Ussing's “Little Chamber”: 60 Years+ Old and Counting

Generally, a Commentary for the Frontiers in Physiology is aimed at a recent research article that has the potential to make a major impact on a particular scientific research field. However, where would the field of epithelial ion transport physiology be without the Ussing chamber? One has to ask that question in the 100th anniversary year of the birth of the great Danish physiologist Hans Henriksen Ussing and his famous “Little Chamber”; which is now over 60 years old. The monumental works of Ussing (1911–2000), starting nearly 65 years ago, ushered in the “age of epithelial ion transport physiology,” which is still flourishing today in numerous epithelial physiology laboratories throughout the world. Indeed, Lindemann (2001) epitomized Ussing’s contribution to science in four simple words “...founder of epithelial transport....” Palmer and Andersen (2008) paid homage to Ussing by writing “...In a sense the field of epithelial polarity began in 1958 with the Koefoed-Johnsen and Ussing paper.” The focus of this Commentary is to highlight the classic paper of Hans Ussing in which he used his now famous “Little Chamber” and the implementation of the fledgling short-circuit current technique to define the active transport of sodium as the source of electric current across the isolated frog skin (Ussing and Zerahn, 1951). This single paper introduced a new research direction that has led to our understanding of many cellular models of ion transport physiology in numerous epithelial tissues of the body. Notably, to quickly appreciate the impact of Ussing’s work, if one types in “Ussing” into PubMed over 2,975 citations are identified at the writing of this manuscript. There are undoubtedly hundreds, if not thousands; of other research papers in which the Ussing chamber technique has been used but not captured by scientific search indices. The reader is referred to recent reviews for the historical prospective of Ussing and technical details of the Ussing chamber short-circuit technique by Larsen (2002), Palmer and Andersen (2008), and Clarke (2009). Prior to the 1951 paper, Ussing had established that the influx of Na+ transport across the frog skin was higher than the out flux of Na+ using double labeling with Na and Na, respectively (Levi and Ussing, 1949). Ussing’s introduction of his Ussing chamber (Ussing and Zerahn, 1951) and implementation of the short-circuit current technique in which he mounted an isolated frog skin epithelium between two fluid-filled chambers (Figure 1, essentially the same design as used today; http://www. ussingchamber.com/) was the beginning of a new age of research. This allowed the first investigation of ion (Na+, in this case) transport physiology of an epithelium. Mounting the frog skin epithelium between the fluidfilled chambers allowed physical access to either side of the epithelial layer for testing modulators of ion transport. Indeed, Ussing and Zerahn used copper, adrenaline and a “neurohypophyseal extract” to examine the modulation of Na transport and the short-circuit current of the frog skin. Ussing and Zerahn, using the short-circuited technique, were able to short-circuit the tissues by driving (voltage clamping) any transepithelial current generated back to zero. This current was indicative of active transport determined by the net transport processes. Ultimately, Ussing and Zerahn demonstrated that the short-circuit current of the frog skin was essentially determined by the active transport of Na+ across the frog skin epithelium. There are five important advances from Ussing and Zerahn’s (1951) paper. First, Ussing and Zerahn applied the shortcircuit technique to an epithelial tissue preparation. Secondly, they describe the apparatus (the Ussing chamber) that enabled them to determine the simultaneous measurement of Na+ transport and electrical current through the frog skin. Third, Ussing and Zerahn bathed the frog skin with identical solutions (high NaCl Ringer’s solution), consequently, there would be no passive ion flow. Therefore, ions that are moved by active transport will continue and the observed short-circuit current will result from a net transport of those ions. Fourth, Ussing and Zerahn developed a hypothesis in which it is possible to calculate the electromotive force for Na+ and also the resistance to the Na+ current from the efflux of Na+ and the short-circuit current. Finally, they reported that the short-circuit current of the frog skin was essentially the net active transport of Na+ based on the Na experiments compared with the short-circuit current measurements. In other words, Na+ influx across the entire frog skin epithelium (from pond water to the blood) dominated the Na+ efflux with very little Na+ being transported from the blood to the pond water.