During the late 1940s and early 1950s, Ussing experimented with the cellular layers – epithelia – that cover the body’s inner and outer surfaces. He used frog skin because it is fairly thin and has just a single layer of epithelial cells. His experiments revealed that ion transport is not purely passive but is an active and regulated process. This insight facilitated the understanding of many physiological functions and disorders, including cystic fibrosis.

Up until then, ions were thought to simply follow concentration gradients across membranes. With inspiration from August Krogh’s (1874–1949) hypothesis from the 1930s, Ussing confirmed that living tissue can use energy to undertake active ion transport. Initially, he was sceptical, because the hypothesis challenged the established view of cells as relatively passive systems that simply follow the laws of physics and chemistry. However, when Ussing developed his own recording methods and found specific experimental evidence, he became convinced – and proved Krogh to be right.

As part of this work, he constructed the device that later became known as the Ussing chamber, which soon became an international standard in physiological research.

With the chamber, Ussing proved that the epithelial cells in the frog skin actively transport sodium ions by using energy from ATP, the cell’s most significant energy molecule. He also documented that chloride and water passively move through the membrane, without needing energy. This laid the basis for distinguishing between active and passive transport in cell biology. 

Ussing’s work marked a breakthrough in the study of biological electricity, which at the time was a fairly understudied topic. In 1949, he introduced the so-called flux-ratio equation for analysing cellular ion transport. It describes the ratio of ions moving into and out of the cell, which varies among different epithelial cells, as does the ratio of active to passive transport paths for ions.

Together with the biophysicist Ove Sten-Knudsen (1919–2007), Ussing later demonstrated that the flux-ratio equation also applies in more-complex membrane systems, provided the ion follows one specific transport path. This meant that the theory is applicable across a wide range of physiological and biophysical fields.

The results paved the way for Jens Christian Skou’s (1918–2018) discovery of the sodium-potassium pump a decade later. The pump actively transports sodium and potassium across the cell membrane and is crucial for processes such as nerve cell signals and muscle contractions. For this discovery, Skou became the first Dane to receive the Nobel Prize in Chemistry, in 1997.

The discovery of active and passive ion transport at a cellular level has had a major impact on both research and development. Cell membrane transport remains an important and active research topic in physiology, biochemistry and medicine. The Ussing chamber is still used today to study how various substances affect ion transport in epithelial cells – particularly in intestines, kidneys and respiratory tracts – and it plays a significant role in pharmaceutical research.

No one would have predicted that studies of frog skin would one day prove crucial to the understanding of the severe pulmonary disorder of cystic fibrosis, in which the chloride transport across the cell membrane is dysfunctional. However, Ussing’s studies of ion transport in frog skin laid the foundation for knowledge that is now used to understand and treat the disease. Today the Ussing chamber continues to be used to test how mutations and medical drugs affect the salt transport in respiratory cells in patients with cystic fibrosis.

The chapter is written by Kristian Sjøgren.