Ionic basis of short-circuit current in toad skin at high hydrostatic pressure

The toad skin and urinary bladder are widely used for the study of water and Na+ transport under high pressure. These tissues can be mounted in Ussing type chambers and ion transport can be measured by evaluating electrical properties of the preparation, e.g., short-circuit current (Isc). The tacit assumption in these experiments is that the preparation behaves in the same manner at high pressure as at 1 ATA; namely, that net Na+ flux is equivalent to Isc. The purpose of the experiments described here was to test that assumption. Toad skins were mounted in an Ussing chamber and Isc was measured as an index of active net Na+ transport under hydrostatic pressures up to 100 ATA. The chamber was modified so that isotopic Na+ flux from the mucosal to serosal compartments could be measured in conjunction with Isc, without decompression. A linear regression of JNa+ms on Isc was computed and found to be described by the equation, JNa+ms = 3.83 + 0.83 Isc; n = 18; r = 0.92. The slope of the line was not significantly different from unity. No correlation was made for JNa+sm because of the difficulty in measuring JNa+sm and JNa+ms in the same skin simultaneously. Independent measurement of JNa+sm demonstrated that this flux accounted for less than 2% of JNa+net. In a second set of experiments, the influence of amiloride on Isc with and without pressure was tested. 10(-4) M amiloride abolished Isc under both circumstances. It is concluded that Isc can be wholly accounted for by net Na+ flux under pressures up to 100 ATA.