Transient permeabilization of cell membranes by ultrasound-exposed microbubbles is related to formation of hydrogen peroxide

Intramembrane cavitation as a unifying mechanism for ultrasound-induced bioeffects

The purpose of this study was to develop a unified model capable of explaining the mechanisms of interaction of ultrasound and biological tissue at both the diagnostic nonthermal, noncavitational (<100 mW · cm(-2)) and therapeutic, potentially cavitational (>100 mW · cm(-2)) spatial peak temporal average intensity levels. The cellular-level model (termed "bilayer sonophore") combines the physics of bubble dynamics with cell biomechanics to determine the dynamic behavior of the two lipid bilayer membrane leaflets. The existence of such a unified model could potentially pave the way to a number of controlled ultrasound-assisted applications, including CNS modulation and blood-brain barrier permeabilization. The model predicts that the cellular membrane is intrinsically capable of absorbing mechanical energy from the ultrasound field and transforming it into expansions and contractions of the intramembrane space. It further predicts that the maximum area strain is proportional to the acoustic pressure amplitude and inversely proportional to the square root of the frequency (ε A,max ∝ P(A)(0.8f - 0.5) and is intensified by proximity to free surfaces, the presence of nearby microbubbles in free medium, and the flexibility of the surrounding tissue. Model predictions were experimentally supported using transmission electron microscopy (TEM) of multilayered live-cell goldfish epidermis exposed in vivo to continuous wave (CW) ultrasound at cavitational (1 MHz) and noncavitational (3 MHz) conditions. Our results support the hypothesis that ultrasonically induced bilayer membrane motion, which does not require preexistence of air voids in the tissue, may account for a variety of bioeffects and could elucidate mechanisms of ultrasound interaction with biological tissue that are currently not fully understood.

Effects of shear stress cultivation on cell membrane disruption and intracellular calcium concentration in sonoporation of endothelial cells

Microbubble facilitated ultrasound (US) application can enhance intracellular delivery of drugs and genes in endothelial cells cultured in static condition by transiently disrupting the cell membrane, or sonoporation. However, endothelial cells in vivo that are constantly exposed to blood flow may exhibit different sonoporation characteristics. This study investigates the effects of shear stress cultivation on sonoporation of endothelial cells in terms of membrane disruption and changes in the intracellular calcium concentration ([Ca(2+)](i)). Sonoporation experiments were conducted using murine brain microvascular endothelial (bEnd.3) cells and human umbilical vein endothelial cells (HUVECs) cultured under static or shear stress (5 dyne/cm(2) for 5 days) condition in a microchannel environment. The cells were exposed to a short US tone burst (1.25 MHz, 8 μs duration, 0.24 MPa) in the presence of Definity™ microbubbles to facilitate sonoporation. Membrane disruption was assessed by propidium iodide (PI) and changes in [Ca(2+)](i) measured by fura-2AM. Results from this study show that shear stress cultivation significantly reduced the impact of ultrasound-driven microbubbles activities on endothelial cells. Cells cultured under shear stress condition exhibited much lower percentage with membrane disruption and changes in [Ca(2+)](i) compared to statically cultured cells. The maximum increases of PI uptake and [Ca(2+)](i) were also significantly lower in the shear stress cultured cells. In addition, the extent of [Ca(2+)](i) waves in shear cultured HUVECs was reduced compared to the statically cultured cells.

Modulation of intracellular Ca2+ concentration in brain microvascular endothelial cells in vitro by acoustic cavitation

Localized delivery of therapeutic agents through the blood-brain barrier (BBB) is a clinically significant task that remains challenging. Ultrasound (US) application after intravenous administration of microbubbles has been shown to generate localized BBB opening in animal models but the detailed mechanisms are not yet fully described. The current study investigates the effects of US-stimulated microbubbles on in vitro murine brain microvascular endothelial (bEnd.3) cells by monitoring sonoporation and changes in intracellular calcium concentration ([Ca(2+)](i)) using real-time fluorescence and high-speed brightfield microscopy. Cells seeded in microchannels were exposed to a single US pulse (1.25 MHz, 10 cycles, 0.24 MPa peak negative pressure) in the presence of Definity microbubbles and extracellular calcium concentration [Ca(2+)](o) = 0.9 mM. Disruption of the cell membrane was assessed using propidium iodide (PI) and change in the [Ca(2+)](i) was measured using fura-2. Cells adjacent to a microbubble exhibited immediate [Ca(2+)](i) changes after US pulse with and without PI uptake and the [Ca(2+)](i) changes were twice as large in cells with PI uptake. Cell viability assays showed that sonoporated cells could survive with modulation of [Ca(2+)](i) and uptake of PI. Cells located near sonoporated cells were observed to exhibit changes in [Ca(2+)](i) that were delayed from the time of US application and without PI uptake. These results demonstrate that US-stimulated microbubbles not only directly cause changes in [Ca(2+)](i) in brain endothelial cells in addition to sonoporation but also generate [Ca(2+)](i) transients in cells not directly interacting with microbubbles, thereby affecting cells in larger regions beyond the cells in contact with microbubbles.

Saving cells from ultrasound-induced apoptosis: quantification of cell death and uptake following sonication and effects of targeted calcium chelation

Applications of ultrasound for noninvasive drug and gene delivery have been limited by associated cell death as a result of sonication. In this study, we sought to quantify the distribution of cellular bioeffects caused by low-frequency ultrasound (24 kHz) and test the hypothesis that Ca(2+) chelation after sonication can shift this distribution by saving cells from death by apoptosis. Using flow cytometry, we quantitatively categorized sonicated cells among four populations: (i) cells that appear largely unaffected, (ii) cells reversibly permeabilized, (iii) cells rendered nonviable during sonication and (iv) cells that appear to be viable shortly after sonication, but later undergo apoptosis and die. By monitoring cells for 6 h after ultrasound exposure, we found that up to 15% of intact cells fell into this final category. Those apoptotic cells initially had the highest levels of uptake of a marker compound, calcein; also had highly elevated levels of intracellular Ca(2+); and contained an estimated plasma membrane wound radius of 100-300 nm. Finally, we showed that chelation of intracellular Ca(2+) after sonication reduced apoptosis by up to 44%, thereby providing a strategy to save cells. We conclude that cells can be saved from ultrasound-induced death by appropriate selection of ultrasound conditions and Ca(2+) chelation after sonication.

Fibrinolytic effects of transparietal ultrasound associated with intravenous infusion of an ultrasound contrast agent: study of a rat model of acute cerebral stroke

The aim of this study was to evaluate the thrombolytic effect of focused transparietal ultrasound in combination with a specific contrast agent (microbubbles) in acute cerebral ischemia. Acute cerebral ischemia was induced in 10 rats by intra-arterial clots injection. Five rats (group 1) were treated with a combination of transparietal ultrasound (probe 2 MHz, acoustic power 500 mW/cm(2)) and intravenous injection of 0.6 mL of the ultrasound contrast agent (UCA) sulfur hexafluoride. Five rats (group 2) were treated by fibrinolytic intravenous infusion (recombinant tissue plasminogen activator). Cerebral cellular energy production was determined by measuring the cellular phosphorylation using phosphorus magnetic spectroscopy before and during ischemia induction and after treatment. Measures were performed on a dedicated 2.35T magnet. The ratio phosphocreatine (P(Cr)) on inorganic phosphate (P(i)), P(Cr)/P(i), estimation of the oxidative phosphorylation metabolism and the intracellular pH (pHi) were measured in the two groups. Compared with the ischemia induction period, both treatments were associated with an increase of P(Cr)/P(i) and pHi values, respectively, +80% and +100% in group 1 (p=0.07) and +100% and +80% in group 2 (p=0.04). There was no significant difference between the two groups for the response treatment. To conclude, treatment with intravenous fibrinolytic infusion and treatment with focused ultrasound in combination with UCA seems to be equally effective in treating acute cerebral ischemia in rats. (E-mail: j.p.tasu@chu-poitiers.fr).

Ultrasound and microbubble-induced intra- and intercellular bioeffects in primary endothelial cells

Recent developments in the field of ultrasound (US) contrast agents have demonstrated that these encapsulated microbubbles can not only be used for diagnostic imaging but may also be employed as therapeutic carriers for localized, targeted drug or gene delivery. The exact mechanisms behind increased uptake of therapeutic compounds by US-exposed microbubbles are still not fully understood. Therefore, we studied the effects of stably oscillating SonoVue microbubbles on relevant parameters of cellular and intercellular permeability, i.e., reactive oxygen species (ROS) homeostasis, calcium permeability, F-actin cytoskeleton, monolayer integrity and cell viability using live-cell fluorescence microscopy. US was applied at 1-MHz, 0.1MPa peak-negative pressure, 0.2% duty cycle and 20Hz pulse repetition frequency to primary endothelial cells. We demonstrated increased membrane permeability for calcium ions, with an important role for H(2)O(2). Catalase, an extracellular H(2)O(2) scavenger, significantly blocked the influx of calcium ions. Further changes in ROS homeostasis involved an increase in intracellular H(2)O(2) levels, protein nitrosylation and a decrease in total endogenous glutathione levels. In addition, an increase in the number of F-actin stress fibers and F-actin cytoskeletal rearrangement were observed. Furthermore, US-exposed microbubbles significantly affected endothelial monolayer integrity, but importantly, disrupted cell-cell interactions were restored within 30min. Finally, cell viability was not affected. In conclusion, these data provide more insight in the interactions between US, microbubbles and endothelial cells, which is important for understanding the mechanisms behind US and microbubble-enhanced uptake of drugs or genes.

Intracellular delivery and calcium transients generated in sonoporation facilitated by microbubbles

Ultrasound application in the presence of microbubbles is a promising strategy for intracellular drug and gene delivery, but it may also trigger other cellular responses. This study investigates the relationship between the change of cell membrane permeability generated by ultrasound-driven microbubbles and the changes in intracellular calcium concentration ([Ca(2+)](i)). Cultured rat cardiomyoblast (H9c2) cells were exposed to a single ultrasound pulse (1MHz, 10-15cycles, 0.27MPa) in the presence of a Definity(TM) microbubble. Intracellular transport via sonoporation was assessed in real time using propidium iodide (PI), while [Ca(2+)](i) and dye loss from the cells were measured with preloaded fura-2. The ultrasound exposure generated fragmentation or shrinking of the microbubble. Only cells adjacent to the ultrasound-driven microbubble exhibited propidium iodide uptake with simultaneous [Ca(2+)](i) increase and fura-2 dye loss. The amount of PI uptake was correlated with the amount of fura-2 dye loss. Cells with delayed [Ca(2+)](i) transients from the time of ultrasound application had no uptake of PI. These results indicate the formation of non-specific pores in the cell membrane by ultrasound-stimulated microbubbles and the generation of calcium waves in surrounding cells without pores.

Microbubbles as ultrasound triggered drug carriers

Originally developed as contrast agents for ultrasound imaging and diagnostics, in the past years, microbubbles have made their way back from the patients' bedside to the researcher's laboratory. Microbubbles are currently believed to have great potential as carriers for drugs, small molecules, nucleic acids, and proteins. This review provides insight into this intriguing new frontier from the perspective of the pharmaceutical scientist. First, basic aspects on the application of ultrasound-targeted microbubble destruction for drug delivery will be presented. Next, we will review the recently applied approaches for manufacturing and drug-loading microbubbles. Important quality issues and characterization techniques for advanced microbubble formulation will be discussed. Finally, we will provide an assessment of the prospects for microbubbles in drug and gene therapy, illustrating the problems and requirements for their future development.

Spatiotemporal effects of sonoporation measured by real-time calcium imaging

To investigate the effects of sonoporation, spatiotemporal evolution of ultrasound-induced changes in intracellular calcium ion concentration ([Ca(2+)](i)) was determined using real-time fura-2AM fluorescence imaging. Monolayers of Chinese hamster ovary (CHO) cells were exposed to a 1-MHz ultrasound tone burst (0.2 s, 0.45 MPa) in the presence of Optison microbubbles. At extracellular [Ca(2+)](o) of 0.9 mM, ultrasound application generated both nonoscillating and oscillating (periods 12 to 30 s) transients (changes of [Ca(2+)](i) in time) with durations of 100-180 s. Immediate [Ca(2+)](i) transients after ultrasound application were induced by ultrasound-mediated microbubble-cell interactions. In some cases, the immediately affected cells did not return to pre-ultrasound equilibrium [Ca(2+)](i) levels, thereby indicating irreversible membrane damage. Spatial evolution of [Ca(2+)](i) in different cells formed a calcium wave that was observed to propagate outward from the immediately affected cells at 7-20 microm/s over a distance >200 microm, causing delayed transients in cells to occur sometimes 60 s or more after ultrasound application. In calcium-free solution, ultrasound-affected cells did not recover, consistent with the requirement of extracellular Ca(2+) for cell membrane recovery subsequent to sonoporation. In summary, ultrasound application in the presence of Optison microbubbles can generate transient [Ca(2+)](i) changes and oscillations at a focal site and in surrounding cells via calcium waves that last longer than the ultrasound duration and spread beyond the focal site. These results demonstrate the complexity of downstream effects of sonoporation beyond the initial pore formation and subsequent diffusion-related transport through the cellular membrane.

Evaluation and comparison of three novel microbubbles: enhancement of ultrasound-induced cell death and free radicals production

Three novel lipid-shell-type microbubbles (MBs), AS-0100, BG6356A and BG6356B, have been evaluated for their impact on ultrasound (US)-induced cell death and free radicals production. Previously studied and well-characterized US exposure conditions were employed in which human myelomonocytic lymphoma U937 cells were exposed to 1MHz pulsed US beam (0.3W/cm(2), 10% duty factor) for 1min with or without MBs. Three different concentrations of each MB were used. Apoptosis and cell lysis were assessed by examining phosphatidylserine externalization and by counting viable cells, respectively, 6h post-exposure. Free radicals production and scavenging activities were evaluated using electron paramagnetic resonance (EPR)-spin trapping. The results showed that only AS-0100 and BG6356A were able to enhance the US-induced apoptosis, mainly by increasing the secondary necrosis. Apoptosis and cell lysis seemed to depend more on mechanical forces exerted by oscillating MBs while free radicals played a trivial role. BG series MBs exhibited pronounced scavenging activities. Generally, despite the need for further optimization, AS-0100 and BG6356A appear to be promising as adjuncts in cases where US-induced cell death is required.

Ultrasound and microbubble-targeted delivery of therapeutic compounds: ICIN Report Project 49: Drug and gene delivery through ultrasound and microbubbles

The molecular understanding of diseases has been accelerated in recent years, producing many new potential therapeutic targets. A noninvasive delivery system that can target specific anatomical sites would be a great boost for many therapies, particularly those based on manipulation of gene expression. The use of microbubbles controlled by ultrasound as a method for delivery of drugs or genes to specific tissues is promising. It has been shown by our group and others that ultrasound increases cell membrane permeability and enhances uptake of drugs and genes. One of the important mechanisms is that microbubbles act to focus ultrasound energy by lowering the threshold for ultrasound bioeffects. Therefore, clear understanding of the bioeffects and mechanisms underlying the membrane permeability in the presence of microbubbles and ultrasound is of paramount importance. (Neth Heart J 2009;17:82-6.).

Ultrasound and microbubble-targeted delivery of macromolecules is regulated by induction of endocytosis and pore formation

Contrast microbubbles in combination with ultrasound (US) are promising vehicles for local drug and gene delivery. However, the exact mechanisms behind intracellular delivery of therapeutic compounds remain to be resolved. We hypothesized that endocytosis and pore formation are involved during US and microbubble targeted delivery (UMTD) of therapeutic compounds. Therefore, primary endothelial cells were subjected to UMTD of fluorescent dextrans (4.4 to 500 kDa) using 1 MHz pulsed US with 0.22-MPa peak-negative pressure, during 30 seconds. Fluorescence microscopy showed homogeneous distribution of 4.4- and 70-kDa dextrans through the cytosol, and localization of 155- and 500-kDa dextrans in distinct vesicles after UMTD. After ATP depletion, reduced uptake of 4.4-kDa dextran and no uptake of 500-kDa dextran was observed after UMTD. Independently inhibiting clathrin- and caveolae-mediated endocytosis, as well as macropinocytosis significantly decreased intracellular delivery of 4.4- to 500-kDa dextrans. Furthermore, 3D fluorescence microscopy demonstrated dextran vesicles (500 kDa) to colocalize with caveolin-1 and especially clathrin. Finally, after UMTD of dextran (500 kDa) into rat femoral artery endothelium in vivo, dextran molecules were again localized in vesicles that partially colocalized with caveolin-1 and clathrin. Together, these data indicated uptake of molecules via endocytosis after UMTD. In addition to triggering endocytosis, UMTD also evoked transient pore formation, as demonstrated by the influx of calcium ions and cellular release of preloaded dextrans after US and microbubble exposure. In conclusion, these data demonstrate that endocytosis is a key mechanism in UMTD besides transient pore formation, with the contribution of endocytosis being dependent on molecular size.

Ultrasound enhanced prehospital thrombolysis using microbubbles infusion in patients with acute ST elevation myocardial infarction: rationale and design of the Sonolysis study

BACKGROUND

Experimental studies have shown that ultrasound contrast agents enhance the effectiveness of thrombolytic agents in the presence of ultrasound in vitro and in vivo. Recently, we have launched a clinical pilot study, called "Sonolysis", to study this effect in patients with ST-elevation myocardial infarction based on proximal lesions of the infarct-related artery.

METHODS/DESIGN

In our multicenter, randomized, placebo controlled clinical trial we will include patients between 18 and 80 years of age with their first ST-elevation myocardial infarction based on a proximal lesion of the infarct-related artery. After receiving a single bolus alteplase 50 mg IV (Actilyse(R) Boehringer Ingelheim GmbH), a loading dose of aspirin 500 mg, and heparin 5000 IU in the ambulance according to the prehospital thrombolysis protocol, patients, following oral informed consent, are randomized to undergo 15 minutes of pulsatile ultrasound with intravenous administration of ultrasound contrast agent or placebo without ultrasound. Afterwards coronary angiography and, if indicated, percutaneous coronary intervention will take place. A total of 60 patients will be enrolled in approximately 1 year.The primary endpoints are based on the coronary angiogram and consist of TIMI flow, corrected TIMI frame count, and myocardial blush grade. Follow-up includes 12-lead ECG, 2D-echocardiography, cardiac MRI, and enzyme markers to obtain our secondary endpoints, including the infarct size, wall motion abnormalities, and the global left ventricular function.

DISCUSSION

The Sonolysis study is the first multicenter, randomized, placebo controlled clinical trial investigating the therapeutic application of ultrasound and microbubbles in acute ST-elevation myocardial infarction patients. A positive finding may stimulate further research and technical innovations to implement the treatment in the ambulance and maybe obtain even more patency at an earlier stage.

TRIAL REGISTRATION

Trialregister NTR161.

Low intensity ultrasound-induced apoptosis in human gastric carcinoma cells

AIM: To investigate the low intensity ultrasound (US)-induced apoptosis in human gastric carcinoma cells and its potential mechanism and to suggest a new therapeutic approach to gastric carcinoma.

METHODS: Human SGC-7901 gastric carcinoma cells were cultured in vitro and irradiated by low intensity US for 10 min at different intensities with different incubation times after irradiation. Morphologic changes were examined under microscope with trypan blue staining and then the percentage of early apoptotic cells was detected by flow cytometry (FCM) with double staining of fluorescein isothiocyanate (FITC)-Annexin V/propidium iodide (PI). Two-dimensional electrophoresis (2DE) was used to get the protein profile and some proteins differently expressed after US irradiation were identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Functional analysis was performed to investigate the mechanism of US-induced cell apoptosis.

RESULTS: The percentage of apoptotic cells increased about 10% after US irradiation (12.0 W/cm², 12 h culture). The percentage of early apoptosis and secondary necrosis in the US-irradiated cells increased with the increased US intensity. Moreover, apoptotic cells increased with the increased culture time after US irradiation and reached its maximum at about 12 h. Several new proteins appeared after US irradiation and were up or down regulated more than 2 times. Some heat shock proteins (HSPs) were found to be associated with the signal process simulating the apoptosis of cells.

CONCLUSION: Low intensity US could induce apoptosis in human gastric carcinoma cells. US-induced apoptosis is related to US intensity/culture time. US-induced apoptosis may be caspases-dependent and endoplasmic reticulum (ER) stress-triggered apoptosis may also contribute to it. Proteomic experimental system is useful in finding the protein alteration in carcinoma cells after US irradiation, helping to develop a new cancer therapy.

Effect of focused ultrasound applied with an ultrasound contrast agent on the tight junctional integrity of the brain microvascular endothelium

Previous studies have investigated a potential method for targeted drug delivery in the central nervous system that uses focused ultrasound bursts combined with an ultrasound contrast agent to temporarily disrupt the blood-brain barrier (BBB). The purpose of this work was to investigate the integrity of the tight junctions (TJs) in rat brain microvessels after this BBB disruption. Ultrasound bursts (1.5-MHz) in combination with a gas contrast agent (Optison) was applied at two locations in the brain in 25 rats to induce BBB disruption. Using immunoelectron microscopy, the distributions of the TJ-specific transmembrane proteins occludin, claudin-1, claudin-5, and of submembranous ZO-1 were examined at 1, 2, 4, 6 and 24 h after sonication. A quantitative evaluation of the protein expression was made by counting the number of immunosignals per micrometer in the junctional clefts. BBB disruption at the sonicated locations was confirmed by the leakage of i.v. administered horseradish peroxidase (HRP, m.w. 40,000 Da) and lanthanum chloride (La(3+), m.w. approximately 139 Da). Leakage of these agents was observed at 1 and 2 h and, in a few vessels, at 4 h after ultrasound application. These changes were paralleled by the apparent disintegration of the TJ complexes, as evidenced by the redistribution and loss of the immunosignals for occludin, claudin-5 and ZO-1. Claudin-1 seemed less involved. At 6 and 24 h after sonication, no HRP or lanthanum leakage was observed and the barrier function of the TJs, as indicated by the localization and density of immunosignals, appeared to be completely restored. This study provides the first direct evidence that ultrasound bursts combined with a gas contrast agent cause disassembling of the TJ molecular structure, leading to loss of the junctional barrier functions in brain microvessels. The BBB disruption appears to last up to 4 h after sonication and permits the paracellular passage of agents with molecular weights up to at least 40 kDa. These promising features can be exploited in the future development of this method that could enable the delivery of drugs, antibodies or genes to targeted locations in the brain.

Microbubbles in ultrasound-triggered drug and gene delivery

Ultrasound contrast agents, in the form of gas-filled microbubbles, are becoming popular in perfusion monitoring; they are employed as molecular imaging agents. Microbubbles are manufactured from biocompatible materials, they can be injected intravenously, and some are approved for clinical use. Microbubbles can be destroyed by ultrasound irradiation. This destruction phenomenon can be applied to targeted drug delivery and enhancement of drug action. The ultrasonic field can be focused at the target tissues and organs; thus, selectivity of the treatment can be improved, reducing undesirable side effects. Microbubbles enhance ultrasound energy deposition in the tissues and serve as cavitation nuclei, increasing intracellular drug delivery. DNA delivery and successful tissue transfection are observed in the areas of the body where ultrasound is applied after intravascular administration of microbubbles and plasmid DNA. Accelerated blood clot dissolution in the areas of insonation by cooperative action of thrombolytic agents and microbubbles is demonstrated in several clinical trials.

Low-intensity ultrasound-exposed microbubbles provoke local hyperpolarization of the cell membrane via activation of BK(Ca) channels

Ultrasound (US) contrast agents have gained wide interest in gene therapy as many researchers reported increased membrane permeability and transfection efficiency by sonoporation in the presence of US contrast agents. We recently demonstrated an increase in cell membrane permeability for Ca2+ in rat cardiomyoblast (H9c2) cells insonified in the presence of microbubbles. In the present study, we specifically investigated whether US-exposed microbubbles have an effect on the cell membrane potential and whether Ca2+-dependent potassium (BK(Ca)) channels are involved. We particularly focused on local events where the microbubble was in contact with the cell membrane. H9c2 cells were cultured on US transparent membranes. US exposure consisted of bursts with a frequency of 1 MHz with a peak-to-peak pressure of 0.1 or 0.5 MPa. Pulse repetition frequency was set to 20 Hz, with a duty cycle of 0.2%. Cells were insonified during 30 s in the presence of Sonovue(trade mark) microbubbles. The membrane potential was monitored during US exposure using the fluorescent dye di-4-aminonaphtylethenylpyridinium (di-4-ANEPPS). The experiments were repeated in the presence of iberiotoxin (100 nM), a specific inhibitor of BK(Ca) channels. Surprisingly, despite the previously reported Ca(2+) influx, we found patches of hyperpolarization of the cell membrane, as reflected by local increases in di-4-ANEPPS mean intensity of fluorescence (MIF) to 118.6 +/- 2.5% (p < 0.001, n = 267) at 0.1 MPa and 125.7 +/- 5.9% (p < 0.001, n = 161) at 0.5 MPa at t = 74 s, respectively, compared with "no US" (100.3 +/- 3.4%, n = 52). This hyperpolarization was caused by the activation of BK(Ca) channels, as iberiotoxin completely prevented hyperpolarization. (MIF(t74) = 100.6 +/- 1.4%; p < 0.001, n = 267) and 0.5 MPa (MIF(t74) = 88.8 +/- 2.0%; p< 0.001, n = 193), compared with 0.1 and 0.5 MPa microbubbles without iberiotoxin. In conclusion, US-exposed microbubbles elicit a Ca2+ influx, which leads to activation of BK(Ca) channels and a subsequent, local hyperpolarization of the cell membrane. This local hyperpolarization of the cell membrane may facilitate uptake of macromolecules through endocytosis and macropinocytosis. (E-mail: ljm.juffermans@vumc.nl).

Ultrasound-induced calcium oscillations and waves in Chinese hamster ovary cells in the presence of microbubbles

This study investigated the effects of ultrasound on the intracellular [Ca(2+)] of Chinese hamster ovary cells in the presence of albumin-encapsulated Optison microbubbles. Cells were exposed to 1 MHz ultrasound (tone burst of 0.2 s duration, 0.45 MPa peak pressure) while immersed in solution of 0.9 mM Ca(2+). Calcium imaging of the cells was performed using digital video fluorescence microscopy and Ca(2+)-indicator dye fura-2AM. Experimental evidence indicated that ultrasound caused a direct microbubble-cell interaction resulting in the breaking and eventual dissolution of the microbubble and concomitant permeabilization of the cells to Ca(2+). These cells exhibited a large influx of Ca(2+) over 3-4 s and did not return to their equilibrium levels. Subsequently, some cells exhibited one or more Ca(2+) oscillations with the onset of oscillations delayed by 10-80 s after the ultrasound pulse. A variety of oscillations were observed including decaying oscillations returning to the baseline value over 35-100 s, oscillations superimposed on a more gradual recovery over 150-200 s, and oscillations continued with increased amplitude caused by a second ultrasound tone burst. The delays in onset appeared to result from calcium waves that propagated across the cells after the application of the ultrasound pulse.

Gene therapy progress and prospects: Ultrasound for gene transfer

Ultrasound exposure (USE) in the presence of microbubbles (MCB) (e.g. contrast agents used to enhance ultrasound imaging) increases plasmid transfection efficiency in vitro by several orders of magnitude. Formation of short-lived pores in the plasma membrane ('sonoporation'), up to 100 nm in effective diameter lasting a few seconds, is implicated as the dominant mechanism, associated with acoustic cavitation. Ultrasound enhanced gene transfer (UEGT) has also been successfully achieved in vivo, with reports of spatially restricted and therapeutically relevant levels of transgene expression. Loading MCB with nucleic acids and/or disease-targeting ligands may further improve the efficiency and specificity of UEGT such that clinical testing becomes a realistic prospect.