Ca²⁺ channels in human erythroblasts. With Ca²⁺ (6 mmol/L) and charybdotoxin (100 nmol/L) in pipette solution and both external and pipette solutions K⁺-free (Materials and Methods), very few upward transitions (inward current) were observed as indicated. In eight of 123 GΩ seals, the average arithmetic unitary current amplitude was 0.287 ± 0.038 pA at Vp of −20 mV. We wish to emphasize that not enough channel openings (events) were present in these eight seals to allow us to perform meaningful fittings with pClamp 5.5.