Optical Measurements of Intracellular Free Magnesium Concentration in Skeletal and Cardiac Muscle Cardiac

将荧光Mg <2+>指示剂furaptra注射到蛙单根骨骼肌纤维中，静息时测量指示剂的荧光信号，并用细胞内游离Mg <2+>浓度（[Mg <2+>]_i）（17 C）进行分析。根据furaptra的荧光激发光谱，如果使用体外获得的解离常数（K_D） （5.5 mM），则校准的[Mg^<2+>]_i水平平均为0.54 mM。然而，如果该指标与Mg^<2+>反应，肌浆中K_D增大两倍，如先前对furaptra- ca ^<2+>反应的建议（Konishi等，1991，J. gengenphysiol . 97: 271-301），计算出的[Mg^<2+>]_i平均为1.1 mM。因此，1.1 mM的值可能代表furaptra对静息肌纤维中[Mg^<2+>]_i的最佳估计。高Mg^<2+>浓度溶液或低Na^+浓度的肌纤维细胞外灌注在4min内未引起[Mg^<2+>]_i的任何可检测的变化。用酶法从大鼠心脏分离心室肌细胞，并将furaptra加载。在单个肌细胞（32 C）中测量荧光信号，并在骨骼肌纤维中分析，静止的单个肌细胞的[Mg^<2+>]_i估计在0.8-1.3 mM之间。高细胞外Mg^<2+>浓度的过量导致[Mg^<2+>]_i在一段时间内缓慢而小的升高。其他实验干预，包括低细胞外Na^+浓度、异丙肾上腺素和CO_2酸中毒，没有引起[Mg^<2+>]_i的可检测变化，而线粒体解偶联剂的应用导致[Mg^<2+>]_i的快速和大量增加。结果表明，1 mM左右的[Mg^<2+>]_i水平在不同的肌肉类型（骨骼肌和心肌）中是相似的。研究还表明，在骨骼肌和心肌中，[Mg^<2+>]_i高度维持在1mm左右，除非细胞内ATP耗尽。

The fluorescent Mg^<2+> indicator, furaptra, was injected into single frog skeletal muscle fibers, and the indicator's fluorescence signals were measured at rest and analyzed in terms of the intracellular free Mg^<2+> concentration ([Mg^<2+>]_i)(17ﾟC). Based on the fluorescence excitation spectrum of furaptra, the calibrated [Mg^<2+>]_i level averaged 0.54 mM, if the value of dissociation constant (K_D) obtained in vitro (5.5 mM) was used. However, if the indicator reacts with Mg^<2+> with a two-fold larger K_D in myoplasm, as previously suggested for the furaptra-Ca^<2+> reaction (Konishi et al, 1991, J. Gen. Physiol. 97 : 271-301), the calculated [Mg^<2+>]_i would average 1.1 mM. Thus, the value 1.1 mM Probably represents the best estimate from furaptra of [Mg^<2+>]_i in resting muscle fibers. Extracellular perfusion of muscle fibers with high Mg^<2+> concentration solution or low Na^+ concentration did not cause any detectable changes in [Mg^<2+>]_i within 4 min.Ventricular myocytes enzymatically isolated from rat heart were loaded with furaptra. The fluorescence signals were measured from single myocytes (32ﾟC), and analyzed as in the skeletal muscle fibers The estimated [Mg^<2+>]_i for the resting single myocytes was between 0.8-1.3 mM. Superfusion with a high extracellular Mg^<2+> concentration caused a very slow and small elevation in [Mg^<2+>]_i over a period of hours. Other experimental interventions, including a low extracellular Na^+ concentration, isoproterenol and CO_2 acidosis, did not cause detectable change in [Mg^<2+>]_i, whereas the application of a mitochondrial uncoupler caused a rapid and large increase in [Mg^<2+>]_i.The results suggest that the [Mg^<2+>]_i level of around 1 mM is similar in different muscle types (skeletal and cardiac muscles). It is also suggested that [Mg^<2+>]_i is highly maintained at around 1 mM in both skeletal and cardiac muscles, unless intracellular ATP is depleted.