a: Dependence of the time-constant τ1 of the linearized system on the two dimensionless kinetic parameters Rb and Rf, shown as a contour plot in the Rb-Rf plane. Numbers on the iso-τ lines indicate the value of the time-constant in seconds. b: Time-course of the calcium-bound indicator signal y(t) in the linear regime, i.e. when eqs. (15) and (16) apply. The inset shows the parameters for the two exponential functions describing the time course: for the decay, and for the increase. If not subject to variation, the parameters in both a and b were as follows: kb = 100.0 [1/sec], kf = 0.1 [1/(nMol*sec)], γ = 20.0 [1/sec], ymax = 1000.0 [nMol] and α0 = 100.0 [nMol/sec]. Consequently, the KD = 1000 nMol, and the dimensionless kinetic parameters Rb and Rf both were 5.0.