Effects of Epo on whole cell currents measured with nystatin perforated patch method. GΩ seals were formed in day 10 BFU-E–derived erythroblasts in IMDM containing FCS and HEPES (both 1%). Without nystatin in pipette solution, typical values for seal resistance were 20 to 50 GΩ. (A) Representative time course of decrease in seal resistance for two erythroblasts under nystatin perforated clamp. Voltage steps (between −40 and +80 mV, 10-mV increments, 100 ms) were applied at the times indicated and seal resistance was calculated. Mean steady-state seal resistance with nystatin-perforated patch was 2.4 ± 0.6 GΩ (n = 9). (B) Whole cell currents were measured only after attainment of minimal steady-state seal resistance, usually at 15 to 20 minutes after initial GΩ seal formation (A). IV relationships in human erythroblasts under nystatin-perforated patch-clamp before (▪, n = 5) and 15 minutes after Epo (2 U/mL) addition (•, n = 6) are shown (mean ± SE). Error bars are not shown if they fall within the boundaries of the symbol. If Epo induced a measurable inward whole cell current, it would be detected as a greater downward deflection at more negative voltages in the IV relationship, similar to what we observed in vasopressin-stimulated rat hepatocytes (Fig 6 in Duszynski et al42 ). This is not the case in Epo-stimulated erythroblasts. IV relationships at 1, 5, 10, and 20 minutes after Epo addition (n = 6 except for 20 minutes where n = 3) are similar and not shown.