Cytoplasmic Free Mg2+ in Rat Ventricular Myocytes Studied with the Fluorescent Indicator Furaptra

Abstract

To estimate cytoplasmic Mg2+ concentration ([Mg2+]i), ventricular myocytes enzymatically isolated from rat hearts were loaded with the fluorescent indicator, furaptra, and the fluorescence signals of single quiescent myocytes were measured at 32°C. The excitation spectrum 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 indicator likely reacts with Mg2+ with a larger KD 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. Superfu-ston with a high extracellular Mg2+ concentration (20 mM) caused a slow and slight elevation in [Mg2+]i over a period of a few hours. Other experimental interventions, including application of a low extracellular Na+ concentration and isoproterenol, and CO2 acidosis, did not cause a detectable change in [Mg2+]s, whereas the application of an uncoupler, a blocker of oxidative phosphorylation in mitochondria, caused a rapid and large increase in [Mg2+]i It is suggested that the [Mg2+]i is tightly maintained at around 1 mM, unless intracellular ATP is depleted. © 1994, PHYSIOLOGICAL SOCIETY OF JAPAN. All rights reserved.