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Intracellular activities of sodium, potassium and chloride ions, aiNa, aiK, and aiCl were measured with ion-selective single-, double- and triple-barrelled micro-electrodes in skin and isolated epithelia of Rana temporaria bathed on both sides with normal or modified physiological saline. Apical and basolateral membrane potentials, psi ac and psi cs and resistance Ra and Rb respectively were also measured and from the latter the fractional resistance of the apical membrane, F(Ra) and voltage divider ratio, delta psi ac/delta psi cs were measured as criteria of satisfactory membrane penetration by the micro-electrodes. Under control conditions, aiNa was 12.3 +/- 0.8 mM, aiK was 70.3 +/- 22 mM and aiCl was 20.3 +/- 1.6 mM with psi ac averaging -38.0 +/- 3.2 mV. When 10(-4) M-amiloride was added to the apical bathing fluid aiNa fell within 10 min to 1.18 +/- 0.1 mM and aiCl to 5.2 +/- 0.9 mM, while aiK increased to 86.2 +/- 3.8 mM as measured from the basolateral border of isolated epithelia. The sodium transport pool of the skin was measured from the fall in aiNa in the presence of amiloride and could be expressed as 33 X 10(-9) mol cm-2 of epithelium. The mean rate of fall of aiNa under these conditions corresponded to an efflux rate at the basolateral border of 30.1 X 10(-9) mol cm-2 min-1 (48 microA cm-2) giving a half-time for turnover of the sodium transport pool of 33 s. Reduction of sodium concentration in the apical fluid from the normal 79 mM-Na to 10, 1 and 0.1 mM caused aiNa to fall in stages to 2 mM. Because psi ac increased in negativity to -101 mV in the process, this driving force for passive sodium accumulation, more than offset the increased sodium gradient opposing sodium influx across the apical border.

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