The sodium-transporting compartment of the epithelium of frog skin.

1. The abdominal frog skin was mounted between two chambers containing Ringer with 1 mM-Na on the outside and 115 mM-Na on the inside. When the Na concentration of the outer solution ([Na](o)) is instantaneously raised from 1 to 50 mM, the short circuit current (I) increases to a new value in less than a second, and becomes essentially time-independent. Only in a few experiments was it observed to increase further, although at a much slower rate.2. At a time t after this increase, the addition of 10(-4)M amiloride to the outer solution produces an exponential decrease of I. The area under this exponential curve is generally taken to reflect the existence of a Na- transporting compartment (NaTC).3. The amount of Na represented by NaTC is a function of t: it increases from 1.7 x 10(-9) mole. cm(-2), at t = 10 sec, to 22.8 x 10(-9) mole. cm(-2) at t = 10 min.4. In view of the fact that (a) I is not a function of the size of the ;NaTC' and (b) that whereas I reaches a steady value in a fraction of a second the size of NaTC keeps increasing for minutes, it is proposed that the ;NaTC' represents an amount of Na which is not located along the main route of transepithelial transport.5. On the assumption that the NaTC is located in a cellular compartment and that, in order to accumulate in this compartment Na should be accompanied by a permeable anion, a series of experiments were performed with Ringer in which Cl(-) was replaced by gluconate. It was observed as expected, that NaTC in gluconate is 164 times smaller than in Cl(-), but I only decreases to one half its value in Cl(-) Ringer.