Simultaneous measurement of [Ca_i] and whole cell current in human erythroblasts. GΩ seal was attained in day 10 BFU-E–derived erythroblasts bathed in PBS. After break in, Vm was set at resting potential of −10 mV22 and the erythroblast was internally dialyzed for 1 (2 cells) or 4 minutes (2 cells) with pipette solution A (Materials and Methods) containing 50 μmol/L fura-2. There was no difference in stability of F350/F380 ratio (R) whether the erythroblast was dialyzed for 1 or 4 minutes. Baseline R was taken, Epo (2 U/mL) added and R was followed for 2 minutes before the addition of ionomycin (1 μmol/L). Our previous experience indicates that R significantly increased in intact erythroblasts 1 minute following Epo addition.8 Simultaneously whole cell current was measured at −10 mV for the duration of fluorescence measurements. (A) F350/F380 ratio (mean ± SE) of 4 erythroblasts before and after Epo or ionomycin addition. Error bars are not shown if they fall within boundaries of the symbol. (B) Whole cell current of one of the four erythroblasts shown in (A). If Epo induced a measurable inward whole cell Ca²⁺ current, it would be detected as a downward deflection in the current tracing, similar to what we observed in vasopressin-treated rat hepatocytes (Fig 5 in Duszynski et al42 ). No such downward deflection is apparent. By contrast, addition of ionomycin caused a large outward (positive) whole cell current.