[Analysis of the kinetics of Ca2+ signals in Ehrlich ascites tumor cells upon the inhibition of the mitochondrial Na+/Ca2+ exchanger].

Signals of cytosolic Ca2+ (Ca2+c) and mitochondrial Ca2+ (Ca2+m) evoked by the activation of purinoreceptors of Ehrlich ascites tumor cells at different extents of inhibition of the mitochondrial Na+/Ca2+ exchanger by tetraphenylphosphonium (TPP+) were investigated. [Ca2+c] was measured by Fura-2 fluorescence, and [Ca2+m] changes were inferred from NAD(P)H fluorescence. The addition of ATP to the cell suspension induced a NAD(P)H response, which replicated Ca2+c signal with some retardation of the peak and a slower decay. In the presence of increasing TPP+ concentrations, NAD(P)H responses evidenced that the rate of [Ca2+m] decay strongly decreases, while the phase of initial rise does not change. The maximal TPP+ dose did not affect [Ca2+c] and NAD(P)H fluorescence in the resting state, as well as ATP-induced [Ca2+c] responses. These data are described in a mathematical model, which accounts for Ca2+ transport through the membranes of endoplasmic reticulum and mitochondria, as well as through the plasma membrane. The model indicates a low rate of the mitochondrial cycle of Ca2+ uptake/efflux at rest and a strong activation of the uptake with increasing [Ca2+c] to which a Hill coefficient of no less than 4 corresponds. Furthermore, the rise of the uptake rate changes in a short time to a decline, and the peak of the rate is markedly ahead of the peak of [Ca2+c].