Intracellular sodium ion activity and sodium transport in rabbit urinary bladder.

1. Intracellular potentials and the intracellular activities of Na+ and K+ were examined using conventional and ion‐selective micro‐electrodes. 2. In animals on a normal diet, the intracellular Na+ activity was 8.6 +/‐ 2.9 mM (mean +/‐ S.D.) with a mean short‐circuit current of 2.8 +/‐ 0.9 microA/cm2. 3. In animals on a low‐Na+ diet, the intracellular Na+ activity was 18.5 +/‐ 9.9 mM with a short‐circuit current of 4.5 +/‐ 1.3 microA/cm2 (mean +/‐ S.D.). 4. There was a correlation between short‐circuit current and intracellular Na+ activity which could be fitted by a saturating hyperbolic relationship. 5. Treatment of the issue with ouabain and amiloride produced an increase and a decrease, respectively, in the intracellular Na+ activity. 6. Treatment with aldosterone produced a large increase in short‐circuit current with a substantial increase in intracellular Na+ activity. 7. Intracellular Na+ activity does not seem to affect apical membrane permeability directly.