Toad urinary bladder as a model for studying transepithelial sodium transport.

Sodium ion transport across tight epithelia has been investigated particularly extensively by studying two model systems: the urinary bladder of the toad and the frog skin. The greatest advantage presented by these models is the capability of monitoring net transepithelial Na+ flux simply, precisely, and instantaneously by measurement of the short circuit current (ISC). Many of the caveats involved in the measurement are discussed in detail. In order to fully characterize the forces driving Na+ movement across the series apical and basolateral membranes, it is necessary to measure intracellular potential and ionic composition. Such measurements are far more easily conducted with frog skin than with toad bladder, using the major biophysical techniques currently available. Regulation of transepithelial Na+ movement across tight epithelia is largely conducted at the apical membranes. This regulation can be clarified by study of the isolated Na+ channels in membrane vesicles. Such vesicles are far more easily prepared from toad urinary bladder than from frog skin. The strengths and potential misappropriations of this technique are considered in detail.