The aim of this paper is twofold. First, to describe a method for the measurement of the unidirectional flux of Na from the outer bathing solution into epithelium (JOT), and second, to describe the use of this method under a variety of experimental conditions in order to obtain some insight into the nature of this flux. The method developed is based on the exposure of a frog skin to a Ringer solution containing 22Na. The exposure is made so that neighboring points along the surface remain in contact with the 22Na solution for gradually longer periods, ranging from 0 to 46 sec. Some 8 to 10 samples of the exposed part are used to obtain the time course of the uptake of 22Na and this time course is used, in turn, to evaluate JOT. This flux is then studied in skins mounted between two identical Ringer solutions with 115 mM Na (11.25 ± 0.10 [18] µmole·hr-2 cm-2), and in skins mounted with Ringer with 1 mM Na on the outside and 115 mM Na on the inside (0.43 ± 0.05 [18] µmole·hr-1·cm-2. From the observations that the flux is much larger than the net Na flux across the whole skin, that it is inhibited by K+, and is unaffected by ouabain, it is concluded that the penetration of Na+ into the epithelium does not occur by simple diffusion and is not directly dependent on an ouabain-sensitive mechanism. In the course of these experiments it was observed that when the skin was crushed between two chambers the uptake of Na in the neighboring exposed areas was decreased.

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