early to mid-diastole. While the model using the unpressurized geometry as initial configuration was specially designed to satisfy the requirements of continuum mechanics for large deformations of hyperelastic materials, it also clearly demonstrated that dry models can be adequate to analyze valve dynamics. Although improvements are still needed, the advanced modeling and validation techniques used herein contribute toward improved and quantified accuracy over earlier simplified models.

A Theoretical Study of Mechanical Stability of Arteries

This study proposes a mathematical model for studying stability of arteries subjected to a longitudinal extension and a periodic pressure. An artery was considered as a straight composite beam comprised of an external thick-walled tube and a fluid core. The dynamic criterion for stability was used, based on analyzing the small transverse vibrations superposed on the finite deformation of the vessel under static load. In contrast to the case of a static pressurization, in which buckling is only possible if the load produces a critical axial compressive force, a loss of stability of arteries under periodic pressure occurs under many combinations of load parameters. Instability occurs as a parametric resonance characterized by an exponential increase in the amplitude of transverse vibrations over several bands of pressure frequencies. The effects of load parameters were analyzed on the basis of the results for a dynamic and static stability of a rabbit thoracic aorta. Under normal physiological loads the artery is in a stable configuration. Static instability occurs under high distending pressures and low longitudinal stretch ratios. When the artery is subjected to periodic pressure, an independent increase in the mean pressure, amplitude of the periodic pressure, or frequency, most often, but not always, increases the risk of stability loss. In contrary, an increase in longitudinal stretch ratio most likely, but not certain, stabilizes the vessel. It was shown that adaptive geometrical remodeling due to an increase in mean pressure and flow does not affect artery stability.

Regeneration of static-load-degenerated articular cartilage extracellular matrix by vitamin C supplementation

The effect of a physiological dose of vitamin C (100 microg/ml) on goat articular cartilage chondrocytes cultured in an alginate matrix and subjected to static pressurization of 2.4 MPa was investigated. Biochemical analyses of DNA, glycosaminoglycan (GAG), collagen and protease activity were carried out in various matrix fractions, i.e. cellular matrix (CM) and further removed matrix (FRM), and in culture medium. The treatment of chondrocytes with vitamin C after static pressure increased the GAG content in both CM and FRM (P < 0.03) as compared with control or vitamin C/ static load alone. The collagen content of chondrocytes treated with vitamin C alone and vitamin C after static load also increased significantly in FRM (P < 0.003) as compared with control and static load alone. The specific activity of protease in CM and FRM decreased after vitamin C supplementation both with and without static pressure relative to control (P < 0.003). Transmission electron-microscopic images showed a mixed population of spherical and elliptical chondrocytes when vitamin C was added after static load as compared with static load alone where only elliptical cells were seen. Abundant pericellular and collagen fibrils were seen in this group of chondrocytes as compared with all other groups and the control. The results thus show that, in vitro, vitamin C supplementation of chondrocytes after static loading has the potential to reduce the morphological and biochemical degeneration of chondrocytes caused by static loading, thereby improving the cellular health and functioning of articular cartilage.

Differential Mechanical Activation of Mitogen-Activated Protein Kinases in Intact Human Blood Vessels

Background. Vascular tissue remodels in response to a variety of hemodynamic factors, often transduced through activation of mitogen-activated protein kinases such as extracellular signal-related kinase (ERK1/2) and c-jun N-terminal kinase (JNK). This study tests the hypothesis that these kinases are involved in mechanical signal transduction in intact human arteries and veins. Methods. Unused portions of human saphenous vein and radial artery were obtained fresh at the time of peripheral or coronary bypass. A sample of the vessel was immediately snap frozen (control 0) and the remainder separated into three segments. One segment was placed in sterile medium and left undisturbed for 2 h (control 2), one was perfused with sterile medium for 2 h at a steady rate of 150 ml/min, yielding shear stress values of 8–20 dyne/cm 2 (flow), and one was statically pressurized without flow at 110 mm Hg for 2 h (pressure). After treatment, samples were tested for phosphorylated ERK1/2 and JNK using Western blot. Results. Two hours of culture produced mild increases in ERK1/2 activity in both vessel types. Stimulation with continuous rapid flow produced significantly increased ERK1 activity and a nearly 100% increase in ERK2 in veins. Static pressurization also stimulated ERK1/2, although slightly less than continuous flow. ERK1/2 phosphorylation was only mildly increased in flow-stimulated radial arteries, and exposure to normal systemic pressure showed no appreciable effect. Significant phosphorylation of JNK was not observed in either vessel. Conclusion. ERK1/2 phosphorylation is increased in human saphenous veins and radial arteries exposed to the hemodynamic conditions of arterial grafting. This pathway may be involved in the transduction of external stimuli leading to remodeling.

The mechanics of harmonic gas pressurization and failure of lava domes

Summary Solutions are developed for the harmonic interior pressurization of idealized hemispherical lava domes to determine the evolution of instability with time. Integrated gas pressure distributions are evaluated to determine uplift on inclined planes of potential detachment that traverse the dome, and to evaluate the limit-equilibrium stability of blocks isolated by these planes. Average pressure distributions show an initial transient rise to peak dynamic gas pressures that corresponds to a harmonic steady state. Progress towards potential instability mirrors this rise, and oscillates with the harmonic steady state. The timing of pressurization of the dome lavas is modulated by a diffusive time. The magnitude of this pressurization is modulated by dome geometry and the magnitude and non-dimensional frequency of the interior inflation. Two distinct pressurization behaviours are apparent: the first, at high relative inflation frequencies, is where the dynamic steady state corresponds to static pressurization at the mean interior pressure; and the second, at low relative inflation frequencies, is where the dynamic state synchronously responds to the oscillating interior-pressure boundary condition, and peak uplift is proportional to the mean pressure plus the maximum harmonic overpressure. These two conditions are separated by a frequency shift, or change in fluid diffusivity, of only two orders of magnitude, and are significant because the destabilizing uplift force may increase by a factor of two, where all other parameters, except excitation frequency or diffusivity, remain fixed. These results are applied to stability evaluations of the lava dome at the Soufriere Hills volcano, to examine failure mechanisms potentially consistent with the observed delay in collapse. Linear models incorporating time-invariant strength and transport parameters are only able to represent collapse for interior gas pressures at the upper limit of observed magnitudes. Conversely, behaviours incorporating fatigue-weakening, or hysteretic permeability enhancement, of the dome rocks are able to approach failure for modest pressurization magnitudes, to match observed delays in collapse and potentially to explain why some domes collapse explosively whereas others do not.

The mechanics of harmonic gas pressurization and failure of lava domes

Solutions are developed for the harmonic interior pressurization of idealized hemispherical lava domes to determine the evolution of instability with time. Integrated gas pressure distributions are evaluated to determine uplift on inclined planes of potential detachment that traverse the dome, and to evaluate the limit‐equilibrium stability of blocks isolated by these planes. Average pressure distributions show an initial transient rise to peak dynamic gas pressures that corresponds to a harmonic steady state. Progress towards potential instability mirrors this rise, and oscillates with the harmonic steady state. The timing of pressurization of the dome lavas is modulated by a diffusive time. The magnitude of this pressurization is modulated by dome geometry and the magnitude and non‐dimensional frequency of the interior inflation. Two distinct pressurization behaviours are apparent: the first, at high relative inflation frequencies, is where the dynamic steady state corresponds to static pressurization at the mean interior pressure; and the second, at low relative inflation frequencies, is where the dynamic state synchronously responds to the oscillating interior‐pressure boundary condition, and peak uplift is proportional to the mean pressure plus the maximum harmonic overpressure. These two conditions are separated by a frequency shift, or change in fluid diffusivity, of only two orders of magnitude, and are significant because the destabilizing uplift force may increase by a factor of two, where all other parameters, except excitation frequency or diffusivity, remain fixed. These results are applied to stability evaluations of the lava dome at the Soufriere Hills volcano, to examine failure mechanisms potentially consistent with the observed delay in collapse. Linear models incorporating time‐invariant strength and transport parameters are only able to represent collapse for interior gas pressures at the upper limit of observed magnitudes. Conversely, behaviours incorporating fatigue‐weakening, or hysteretic permeability enhancement, of the dome rocks are able to approach failure for modest pressurization magnitudes, to match observed delays in collapse and potentially to explain why some domes collapse explosively whereas others do not.

Fractographic study of high-density polyethylene pipe

High-density polyethylene (HDPE) pipe is now being used as an alternative to medium-density polyethylene (MDPE) for gas, water, sewage and waste-water distribution systems. Laboratory tests appear to show that HDPE is more able to suppress rapid crack propagation (RCP), whilst remaining sufficient resistance under the operational circumstances that lead to the type of slow crack growth observed in service failures. There have been many fractographic studies on MDPE pipe materials, actual pipe and fittings, but little on HDPE. A fractographic study of the type of HDPE pipe in current production has been undertaken. For these tests, whole pipe sections were subjected to either static or dynamic internal (water) pressurization fatigue loading. Failure mechanisms are discussed based on the fracture morphologies resulting from these tests. A further argument for good resistance of HDPE pipe to RCP is suggested. © 1998 Chapman & Hall

Hsp70 accumulation in chondrocytic cells exposed to high continuous hydrostatic pressure coincides with mRNA stabilization rather than transcriptional activation.

In response to various stress stimuli, heat shock genes are induced to express heat shock proteins (Hsps). Previous studies have revealed that expression of heat shock genes is regulated both at transcriptional and posttranscriptional level, and the rapid transcriptional induction of heat shock genes involves activation of the specific transcription factor, heat shock factor 1 (HSF1). Furthermore, the transcriptional induction can vary in intensity and kinetics in a signal- and cell-type-dependent manner. In this study, we demonstrate that mechanical loading in the form of hydrostatic pressure increases heat shock gene expression in human chondrocyte-like cells. The response to continuous high hydrostatic pressure was characterized by elevated mRNA and protein levels of Hsp70, without activation of HSF1 and transcriptional induction of hsp70 gene. The increased expression of Hsp70 was mediated through stabilization of hsp70 mRNA molecules. Interestingly, in contrast to static pressurization, cyclic hydrostatic loading did not result in the induction of heat shock genes. Our findings show that hsp70 gene expression is regulated posttranscriptionally without transcriptional induction in chondrocyte-like cells upon exposure to high continuous hydrostatic pressure. We suggest that the posttranscriptional regulation in the form of hsp70 mRNA stabilization provides an additional mode of heat shock gene regulation that is likely to be of significant importance in certain forms of stress.