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1. The rate of volume flow across frog skin induced by an osmotic gradient was measured when normal (7.4) and low pH (2.28) solutions bathed the outside. The osmotic permeabilities (Pos) were 2.4 +/- 0.4 and 4.8 +/- 1.0 micrometer/sec, respectively. The change in Pos induced by low pH was reversible. 2. Volume flow in the absence of an osmotic gradient was measured at normal and low pH. Values were 0.69 +/- 0.13 and 1.1 +/- 0.2 microliter/hr. cm2, respectively; the paired differences were significant (P less than 0.0025). This change in rate was partially reversible upon return to normal pH. 3. The potential difference (V) and short-circuit current (Is) across skins were measured under several conditions and the following equivalent parameters in a simplified electrical model were computed: total resistance (Rt); shunt resistance (Rs); electromotive force of the pump (ENa); and salt transport at open circuit (JNaCl). Representative figures were (a), at pH 7.4: Is = 14 +/- 1.6 microampere/cm2; Rt = 3.3 +/- 0.4 komega.cm2; Rs = 7.2 +/- 1.0 komega.cm2; ENA = 103 +/- 38 mV; JNaCl = 7.2 +/- 1.2 microampere/cm2; (b) at pH 2.28: Is = 8.3 +/- 2.1 microampere/cm2; Rt = 0.46 +/- 0.12 komega. cm2; Rs = 0.65 +/- 0.06 komega.cm2; ENa = 59 +/- 25 mV; JNaCl = 9.4 +/- 3.3 microampere/cm2. 4. From the electrical parameters measured concomitantly with the rate of fluid transport in given experiments, the expected salt concentration of the transported fluid was 0.30 +/- 0.08 and 0.38 +/- 0.08 mole/l. at normal and low pH, respectively, or some 3-4 times hyperosmotic with respect to the medium. 5. Treatment with low pH on the outside has been found to open the intercellular junctions in previous studies. The present results suggest that, if such an effect occurs, it is localized only to a small fraction of the cell perimeter. Making certain assumptions that fraction could be as low as 0.003. 6. Low pH on the outside reversibly changes the electrical parameters of a 'tight' epithelium like the frog skin into values more typical of 'intermediate' epithelia; both the total and shunt resistances decrease to about 0.1 of their normal values. These changes do not apparently affect the osmolarity of the transported fluid.

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