FIGURE 4.

Effects of [Ca²⁺]_i and/or [Ca²⁺]_o on the initial Δ[Mg²⁺]_i/Δt in the Mg²⁺-loaded myocytes. (A–F) Records from six separate experiments in which the Mg²⁺ efflux was induced by addition of extracellular Na⁺ (140 mM) at various [Ca²⁺]_o with or without intracellular BAPTA loading. The cells were loaded with Mg²⁺ in the Mg-loading solution for 3 h with (B and C) or without (A, D–F) concomitant loading of BAPTA, and Tyrode's solution that contained 0–2 mM [Ca²⁺]_o (amounts indicated in the panels) was introduced at the times shown by arrows. In this and subsequent figures (Figs. 6 and 8), [Mg²⁺]_i traces have been smoothed with adjacent averaging of 51 data points (10 s) to reduce noise, whereas a solid line was drawn by the linear least-squares fit to unsmoothed data points for 180 s; the initial Δ[Mg²⁺]_i/Δt estimated from the slope is indicated (μM/s) near the trace. Cells 070904 (A), 070304 (B), 090304 (C), 072104 (D), 081404 (E), and 082704 (F).