Effects of metabolic inhibition and intracellular pH on the Na⁺-dependent Mg²⁺ efflux in rat ventricular myocytes

Abstract

Effects of metabolic inhibition and intracellular pH on the Na⁺-dependent Mg²⁺ efflux in rat ventricular myocytesMichiko, Tashiro; Masato, Konishi (Dept. physiol., Tokyo Med. Univ., Tokyo, Japan)We measured cytoplasmic Mg²⁺ concentration ([Mg²⁺]_i) with the fluorescent indicator furaptra under Ca²⁺-free conditions. Treatment of the cells with 1 μM FCCP (10-15 min) or 5 mM KCN (60-90 min) in the absence of extracellular Na⁺ significantly increased [Mg²⁺]_i from ∼1 mM to ∼2.5 mM. The Na⁺-dependent Mg²⁺ efflux activity was estimated from the initial rate of in [Mg²⁺]_i (initial Δ [Mg²⁺]_i/Δt) for 30-150 s after introduction of extracellular Na⁺. The Mg²⁺ efflux activity was largely reduced, on average, by 87.6%(n=8) in FCCP-treated cells and 90.5% (n=4) in KCN-treated cells, compared with that in intact cells with comparable [Mg²⁺]_i. [Na⁺]_i measured with a Na⁺indicator SBFI was, on average, 5.0-10.5 mM (n=4) within the time range for initial Δ[Mg²⁺]_i/Δt measurements, which is much lower than that required for 50% inhibition of the Mg²⁺ efflux (–40 mM). Clamping intracellular pH to 7.15 by application of nigericin did not reverse the inhibition of the Mg²⁺ efflux. In intact cells, intracellular alkalosis upon application of 20 mM NH₄Cl did not significantly change, while intracellular acidosis upon removal of NH₄Cl significantly reduced the Mg²⁺ efflux, on average, by 54.5% (n=4). These results suggest that the Na⁺-dependent Mg²⁺ efflux critically requires cellular metabolism, although intracellular acidosis caused by metabolic inhibition may also contribute partly to the inhibition of the Mg²⁺ efflux. [J Physiol Sci. 2008;58 Suppl:S209]