CYTOPLASMIC FREE MG2+ IN RAT VENTRICULAR MYOCYTES STUDIED WITH THE FLUORESCENT INDICATOR FURAPTRA

To estimate cytoplasmic Mg2+ concentration ([Mg2+](i)), ventricular my ocytes enzymatically isolated from rat hearts were loaded with the flu orescent indicator, furaptra, and the fluorescence signals of single q uiescent myocytes were measured at 32 degrees C. The excitation spectr um of furaptra measured in the myocytes was well-fitted by the spectra obtained in vitro; thus it was possible to calibrate the fluorescence signal in terms of [Mg2+](i). The analysis implied that about 20% of the indicator molecules were Mg2+-bound. Considering that the indicato r likely reacts with Mg2+ with a larger K-D value in cytoplasm than in vitro (by a factor of 1.2-2 as suggested for mouse and frog skeletal muscles), the [Mg2+](i) for the resting single myocytes was estimated to be within 0.8-1.3 mM. Superfusion with a high extracellular Mg2+ co ncentration (20 mM) caused a slow and slight elevation in [Mg2+](i) ov er a period of a few hours. Other experimental interventions, includin g application of a low extracellular Na+ concentration and isoproteren ol, and CO2 acidosis, did not cause a detectable change in [Mg2+](i), whereas the application of an uncoupler, a blocker of oxidative phosph orylation in mitochondria, caused a rapid and large increase in [Mg2+] (i). It is suggested that the [Mg2+](i) is tightly maintained at aroun d 1 mM, unless intracellular ATP is depleted.