No abstract

Longitudinal muscle strips dissected from tenia cecum of guinea pig were loaded with the Mg2+ indicator, furaptra, and the relation between the fluorescent ratio signal (R) and cytoplasmic free Mg2+ concentration ([Mg2+]i) was studied in smooth muscle cells at 25 degrees C. After the application of ionophores (4-bromo-A23187, monensin, and nigericin), a small immediate offset of R (deltaRjump) was followed by a slow change in R (deltaRslow), which reached a steady level within 2-5 h. The deltaRjump was independent of Mg2+ concentration in solution ([Mg2+]o), and was thought to be unrelated to the change in [Mg2+]i. The direction of the deltaRslow depended on [Mg2+]o with a reversal at approximately 1 mM [Mg2+]o. The intracellular calibration curve was constructed from the steady levels of deltaRslow, and the dissociation constant was 5.4 mM. With the intracellular calibration curve and correction for the deltaRjump, basal [Mg2+], was estimated to be 0.98 +/- 0.05 mM (mean +/- SE, n = 12). When the same calibration was applied to A7r5 cells and rat ventricular myocytes, estimates of basal [Mg2+]i of these cells were 0.74 +/- 0.02 mM (n = 33) and 1.13 +/- 0.06 mM (n = 9), respectively. These results suggest that the basal [Mg2+] level is approximately 1 mM at least in some types of smooth muscle cells, as generally found in striated muscles.

In cardiac myocytes, the cytoplasmic-free concentration of Mg 2ϩ ([Mg 2ϩ ] i ) is maintained at or slightly lower than 1.0 mM [1][2][3][4], a level several hundred fold lower than that expected from its passive distribution. It follows that Mg 2ϩ must be actively extruded from the cells to counterbalance Mg 2ϩ influx driven by the electrochemical gradient across the cell membrane.As it is such an active extrusion pathway, it has been postulated that a Na ϩ -Mg 2ϩ exchange that utilizes energy from Na ϩ influx plays an important role in cardiac myocytes [5][6][7] as well as in other cell types (for review, see Flatman [8] and Romani and Scarpa [9]).However, experimental evidence of the Na ϩ -Mg 2ϩ exchange in cardiac myocytes is controversial [3,4,10], and detailed properties of the transport still remain largely unknown.The present study was aimed to determine, under control of the membrane potential, if Na ϩ -Mg 2ϩ exchange plays an essential role in cardiac myocytes, and how the membrane potential, over a wide range, modulates the transport of Mg 2ϩ across the cell membrane. This information is one of the important clues to the elucidation of Na ϩ -Mg 2ϩ exchange stoichiometry. Flatman et al. [11] studied, for the first time, Na ϩ -

The fluorescent Mg(2+) indicator furaptra (mag-fura-2) was introduced into single ventricular myocytes by incubation with its acetoxy-methyl ester form. The ratio of furaptra's fluorescence intensity at 382 and 350 nm was used to estimate the apparent cytoplasmic [Mg(2+)] ([Mg(2+)](i)). In Ca(2+)-free extracellular conditions (0.1 mM EGTA) at 25 degrees C, [Mg(2+)](i) averaged 0.842 +/- 0.019 mM. After the cells were loaded with Mg(2+) by exposure to high extracellular [Mg(2+)] ([Mg(2+)](o)), reduction of [Mg(2+)](o) to 1 mM (in the presence of extracellular Na(+)) induced a decrease in [Mg(2+)](i). The rate of decrease in [Mg(2+)](i) was higher at higher [Mg(2+)](i), whereas raising [Mg(2+)](o) slowed the decrease in [Mg(2+)](i) with 50% reduction of the rate at approximately 10 mM [Mg(2+)](o). Because a part of the furaptra molecules were likely trapped inside intracellular organelles, we assessed possible contribution of the indicator fluorescence emitted from the organelles. When the cell membranes of furaptra-loaded myocytes were permeabilized with saponin (25 microg/ml for 5 min), furaptra fluorescence intensity at 350-nm excitation decreased to 22%; thus approximately 78% of furaptra fluorescence appeared to represent cytoplasmic [Mg(2+)] ([Mg(2+)](c)), whereas the residual 22% likely represented [Mg(2+)] in organelles (primarily mitochondria as revealed by fluorescence imaging). [Mg(2+)] calibrated from the residual furaptra fluorescence ([Mg(2+)](r)) was 0.6-0.7 mM in bathing solution [Mg(2+)] (i.e., [Mg(2+)](c) of the skinned myocytes) of either 0.8 mM or 4.0 mM, suggesting that [Mg(2+)](r) was lower than and virtually insensitive to [Mg(2+)](c). We therefore corrected furaptra fluorescence signals measured in intact myocytes for this insensitive fraction of fluorescence to estimate [Mg(2+)](c). In addition, by utilizing concentration and dissociation constant values of known cytoplasmic Mg(2+) buffers, we calculated changes in total Mg concentration to obtain quantitative information on Mg(2+) flux across the cell membrane. The calculations indicate that, in the presence of extracellular Na(+), Mg(2+) efflux is markedly activated by [Mg(2+)](c) above the normal basal level (approximately 0.9 mM), with a half-maximal activation of approximately 1.9 mM [Mg(2+)](c). We conclude that [Mg(2+)](c) is tightly regulated by an Mg(2+) efflux that is dependent on extracellular [Na(+)].

Cytoplasmic concentration of Mg(2+) ([Mg(2+)](i)) was measured with a fluorescent indicator furaptra in ventricular myocytes enzymatically dissociated from rat hearts (25 degrees C). To study Mg(2+) transport across the cell membrane, cells were treated with ionomycin in Ca(2+)-free (0.1 mM EGTA) and high-Mg(2+) (10 mM) conditions to facilitate passive Mg(2+) influx. Rate of rise of [Mg(2+)](i) due to the net Mg(2+) influx was significantly smaller in the presence of 130 mM extracellular Na(+) than in its absence. We also tested the extracellular Na(+) dependence of the net Mg(2+) efflux from cells loaded with Mg(2+). After [Mg(2+)](i) was raised by ionomycin and high Mg(2+) to the level 0.5-0.6 mM above the basal value ( approximately 0.7 mM), washout of ionomycin and lowering extracellular [Mg(2+)] to 1.2 mM caused rapid decline of [Mg(2+)](i) in the presence of 140 mM Na(+). This net efflux of Mg(2+) was completely inhibited by withdrawal of extracellular Na(+) and was largely attenuated by imipramine, a known inhibitor of Na(+)/Mg(2+) exchange, with 50% inhibition at 79 microM. The relation between the rate of net Mg(2+) efflux and extracellular Na(+) concentration ([Na(+)](o)) had a Hill coefficient of 2 and [Na(+)](o) at half-maximal rate of 82 mM. These results demonstrate the presence of Na(+) gradient-dependent Mg(2+) transport, which is consistent with Na(+)/Mg(2+) exchange, in cardiac myocytes.