12:1/ Destructive Effects of High Intensity Ultrasound 221 



Ultrasonic destruction of single cells in suspension occurs in the 

 presence of a phenomenon known as cavitation. Cavitation refers to the 

 rupturing of the suspending medium, forming small bubbles or cavities 

 whose density is negligible compared to the density of water. In 

 general, these cavities are filled with air or other dissolved gases. If a 

 gas-free liquid is used, the liquid vapor fills the cavities. During each period 

 of the ultrasonic wave, these bubbles expand rapidly as the pressure 

 decreases and then collapse to a much smaller volume as it increases. 



The rupture of single cells can be applied for various purposes. One 

 is to extract protein catalysts known as enzymes from whole cells. 

 Enzyme extractions are important in order to study these catalysts in as 

 pure a form as possible. However, the entire procedure is misleading 

 unless the final, purified enzyme molecules are very similar to the original 

 ones within the cell. In general, if several methods of extraction give 

 purified enzyme molecules with the same chemical and physical pro- 

 perties, one tends to believe that the purified enzyme molecules are 

 similar to those in the whole cell. Ultrasonic rupture of cells can often 

 be used to confirm the validity of other methods of enzyme extraction. 

 In a few known examples, enzyme extraction by ultrasonic techniques 

 gives a higher yield than other methods. For instance, this is a very 

 efficient method of preparing verdoperoxidase from white blood cells. 



Similarly, subcellular particles can be extracted which have unique 

 properties. In certain bacteria, it has been possible to obtain particu- 

 late fractions which will still synthesize proteins from amino acids. 

 Gale has found the highest yield of such subcellular particles from cells 

 fractured with ultrasonically generated cavitation. Other small particles 

 made from ultrasonically ruptured, intracellular organelles called 

 mitochondria have been used to study the mitochondrial structure. 



A more basic approach to ultrasonic rupture of single cells in suspen- 

 sion is to investigate the physical parameters of the cell which control its 

 sensitivity in a cavitating ultrasonic field. Measurements of the relative 

 rates of destruction of different cell types indicate their relative fragility. 

 It should be possible to relate these to other physical parameters of the 

 cell structure. Approaches to this problem are considered both in this 

 chapter and in the following one, Chapter 13. 



A different type of ultrasonic destruction occurs when a portion of the 

 central nervous system of a vertebrate is exposed to a high intensity 

 ultrasonic beam. In this case, specific use is made of the very small 

 wavelength to bring the ultrasonic field to focus in a small region of the 

 brain. The evidence concerning the mode of action is not clear-cut, 

 but the results are, nonetheless, very useful both for neurosurgery and for 

 physiological studies of the actions of specific regions of the nervous 

 system. 



