title: Loading rat heart myocytes with Mg2+ using low-[Na+] solutions.
abstract: The objective of our study was to investigate how Mg2+ enters mammalian cardiac cells. During this work, we found evidence for a previously undescribed route for Mg2+ entry, and now provide a preliminary account of its properties. Changes in Mg2+ influx into rat ventricular myocytes were deduced from changes in intracellular ionized Mg2+ concentration ([fMg2+]i) measured from the fluorescence of mag-fura-2 loaded into isolated cells. Superfusion of myocytes at 37 degrees C with Ca2+-free solutions with both reduced [Na+] and raised [Mg2+] caused myocytes to load with Mg2+. Uptake was seen with solutions containing 5 mm Mg2+ and 95 mm Na+, and increased linearly with increasing extracellular [Mg2+] or decreasing extracellular [Na+]. It was very sensitive to temperature (Q(10) > 9, 25--37 degrees C), was observed even in myocytes with very low Na+ contents, and stopped abruptly when external [Na+] was returned to normal. Uptake was greatly reduced by imipramine or KB-R7943 if these were added when [fMg2+]i was close to the physiological level, but was unaffected if they were applied when [fMg2+]i was above 2 mm. Uptake was also reduced by depolarizing the membrane potential by increasing extracellular [K+] or voltage clamp to 0 mV. We suggest that initial Mg2+ uptake may involve several transporters, including reversed Na+-Mg2+ antiport and, depending on the exact conditions, reversed Na+-Ca2+ antiport. The ensuing rise of [fMg2+]i, in conjunction with reduced [Na+], may then activate a new Mg2+ transporter that is highly sensitive to temperature, is insensitive to imipramine or KB-R7943, but is inactivated by depolarization.
authors.affiliation: Centre for Integrative Physiology, The University of Edinburgh, Hugh Robson Building, George Square, Edinburgh EH8 9XD, UK. peter.flatman@ed.ac.uk.
authors.forename: Hasan A
authors.initials: HA
authors.lastname: Almulla
authors.forename: Peter G
authors.initials: PG
authors.lastname: Bush
authors.forename: Michael G
authors.initials: MG
authors.lastname: Steele
authors.forename: David
authors.initials: D
authors.lastname: Ellis
authors.forename: Peter W
authors.initials: PW
authors.lastname: Flatman
mesh_terms: D018759:Adrenergic Uptake Inhibitors / Q000494:pharmacology; D000818:Animals; D000889:Anti-Arrhythmia Agents / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002463:Cell Membrane Permeability / Q000187:drug effects / Q000502:physiology*; D006352:Heart Ventricles / Q000166:cytology; D007099:Imipramine / Q000494:pharmacology; D008274:Magnesium / Q000493:pharmacokinetics* / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism* / Q000493:pharmacokinetics; D019831:Sodium-Calcium Exchanger / Q000502:physiology; D013696:Temperature; D013890:Thiourea / Q000031:analogs & derivatives / Q000494:pharmacology; D016276:Ventricular Function
chemical_list: C101670:2-(2-(4-(4-nitrobenzyloxy)phenyl)ethyl)isothiourea methanesulfonate; D018759:Adrenergic Uptake Inhibitors; D000889:Anti-Arrhythmia Agents; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D013890:Thiourea; D008274:Magnesium; D007099:Imipramine