ABSTRACT : The Nat-dependence of Mg2+ transport in isolated rat ventricular myocytes was investigated using the Mg2+ fluorescent dye mag-fura-2. The dye was used in dual excitation mode (340 nm/ 380 nm) and was calibrated in vitro. Free cytosolic Mg2+ concentration ([Mg2+] ;) in quiescent ventricular myocytes was 0.75 ± 0.05 mM (mean ± S.E.M., n = 77). Myocytes were loaded with Mg2+ by superfusion with a high [Mg2+] solution that was both Nat- and Ca²+-free. On changing to a physiological solution, [Mg2+]; returned to pre-load levels. This reduction was found to depend on the presence and concentration of external Na+ ([Na]) where, increasing [Na]。 increased the rate constant of [Mg2+], reduction. The relationship between [Na], and [Mg2+]; reduction is well described by a straight line of slope 1.02 ± 0.10 mM¹ (mean ± S.E., n = 20). This [Mg2+]; reduction is inhibited by imipramine (200 µΜ). The rate of [Mg2+]; elevation above resting levels was also dependent on [Na], and [Mg2+]. Decreasing [Na] or increasing [Mg2+] 。 increases the maximum rate of [Mg2+], elevation. [Mg2+]; elevation was not inhibited by imipramine (200 µM), in cells loaded such that [Mg2+]; was at least 2 mm In order to investigate the involvement of the Na+/Ca2+ exchanger in the regulation of [Mg2+]i, cells were treated with KB-R7943, an inhibitor of the reverse mode of the Na+/Ca2+ exchanger. [Mg2+]; reduction in Mg2+-loaded myocytes was insensitive to KB-R7943 (20 μΜ). However, the same concentration of the drug inhibited [Mg2+]i elevation, but only when applied at modest Mg2+ loads ([Mg2+]; < 2 mM). The possible electrogenicity of Mg2+ transport was investigated by depolarising the cell, either by high [K], or by patch clamping. Cell depolarisation increased the rate constant of [Mg2+]; reduction in Mg2+-loaded myocytes and decreased the maximum rate of [Mg2+]; elevation in cells superfused with the high [Mg2+] solution. The results suggest that Mg2+ is expelled from these cardiac cells through an imipramine- and Na-sensitive sarcolemmal Mg2+ transporter, possibly a Na+/Mg2+ antiport in which, the influx of one Na* ion is required for the extrusion of one Mg2+ 2+ ion. Also, some Mg uptake probably occurs through reversal of the putative antiport, but almost certainly other uptake mechanisms exist, possibly including the Na+/Ca2+ exchanger, at least under the experimental conditions used in this study.