Estimation of intracellular [Ca2+] by nonlinear indicators. A quantitative analysis

When spatial gradients of intracellular free [Ca2+] are present, intracellular calcium indicators that have a nonlinear response to [Ca2+] may yield an estimate of [Ca2+] that differs from the spatial average [Ca2+]. We present two rules that provide (a) general criteria to distinguish those classes of indicators that will yield an overestimate of spatial average [Ca2+] from those that will yield an underestimate, and (b) limits on the extent to which spatial average [Ca2+] might be over- or underestimated. These rules are used to interpret quantitatively the aequorin luminescence signals obtained from cardiac ventricular myocardium.