Effects of membrane potential on Na+ -dependent Mg2+ extrusion from rat ventricular myocytes.
To study Mg2+ transport across the cell membrane, the cytoplasmic concentration of Mg2+ ([Mg2+](i)) in rat ventricular myocytes was measured with the fluorescent indicator furaptra (mag-fura-2) under Ca2+ -free conditions (0.1 mM EGTA) at 25 degrees C. The fluorescence ratio signal of furaptra was converted to [Mg2+](i) using calibration parameters previously estimated in myocytes (Watanabe and Konishi, Pflügers Arch 442: 35-40, 2001). After [Mg2+](i) was raised by loading the cells with Mg2+ in a solution containing 93 mM Mg(2+), the cells were voltage-clamped at a holding potential of -80 mV using the perforated patch-clamp technique with amphotericin B. At the holding potential of -80 mV, the reduction of extracellular Mg2+ to 1.0 mM caused a rapid decrease in [Mg2+](i) only in the presence of extracellular Na(+). The rate of the net Mg2+ efflux appeared to be dependent on the initial level of [Mg2+](i); the decrease in [Mg2+](i) was significantly faster in the myocytes markedly loaded with Mg2+. The rate of decrease in [Mg2+](i) was influenced little by membrane depolarization from -80 to -40 mV, but the [Mg2+](i) decrease accelerated significantly at 0 mV by, on average, approximately 40%. Hyperpolarization from -80 to -120 mV slightly but significantly slowed the decrease in [Mg2+](i) by approximately 20%. The results clearly demonstrate an extracellular Na(+)- and intracellular Mg2+ -dependent Mg2+ efflux activity, which is consistent with the Na(+)-Mg2+ exchange, in rat ventricular myocytes. We found that the apparent rate of Mg2+ transport depends slightly on the membrane potential: facilitation by depolarization and inhibition by hyperpolarization with no sign of reversal between -120 and 0 mV.
Tashiro
Michiko
M
Department of Physiology, Tokyo Medical University, 6-1-1 Shinjuku, Shinjuku-ku, Tokyo, 160-8402 Japan.
Tursun
Pulat
P
Miyazaki
Takefumi
T
Watanabe
Masaru
M
Konishi
Masato
M
D000666:Amphotericin B / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology*; D005260:Female; D016257:Fura-2 / Q000031:analogs & derivatives*; D006352:Heart Ventricles / Q000378:metabolism; D007473:Ion Channels / Q000502:physiology; D008274:Magnesium / Q000378:metabolism*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D008856:Microscopy, Fluorescence / Q000379:methods; D032383:Myocytes, Cardiac / Q000378:metabolism*; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000493:pharmacokinetics*; D013050:Spectrometry, Fluorescence / Q000379:methods
D007473:Ion Channels; C058776:2-(2-(5-carboxy)oxazole)-5-hydroxy-6-aminobenzofuran-N,N,O-triacetic acid; D000666:Amphotericin B; D012964:Sodium; D008274:Magnesium; D016257:Fura-2