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LAB27 - The Journal of Experimental Biology 207 1439-1452...

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What might be termed the ‘modern era’ of epithelial ion transport began nearly 70 · years ago with the observation that metabolically dependent net Cl (presumably NaCl) movement occurred across isolated frog skin bathed on both surfaces by Ringer solutions (Huf, 1936). This was followed shortly after by experiments showing that freshwater (FW) animals, including frogs, could absorb both Na + and Cl from solutions as dilute as 10 –5 · mol · l –1 (Krogh, 1937, 1938). But Krogh showed more than this. His work also demonstrated that the uptake of Na + and Cl were independent; Cl was absorbed from salts of impermeant cations and Na + was absorbed from salts of non-penetrating anions. Although he made no electrical measurements, he realized the significance of these observations and suggested that the influx of each ion was accompanied by the efflux of an ion of like sign. This was the first step in working out a mechanism for NaCl absorption in FW animals. During the decade following the end of the Second World War, the basic concepts and experimental approaches to ion movement across epithelia were developed, most of them in the laboratory of Hans Ussing. A rigorous criterion for active transport, the flux ratio equation, was described (Ussing, 1949), voltage clamping allowed relatively easy measurement of ion- induced currents, and isotopes were used to identify the current (Ussing and Zerahn, 1951). The concept of exchange diffusion was introduced (Levi and Ussing, 1948). A more thorough description of Ussing’s research career has been published recently (Larsen, 2002), but it is worth emphasizing here that elements of this framework are still used in studying epithelial ion movements. It is also worth noting that, with the exception of exchange diffusion (from a study of muscle), the preparation used in these studies was the isolated frog skin, usually bathed on both sides by Ringer solution. The importance of this model system can’t be overemphasized, as will be noted again later. In addition to these conceptual and methodological tools, Ussing’s laboratory also made two observations that became key in providing a definitive model of Na + transport across frog skin. First, they showed that transfer of Na + from the apical (external) bathing solution into the epithelial cells was passive (Koefoed Johnsen and Ussing, 1958), an observation that led ultimately to the description of the amiloride-sensitive ‘epithelial Na channel’ (ENaC; cf. Smith and Benos, 1991). In addition, they showed that extrusion of Na + from cell to internal bathing medium was mediated by a ouabain-sensitive mechanism, later shown to be the Na + /K + -ATPase (Koefoed Johnsen, 1957; Koefoed Johnsen and Ussing, 1958). The use of a high [Cl ] medium (Ringer) to bath the apical surface of the skin opened channels permeable to Cl . The behavior of these Cl channels has been described in detail (Larsen, 1991), but their role in the animal is still not clear; frogs (except for Rana cancrivora ) are not exposed to bathing media with [Cl ]>100 · mmol · l –1 . In FW, the channels are
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