Evidence against norepinephrine-stimulated efflux of mitochondrial Mg2+ from intact cardiac myocytes.
We investigated the hypotheses that norepinephrine stimulates Mg2+ efflux from intact isolated adult rat ventricular cardiomyocytes and that adenosine 3',5'-cyclic monophosphate stimulates Mg2+ efflux from permeabilized myocytes and isolated mitochondria. Norepinephrine stimulation of Mg2+ release from cardiac myocytes was observed only when cells at approximately 20 mg protein/ml in Mg(2+)-containing buffer were diluted 50- to 60-fold into an Mg(2+)-free medium. Under these conditions, > 30% of total cellular lactic acid dehydrogenase activity was also released, indicating that a significant portion of the cells had died. In other protocols, where Mg2+ efflux from myocytes was not observed, extracellular Mg2+ removal and administration of 10 microM norepinephrine increased 45Ca2+ accumulation by cells in suspension. In single myocytes, Mg2+ removal and norepinephrine administration increased intracellular free [Ca2+] as measured by fura-2 fluorescence microscopy, and this was accompanied by vigorous spontaneous contractile activity followed by Ca2+ overload hypercontracture. With permeabilized myocytes and isolated mitochondria from a variety of sources, adenosine 3',5'-cyclic monophosphate did not stimulate Mg2+ efflux. These results suggest that recent evidence for direct hormonal regulation of myocardial Mg2+ homeostasis may need to be reevaluated.
Altschuld
R A
RA
Department of Medical Biochemistry, Ohio State University, Columbus 43210-1218.
Jung
D W
DW
Phillips
R M
RM
Narayan
P
P
Castillo
L C
LC
Whitaker
T E
TE
Hensley
J
J
Hohl
C M
CM
Brierley
G P
GP
D000818:Animals; D002021:Buffers; D002118:Calcium / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D004072:Digitonin / Q000494:pharmacology; D007770:L-Lactate Dehydrogenase / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism*; D008297:Male; D008928:Mitochondria / Q000378:metabolism*; D009200:Myocardial Contraction; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D002021:Buffers; D002217:Carbachol; D000242:Cyclic AMP; D007770:L-Lactate Dehydrogenase; D008274:Magnesium; D004072:Digitonin; D002118:Calcium; D009638:Norepinephrine