FIGURE 2.

(Left) Estimation of the initial rates of decrease in [Mg²⁺]_i in three myocytes which were loaded with Mg²⁺ to different levels of [Mg²⁺]_i. After the Mg²⁺ loading in the high-Mg²⁺ and low Na⁺ solution, Ca²⁺-free Tyrode's solution was introduced to induce Mg²⁺ efflux, as indicated at the top. Cells 051203 (top), 060303 (middle), and 062403 (bottom). (Right) Values of the initial Δ[Mg²⁺]_i/Δt estimated as shown in the left panel are plotted as a function of initial [Mg²⁺]_i (defined as [Mg²⁺]_i at the first point of the fitted line). The myocytes were loaded with Mg²⁺ in solutions containing either 93 mM Mg²⁺ (open circles) or 24 mM Mg²⁺ (solid circles) for 2–5 h. Mean basal [Mg²⁺]_i measured before Mg²⁺-loading in these myocytes was 0.855 mM (n = 23), and is plotted at zero initial Δ[Mg²⁺]_i/Δt (⊕). Also shown (x symbols) are data taken from Figs. 4 and 6 of Tashiro et al. (2002); the myocytes were loaded with Mg²⁺ in the 93 mM Mg²⁺-containing solution, and the initial Δ[Mg²⁺]_i/Δt values were estimated under voltage-clamp at the holding potential of −80 mV. A solid curve indicates the least-squares fit to the data points (open circles and solid circles) by the Hill-type curve (Eq. 2) with the parameters shown in the panel.