Radioactive ⁸⁶Rb⁺ and ²²Na⁺ uptake in oocytes expressing human Na/K pumps. (A) Mean ⁸⁶Rb⁺ uptake in Na⁺-loaded oocytes, 4 d after injection, during 5-min incubation in 125 mM Na⁺_o with 4.5 mM Rb⁺_o, either in the absence (open bars) or presence (striped bars) of 100 µM ouabain. (B) Mean ⁸⁶Rb⁺ uptake in oocytes, 4 d after injection, which were not Na⁺ loaded, during 15-min incubations in the same ionic conditions as in A. In both A and B, ⁸⁶Rb⁺ uptake was also measured in uninjected oocytes from the same batches. (C) Mean ²²Na⁺ uptake during a 2-h incubation in 125 mM Na⁺_o by oocytes expressing wild-type or mutant pumps, 4–5 d after injection in the absence (open bars) or presence (striped bars) of 100 µM ouabain. The number of oocytes is indicated in parentheses above each column. Error bars represent SEM.