Cross-talk between sodium and potassium channels in tight epithelia

Abstract

The barrier epithelia of amphibians such as frog skin and toad urinary bladder play an essential role in electrolyte, acid/base and water balance of the whole animal. These epithelia have junctions of high electrical resistance between the outermost living cells which render the epithelium tight to paracellular flow of Na + or K + , thus allowing vectorial transepithelial transport of these ions. The Na + absorption function is carried out by granular or principal cells via Na + channels in the apical membranes and an Na,K-ATPase pump in the basolateral membranes; K + is recyled across this barrier via K + -selective channels. The multilayered epithelium of frog skin functions as a syncytial Na + transport compartment [1, 2]. The granular cells, along with the deeper spinosum and germinativum cells, form the syncytium. The cells in deeper layers are coupled to the first reactive cell layer by gap junctions and although the cells in the different layers share the Na transport load, only the granular cell layer has a polarized apical membrane containing amiloride-sensitive Na + channels. The amphibian skin and urinary bladder actively secrete hydrogen ions under appropriate electrochemical gradients [3, 4]. Active H + secretion is restricted to the mitochondria-rich cells [5–8]. The amphibian skin and urinary bladder share common Na + and H + transport properties with distal renal tubule, and have been used for the past 50 years as highly successful models in studies of the mechanisms and regulation of ion transport across tight-junction epithelia [9]. A vast amount of information is available on the mechanisms and regulation of Na + , K + , Cl - , H + and water transport systems in amphibian skin and urinary bladder [10–13]. Indeed, many of the first descriptions of epithelial transport mechanisms and the cellular action of natriferic and anti-diuretic hormones and diuretic drugs were first described in these epithelia.