Figure 4. I_NSC activated by S1P in HUVECs.

Aa, 0.3 μm SPH, a negative control lipid, had no effect on membrane currents at a holding potential of -50 mV. Ab, this cell was voltage clamped at -50 mV while [Ca²⁺]_i was monitored. The ramp waveform (0.96 V s⁻¹) was applied every 10 s. Ab, the change in the holding current and the corresponding fura-2 signal (F₃₄₀/F₃₈₀ ratio) in the whole cell are plotted against time. The Ca²⁺ images indicated by 1-3 in the upper panel were taken at the times shown in the lower panel. Sampling frequency of Ca²⁺-imaging apparatus was 1 Hz. The calibration of fluorescence ratio is indicated by the colour scale on the right. B, summary of the onset of the elevation of [Ca²⁺]_i and activation of I_S1P. The onset time of I_S1P was normalized to 0 s in four separate cells. The amplitude of I_S1P at -50 mV was averaged in the upper panel and the corresponding change of fluorescence ratio is plotted against time in the lower panel. C, the current-voltage (I–V) relationship of I_NSC activated by S1P under the physiological experimental conditions. This I–V relationship was obtained by subtracting the net membrane current in the presence of S1P (β in Ab) from that in its absence (α in Ab).