Figure 4

Driving of oscillatory activity by Cyt-facing vs. µd-facing IP₃Rs. Upon IP₃ stimulation (t = 50 s), flux through each of the two pools of IP₃Rs was clamped to its time-averaged value. \({[{{\rm{Ca}}}^{2+}]}_{{\rm{Cyt}}}^{{\rm{eff}}}\) profiles following an IP₃ stimulus of 0.2 µM (region α; A) and 0.7 µM (region β; B) are shown for control (black curve), after clamping IP₃Rs facing the Cyt (red curve), or after clamping IP₃Rs facing the µd (blue curve). The activity of IP₃Rs facing the µd is required for oscillatory activity at low [IP₃] (blue curve in A), whereas the Cyt-facing IP₃R activity drives the oscillations at higher [IP₃] (red curve in B).