Experiments Explained: Electrophysiology
I have science on the brain after being away from the lab for over a week during the holidays. So I thought I would introduce a new topic series about lab techniques. Every lab is different and we all use different types of experiments to answer our very specific questions. The lab I am in is particularly interested in how different ion conducting proteins, called ion channels, work in certain contexts. Now, brace yourselves because here come some of the nitty gritty details. Scientists still debate the number of proteins but the proposed number at this time ranges between thousands to billions of different proteins. Proteins are elaborately folded sequences of amino acids (like tryptophan that is commonly rumored to make us sleepy after eating holiday turkey). These proteins are found everywhere in the body and the ones I am interested in are found on the cell membrane, which is a thin layer separating the inside and outside of the cells in the heart. Below, I've used the example of an ion channel that lets potassium ions travel from the inside of a cell to the outside of a cell. This potassium channel is only one of many different channels found on the surface of individual heart cells (cardiomyocytes).
When ions cross the cell membrane they carry a charge with them and this ends up changing the voltage inside of the cells, measured in volts (like a 9 volt battery for example). We actually use a smaller unit of measure called the millivolt (mV) because we are studying very small changes in voltage. The change in voltage allows an electrical signal to take place in the heart cell, which causes the heart to contract, pumping blood out of the heart and throughout the rest of the body. This electrical signal is called the action potential and it happens in cells like heart cells and neurons which are located in the brain.
We look at genetic diseases that decrease the amount of potassium ions that travel outside of the heart cell and this creates an abnormal action potential. When this happens it's easier to trigger potentially deadly types of cardiac arrhythmia, or irregular heartbeat. We can take a sequence of RNA (a strand of amino acids that encodes the exact protein found in the human heart) for a particular channel (in this example, a potassium channel again) and inject this RNA into frog eggs. The molecules inside of the frog egg let the ion channel protein assemble so that we get thousands (if not more) of our one channel on the surface of the frog egg. To look precisely at changes in the number of ions that travel across the cell membrane, I use a technique called electrophysiology and more specifically a technique known as two-electrode voltage clamp. To make a long and complicated story short, a different voltage inside of the cell means a different amount of potassium ions that go through the channel. We poke the cell with microscopically thin glass pipettes and inject different current so that we are "clamping" the cell at a particular voltage, or keeping the voltage the exact same. When this happens we can see how many ions are crossing the cell membrane in the form of an electrical current. We can see how these currents are different when the ion channel protein is mutated or when we apply different drugs that increase or decrease the ability of the channel to let ions pass through.
In a nutshell, two-electrode voltage clamp is what I spend a majority of my time doing. Electrophysiology is a beautiful and powerful tool to study the electrical changes taking place in real time in both health and disease.