Evidence against norepinephrine-stimulated efflux of mitochondrial Mg2+ from intact cardiac myocytes

R. A. Altschuld, D. W. Jung, R. M. Phillips, P. Narayan, L. C. Castillo, T. E. Whitaker, J. Hensley, C. M. Hohl and G. P. Brierley Department of Medical Biochemistry, Ohio State University, Columbus 43210-1218. 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.