Potassium activities in epithelia

In general, intracellular K+ appears to be compartmentalized. This phenomenon does not seem to characterize cytoplasm per se, but probably reflects the processes of sequestration and ion exclusion characterizing certain as yet unidentified organelles. The cell nucleus does not appear to participate significantly in these processes. Measurement of intracellular potassium activity (alpha K)c in small epithelial cells is complicated by significant technical problems. Recent experimental maneuvers designed to circumvent these problems have led to substantially higher estimates of (alpha K)c under baseline conditions. The time courses of short circuit current (SCC) and (alpha K)c in toad urinary bladder have been correlated under two experimental conditions. After removing external K+ or after adding ouabain, both parameters are depressed. However, the time courses of SCC and (alpha K)c are very different following return to baseline conditions. The data suggest: 1) that the processes of cell K+ accumulation and transepithelial Na+ transport are not linked with a fixed stoichiometry, and 2) if a reduction in cytosolic K+ activity does inhibit transepithelial Na+ transport, its role is indirect.