Jump to content
Powered by

Small air bubbles with a huge impact

Ion channels play an important role in the communication of an organism. These proteins form small pores in the cell membrane allowing charged particles like potassium or sodium ions to flow into and out of the cell. In so doing, they confer, amongst other things, the electrical activity of nerve and muscle cells. The malfunction of ion channels might have serious health effects. That is why it is necessary to understand their structure and function. Scientists from the Max Planck Institute for Metal Research in Stuttgart and their colleagues from the Rush Medical School in Chicago and the Miller School of Medicine at the University of Miami have identified a physical mechanism that gives a new explanation as to how ion channels open and close.

In order to lift a cup of tea or coffee, raise it to the mouth and sip it for breakfast, the brain has to process information about the temperature and weight of the cup as well as about the position of the hand. This results in the transmission of commands to the arm muscles that coordinate the movement of the cup to the mouth. The information between hand and brain is exchanged by way of axons. Although these movements are carried out many times every day and are hardly noticed, a plethora of breathtaking events on the microscopic scale make such movements possible.

The information along the axons is transmitted by way of action potentials, i.e. waves of membrane voltage that travel on the cell membrane, enabling sodium ions to initially enter the cells and potassium ions to leave the cell, thereby restoring equilibrium. The cell membrane is normally impermeable to ions. For sodium and potassium ions to be able to pass through the cell membrane, nature has developed specific proteins, so-called ion channels, microscopic pores in the membrane that enable certain ions to flow through. The channels can differentiate between sodium and potassium ions. The narrowest point – the ion channel shown in the picture below has a diameter of only approximately three Ångström (1 Å = 10-7 mm) – thereby acts as selective filter.
When the gate is open (left), a sufficient quantity of water molecules is available in order to envelop the ions that are seeking to diffuse through the channel. If the gate is closed (right), the diameter of the pore decreases and the water molecules are
When the gate is open (left), a sufficient quantity of water molecules is available in order to envelop the ions that are seeking to diffuse through the channel. If the gate is closed (right), the diameter of the pore decreases and the water molecules are pushed away due to the slight hydrophobicity of the inner side of the channel wall. Bubbles form that act as an ion barrier. (Figure: Max Planck Society)
The gate of the pore, which acts as selective filter, has a greater diameter (12 Ångström). By reacting to the changing membrane voltage, ion channels can increase or reduce the diameter of their gate. This conformational change is however not always sufficient for preventing the flow of ions. Another important detail is that the gates are typically slightly hydrophobic. When the opening of the gate is large enough, then the interaction of water molecules and the protein only plays a subordinate role because each water molecule is surrounded on average by several water molecules.

Small bubbles with a huge impact

If the opening of the gate becomes smaller, then the interaction between water molecules and the protein becomes more important. Once the gate has decreased to a specific size, the probability of finding water inside the gate is very low due to the repulsive effect between the water molecules and the protein. Instead, a small bubble forms that has a huge impact: The bubbles create a kind of vacuum that prevents the flow of the ions through the channels, thereby closing the gate. “Indeed, many experimental observations on how ion channels open and close can now be understood with what we call ‘bubble gating’,” said Roland Roth, further explaining that the effect of restricted fluids has been known for a long time in physics, but now also helps to explain a biological phenomenon.

It is interesting to note that the bubble-gating model enables the scientists to explain the numbing effect of inert gases like xenon. Xenon is a perfect narcotic when mixed in the right concentration with air that is breathed. “Since xenon is very slow in chemical terms, mechanisms based on specific chemical bindings can be excluded as an explanation of its narcotic effect,” said the young biophysicist. “But our bubble-gating calculations have shown that xenon increases the probability of bubble formation at low concentrations, even though the gate is still wide open.”
The model helps summarise and theoretically investigate numerous known phenomena. Therefore, the new model not only expands the scientists’ possibilities of looking at the fascinating processes happening in neurones when people are lifting tea or coffee cups to the mouth, but also provides them with new possibilities for investigating the effects of anaesthesia and drugs.

Source: Max Planck Society - 11.03.2008

Original publication: Roland Roth, Dirk Gillespie, Wolfgang Nonner, Bob Eisenberg. Bubbles, Gating, and Anesthetics in Ion Channels. Biophys. Journal BioFAST, 30 January 2008

Further information:
Roland Roth
Max Planck Institute for Metal Research, Stuttgart
Tel.: +49 711 689-1907
E-mail: Roland.Roth@mf.mpg.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/small-air-bubbles-with-a-huge-impact