Effects of intracellular and extracellular Ca²⁺ on Mg²⁺ efflux from rat ventricular myocytes

Abstract

Intracellular Mg ²⁺ concentration ([Mg²⁺]_i) was measured with the fluorescent indicator furaptra (mag-fura-2) at 25°C. After the cells were loaded with Mg²⁺ in a high-Mg²⁺ and low-Na⁺ solution for 3 h, reduction of extracellular Mg²⁺ to 1 mM caused a rapid decrease in [Mg²⁺]_i in the presence of 140 mM Na⁺ and 0.1 mM EGTA (extracellular Na⁺-dependent Mg²⁺ efflux). To study the effects of intracellular Ca²⁺ concentration on the Mg²⁺ efflux, the initial rate of decrease in [Mg²⁺]_i (initial Δ[Mg²⁺]_i/Δt) was measured in the cells heavily loaded with a Ca²⁺ chelator, either BAPTA or dimethyl BAPTA, by incubation with its acetoxy-methyl ester for 3.5 h. Intracellular loading of the Ca²⁺ chelator did not significantly change the initial Δ[Mg²⁺]_i/Δt: 109.7±8.2% (7 BAPTA-loaded cells) and 102.2±10.3% (4 dimeth yl BAPTA-loaded cells) of the control measured in the absence of the intrace llular chelator. When the Mg²⁺ efflux was induced at various extracellular Ca²⁺ concentrations ([Ca²⁺]_o) and in the absence of the intracellular Ca²⁺ chelator, the initial Δ[Mg²⁺]_i/Δt values were essentially unchanged at [Ca ²⁺]_o up to 1 mM (101.4±7.7%, n=5), but were significa ntly reduced at 2 mM [Ca²⁺]_o (62.0±9.0%, n=5). This reduction at 2 mM [Ca²⁺]_o was abolished by intracellu lar loading of BAPTA (99.6±7.6%, n=6), suggesting that intracellular Ca²⁺ overload, rather than extracellular Ca²⁺ itself, caused the apparent reduction of Mg²⁺ efflux rate. We conclude that the net Mg²⁺ efflux can be driven in the essential absence of intracellular Ca²⁺ and is not modulated by changes in [Ca²⁺]_o. [Jpn J Physiol 55 Suppl:S76 (2005)]