Regulation of magnesium uptake and release in the heart and in isolated ventricular myocytes.
Perfused rat hearts release or accumulate approximately 10% of total Mg2+ content when stimulated with norepinephrine (NE) or carbachol, respectively. Collagenase-dispersed rat ventricular myocytes increase or decrease total cell Mg2+ by 1 mM within 5 minutes when stimulated with these same transmitters. Measurements of Mg2+ transport using 28Mg or atomic absorbance spectrophotometry indicate that the rate and the extent of both stimulated Mg2+ efflux and influx are independent of the concentration of extracellular Mg2+ (0 to 1.2 mM). Mg2+ release induced by NE is rapidly reversed by the addition of carbachol, and Mg2+ uptake induced by carbachol is reversed by NE. Decreasing extracellular Na+ or Ca2+ decreases or abolishes Mg2+ efflux from myocytes. Cd2+ or other Ca2+ channel blockers also inhibit Mg2+ efflux in the presence of a physiological concentration of extracellular Ca2+. Replacement of extracellular Ca2+ with Sr2+ or with Mn2+ decreases or abolishes both stimulated efflux and influx of Mg2+. Redistribution of 85Sr in myocytes and in the supernatant indicates that under those conditions Sr2+ is released or accumulated by NE or carbachol in a manner similar to that of Mg2+. Hence, at least in the case of Sr2+, the inhibition of Mg2+ fluxes can be explained by the transport of Sr2+ rather than Mg2+ through the transport(s) systems. By contrast, replacement of extracellular Ca2+ with Ba2+ inhibits stimulated Mg2+ uptake but not Mg2+ release. These results indicate that cardiac myocytes have a major pool of Mg2+ that can be rapidly mobilized upon hormonal stimulation. The net uptake and release of Mg2+ are quantitatively similar and appear to be independent of the extracellular Mg2+ concentrations but are affected, to various degrees, by the presence of other cellular or extracellular cations.
Romani
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Department of Physiology and Biophysics, Case Western Reserve University, School of Medicine, Cleveland, OH 44106.
Marfella
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Scarpa
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D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000502:physiology; D002217:Carbachol / Q000037:antagonists & inhibitors / Q000494:pharmacology; D003593:Cytoplasm / Q000378:metabolism; D005110:Extracellular Space / Q000378:metabolism; D066298:In Vitro Techniques; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D009638:Norepinephrine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000502:physiology; D013324:Strontium / Q000378:metabolism; D014700:Verapamil / Q000494:pharmacology
D002104:Cadmium; D002217:Carbachol; D012964:Sodium; D014700:Verapamil; D008274:Magnesium; D009543:Nifedipine; D002118:Calcium; D009638:Norepinephrine; D013324:Strontium