Intracellular calibration of the fluorescent Mg2+ indicator furaptra in rat ventricular myocytes

Single ventricular myocytes enzymatically isolated from rat hearts were loaded with the Mg2+ indicator furaptra, and the relationship between the fluorescence ratio signal (R) and the intracellular fr ee concentration of Mg2+ ([Mg2+](i)) was studied in situ at 25 degreesC. After the application of ionophores (ionomycin, monensin, nigericin and valinomycin), an immediate change in furaptra R was noted, followed by a slow change in R that reached a steady level in 2-4 h. The direction of the early change in R that accompanied rigor contraction was independent of the extracellular. Mg2+ concentration ([Mg2+](o)), and was consistent with the breakdown of ATP and release of bound Mg2+ The intracellular calibration curve was constructed from the steady levels of X obtained at various [Mg2+](o) between 0 and 47 mM. The dissociation constant of intracellular furaptra was estimated to be 5.3 mM, which was 44% higher than that determined in salt solutions (3.7 mM). The basal [Mg2+](i) of rat ventricular myocytes calculated with the intracellular. curve averaged 0.91 mM.