Basal intracellular free Mg2+ concentration in smooth muscle cells of guinea pig tenia cecum: Intracellular calibration of the fluorescent indicator Furaptra

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 (Delta R-jump) was followed by a slow change in R (Delta R-slow), which reached a steady level within 2-5 h. The Delta R-jump was independent of Mg2+ concentration in solution ([Mg2+](o)) and thought to be unrelated to the change in [Mg2+](i). The direction of the Delta R-slow depended on [Mg2+](o) with a reversal at similar to 1 mM [Mg2+](o). The intracellular calibration curve was constructed from the steady levels of Delta R-slow, and the dissociation constant was 5.4 mM. With the intracellular calibration curve and correction for the Delta R-jump basal [Mg2+](i) 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+](i) level is similar to 1 mM at least in some types of smooth muscle cells, as generally found in striated muscles.