Dual probe fluorescence monitoring of intracellular free calcium during ischemia in mouse heart by using continuous compensation for pH dependence of the dissociation constant of Fura-2, and the interference of myoglobin

Mitochondrial damage is the main source of cellular injury upon ischemia-reperfusion, and calcium loading has been implicated in this phenomenon. The use of optical probes for calcium monitoring of the intact heart is hampered by internal filter effects of intracellular hemoproteins, endogenous fluorescence, and their sensitivity to pH. We describe here a method for measurement of intracellular free calcium in isolated myoglobin-deficient perfused mouse hearts under conditions of large intracellular pH fluctuations by simultaneous fluorescence monitoring of the calcium-probe Fura-2 and the pH probe BCECF through dual wavelength excitation of both probes. In myoglobin-containing mouse heart endogenous chromophores interfere with Fura-2 fluorometry. It is shown that a paradoxical decrease in Fura-2 fluorescence occurs during ischemia in isolated mouse hearts. Simultaneous recording of BCECF fluorescence (calibrated against pH measurement with phosphorus NMR) and data reduction based on continual recalculation of the apparent dissociation constant of the calcium-probe complex revealed that a marked increase in intracellular free calcium occurs, and that the Fura-2 fluorescence decrease was caused by an increase in dissociation constant due to intracellular acidification. Intracellular free calcium rose almost linearly during a 20-min period of ischemia and returned to basal values rapidly upon the commencement of perfusion.