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Fig. 4. Simulation of the effect of a global increase in [gPtdIns(4,5)P₂]. Results have been obtained by integration of equations (1) to (5) with the ER density represented in Fig. 2 and the same parameter values as in Fig. 3, except for the fact that gPtdIns(4,5)P₂ has been used instead of Ins(1,4,5)P₃. Thus the rates of degradation (V_5P and V_3K) have been divided by ten. To simulate the activation of the Ins(1,4,5)P₃R by both gPtdIns(4,5)P₂ and the basal Ins(1,4,5)P₃ level, we must replace the second part of equation (3) by:

with K_{gPtdIns(4,5)P2}=8 µM. These latter changes reflect the facts that gPtdIns(4,5)P₂ is slowly metabolized and that Ins(1,4,5)P₃ and gPtdIns(4,5)P₂ bind to the same site of the receptor, and that gIns(4,5)P₂ has less affinity for the receptor. A basal level of Ins(1,4,5)P₃ equal to 18 nM (corresponding to the stationary state in Ins(1,4,5)P₃ with the basal LC activity) is always present. During the flash time (2 seconds), I_{gPtdIns(4,5)P2}=0.045 µM second^-1 in the whole system. The successive panels show the spatial distribution of Ca²⁺ at the times indicated by the points on the curve on the left.

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