Rhythmic Fluctuations in the Concentration of Intracellular Mg2+in Association with Spontaneous Rhythmic Contraction in Cultured Cardiac Myocytes

Magnesium ions (Mg2+) play a fundamental role in cellular function, but the cellular dynamic changes of intracellular Mg2+ remain poorly delineated. The present study aims to clarify whether the concentration of intracellular Mg2+ possibly changes cyclically in association with rhythmic contraction and intracellular Ca2+ oscillation in cultured cardiac myocytes from neonatal rats. To do this, we performed a noise analysis of fluctuations in the concentration of intracellular Mg2+ in cardiac myocytes. The concentration was estimated by loading cells with either Mg-fluo4/AM or KMG-20/AM. Results revealed that the intensity of Mg-fluo-4 or KMG-20 fluorescence fluctuated cyclically in association with the rhythmic contraction of cardiac myocytes. In addition, the simultaneous measurement of Fura2 and Mg-fluo-4 fluorescence revealed phase differences between the dynamics of the two signals, suggesting that the cyclic changes in the Mg-fluo-4 or KMG-20 fluorescent intensity actually reflected the changes in intracellular Mg2+. The complete termination of spontaneous rhythmic contractions did not abolish Mg2+ oscillations, suggesting that the rhythmic fluctuations in intracellular Mg2+ did not result from mechanical movements. We suggest that the concentration of intracellular Mg2+ changes cyclically in association with spontaneous, cyclic changes in the concentration of intracellular Ca2+ of cardiac myocytes. A noise analysis of the fluctuation of subtle changes in fluorescence intensity could contribute to the elucidation of novel functional roles of Mg2+ in cells.