Cytosolic free magnesium modulates Na/Ca exchange currents in pig myocytes

Objective: Cardiac Na/Ca exchanger (NCX) protein is up-regulated and intracellular free magnesium ([Mg²⁺]_i) is significantly reduced in experimental heart failure. We asked whether changes in [Mg²⁺]_i in a physiologically relevant range could alter the I_NCX. Methods: The nickel-sensitive current was measured in voltage-clamped myocytes (Yorkshire pig; left ventricular) exposed to ramp pulses at 37°C in Tyrode's solution containing ouabain, nifedipine and ±Ni²⁺ (5 mmol/l). The intracellular free [Ca²⁺] and [Mg²⁺] concentrations were set at 50 nmol/l and 1.25 mmol/l (HiMg) or 0.13 mmol/l (LoMg), respectively, through pipette dialysis. Results: Reducing [Mg²⁺]_i resulted in a significant increase in both outward and inward Ni-sensitive current without a shift in the reversal potential. This effect was not due to the inadvertent reduction of intracellular free [ATP] secondary to binding of ATP to Mg²⁺; reducing intracellular [ATP] in LoMg cells from 1.35 mmol/l to 0.18 mmol/l did not affect I_NCX. The intracellular free [Ca²⁺] was raised from 50 to 200 nmol/l, resulting in augmented inward and outward current due to calcium activation. HiMg attenuated both inward and outward currents significantly compared to LoMg, suggesting that [Mg²⁺]_i competes with [Ca²⁺]_i at the allosteric regulatory site. Conclusion: Cytosolic free magnesium modulates the I_NCX over a physiologic range independent of [ATP]_i. Reduced [Mg²⁺]_i in heart failure could contribute to altered calcium regulation of the NCX, contributing to the altered heart failure phenotype through enhanced NCX activity.