Intracellular K+activity in the epithelium of the isolated bullfrog urinary bladder was measured with a K+-selective liquid ion exchanger microelectrode. Using Ussing's apparatus, the short circuit current (SCC) as well as the potential difference (PD) across the bladder was also determined in Ringer solution on both sides of the epithelium with or without certain metabolic inhibitors.
The mean values obtained from 76 control experiments were-25.1mV for PD (the mucosal side negative), and 49.3 μA for the SCC (current from the mucosal to the serosal side). The serosal and mucosal membrane potential (sEm and mEm) was-12.4 and 12.7mV with respect to the serosal and mucosal surface, respectively. The normal intracellular K+ activity was found to be 39.3mM. Assuming that the intracellular ionic strength was equal to the extracellular one, the predicted K+ concentration was 52mM. The calculated K+ equilibrium potential, EK, was-67. 2mV with respect to the serosal surface, which was far more negative than the measured value of sEm or mEm. Addition of ouabain to and removal of K+ from the bathing media effectively reduced the intracellular K+activity in the free form, but not in the bound form. Ethacrynic acid and rotenone markedly depressed the membrane potential disproportionately to the cellular K+. Though significant correlations were observed between PD and SCC, sEm, and
mEm, and so between SCC and sEm, the EK tended to vary independently of either sEm or SCC. The above findings indicate that K+must be actively accumulated into the cell against the electrochemical gradient, and further, more than half of the cellular K might exist in a restricted (bound or compartmentalized) form.