Figure 3.

Acceleration of decay kinetics in spontaneous Ca²⁺ waves indicates increased SERCA2a function in T₃-treated HL-1 cells. Spontaneous Ca²⁺ waves in HL-1 cells that were either treated with T₃ (1 μm) or vehicle control for 72 h were analyzed for their kinetic properties. A, Representative images of the fluo-4 fluorescent signal from control Hl-1 cells (top panels) with a trace (bottom panels, black line) of the mean fluorescent signal from the white boxed area. The trace data presented has been smoothed by a fast Fourier transform filter (4 point). A gray dashed line shows the function used to fit the fluorescence signal trace for analysis of kinetic properties. B, Same as in A for T₃-treated HL-1 cells. C, T₃ treatment (open bar) resulted in a decrease in the decay time constant (DTC) of Ca²⁺ waves when compared with a vehicle control (black bars). This suggests that Ca²⁺ clearance from the cytosol was accelerated due to T₃ treatment, which is consistent with an increase of SERCA2a activity, resulting in increased uptake of Ca²⁺ into the sarcoplasmic reticulum. D, T₃-treated cells also displayed a decreased duration of individual Ca²⁺ waves, which is also consistent with accelerated Ca²⁺ clearance for the cytosol due to elevated SERCA2a activity. E, The amplitude of individual Ca²⁺ waves in T₃-treated cells was not significantly altered from those of vehicle-treated cells. Therefore, it is not likely that the alterations to the kinetics of the Ca²⁺ waves is due to alterations in the Ca²⁺ release machinery of, Ca²⁺ storage within the sarcoplasmic reticulum. Data are presented as mean ± sem. Statistical significance is determined by t test (**, P < 0.01).