Characterization of cell membrane response to ultrasound activated microbubbles

Contrast agents for ultrasound imaging, composed of tiny gas microbubbles, have become a reality in clinical routine. They are extensively used in radiology for detection and characterization of various tumors and in cardiology for left ventricular opacification. Recent experimental studies showed that ultrasound waves in combination with contrast agent microbubbles increase transiently cell membrane permeability in a process known as sonoporation. This effect is thought to allow foreign molecules to enter the cell. In that context, we explored the cell membrane's responses to microbubbles' oscillations as the mechanism is not completely understood. Breast cancer sell line in combination with contrast microbubbles were used. Ultrasound was applied using a transducer of 1 MHz center frequency transmitting a 10-cycle burst of different acoustic pressures repeated every 100 mu s. Patch-clamp technique in whole cell configuration was used to explore transmembrane ion exchange through the variations in membrane potential. To characterize the activated ion channels, the variations of the intracellular calcium (Ca2+) concentration were explored using a fluorescent marker. The results revealed that ultrasound stimulation induces a rapid hyperpolarization of cell membrane potential when the microbubble is in direct contact with the cell, but the potential returned to its initial value when ultrasound stimulation stopped. The change in cell membrane potential indicates the activation of specific ion channels and depends on the quality of microbubble adhesion to the cell membrane. Microbubbles were shown to induce a mechanical stretch activating BKCa channels. Simultaneous Ca2+ measurements indicate a slow and progressive Ca2+ increase that is likely a consequence of BKCa channels opening not a cause. These results demonstrate that microbubbles' oscillations under ultrasound activation entail modulation of cellular function and signaling by triggering the modulation of ionic transports through the cell membrane. Cells response to the mechanical stretch caused by gentle microbubble oscillations is characterized by the opening of BKCa stretch channels and a Ca2+ flux, which might potentially trigger other cellular responses responsible for membrane sonopermeabilization.