Fig. 5. Membrane potential and Ca²⁺ influx through window current during myoblast fusion. (A) Model of the events leading to an increase in cytoplasmic Ca²⁺ concentration in fusion-competent myoblasts. The window current was computed from the mean activation and inactivation parameters of T-type Ca²⁺ currents measured in 11 fusion-competent myoblasts (Bijlenga et al., 2000). Ca²⁺ influx increases almost fourfold when the membrane potential is depolarized from -74 to -64 mV. (B) Time course of the cytoplasmic free-Ca²⁺ concentration in fusion-competent myoblasts evaluated with the `inverse-pericam' fluorescent indicator. Images were recorded every 10 seconds. Control (circles): myoblasts transfected with a bicistronic vector containing only the inverse-pericam. AS-HERG (squares): myoblasts transfected with a bicistronic vector expressing HERG antisense and inverse-pericam. (C) Mean fluorescence ratio increase obtained in control and AS-HERG treated fusion-competent myoblasts.