%**********************************************************************************
%
%                  CalC version 4.3.0, script "XY.ode.par"
%                     Victor Matveev, February 26, 2002
%
%        Supplemental material for the Letter to the Editor of Biophys. J.
%           "New and Corrected Simulations of Synaptic Facilitation"
%                    by V. Matveev, A. Sherman and R. Zucker
%
% This CalC script implements the Ca2+ binding schemes for the secretory (X)
% and facilitation (Y) sensors of the model by Tang et al. (2000), p. 2743.
%
% This script is used by the main parent scripts "STF.growth.par",
% "STF.decay.stage1.par" and "STF.decay.stage2.par"
%
%**********************************************************************************
 
% We denote [Ca2+] at the X-site as "CaX", and [Ca2+] at the Y-site as "CaY"; these
% two variables are defined in the scripts describing the reaction-diffusions PDEs
% ("quoted.pde.par", "actual.pde.par", "modified.pde.par" and "tortuosity.pde.par).
 
 
% 1. First, we define the equations for the secretory X-sensor (we denote "C X" by "CCX"
% and so forth):                                                            2
 
dCX/dt   = - koff CX + 3 kon X CaX - 2 kon CX CaX + 2 koff CCX
dCCX/dt  = 2 kon CX CaX - 2 koff CCX - kon CCX CaX + 3 koff CCCX
dCCCX/dt = kon CCX CaX - 3 koff CCCX
 
X  := 1 - CX - CCX - CCCX                 % Conservation constraint for the X-sensor
CX(0) = 0  ;  CCX(0) = 0  ;  CCCX(0) = 0  % Initial conditions
 
 
% 2. Next, we define the equation for the facilitation Y-sensor:
 
dCY/dt = Y.kon CaY ( 1 - CY ) - Y.koff CY
CY(0) = 0
 
 
% 3. Finally, the equation for the release rate:
 
dRelease/dt = k2 CCCX CY - k3 Release
Release(0) = 0
 
 
% 4. Now we define the rate constants in the above system of equations (Y.kon will be
% redefined for the simulations with tortuosity, Figs. 2D and 3D; in that case the
% Y-sensor affinity is changed to 9 uM)
 
koff = 100     ;  kon = 0.5      % For the X-sensor affinity = koff / kon = 200 uM
Y.koff = 0.555 ;  Y.kon = 0.185  % For the Y-sensor affinity = Y.koff / Y.kon = 3 uM
k2 = 1         ;  k3 = 10        % These define kinetics of the release rate variable
 
%==================================================================================
%                                  T H E   E N D
%==================================================================================