Heart mitochondria contain a nNa+/Ca2+ antiport that participates in the regulation of matrix [Ca2+]. Based largely on a single study (Brand, M. D. (1985) Biochem. J. 229, 161-166), there has been a consensus that this antiport promotes the electroneutral exchange of two Na+ for one Ca2+. However, a recent study in our laboratory (Baysal, K., Jung, D. W., Gunter, K. K., Gunter, T. P., and Brierley, G. P. (1994) Am. J. Physiol. 266, C800-C808) has shown that the Na(+)-dependent efflux of Ca2+ from heart mitochondria has more energy available to it than can be supplied by a passive 2Na+/Ca2+ exchange. We have therefore re-examined Brand's protocols using fluorescent probes to monitor matrix pH and free [Ca2+]. Respiring heart mitochondria, suspended in KCl and treated with ruthenium red to block Ca2+ influx, extrude Ca2+ and establish a large [Ca2+]out:[Ca2+]matrix gradient. The extrusion of Ca2+ under these conditions is Na(+)-dependent and diltiazem-sensitive and can be attributed to the nNa+/Ca2+ antiport. Addition of nigericin increases the membrane potential (delta psi) and decreases delta pH to 0.1 or less, but has virtually no effect on the magnitude of the [Ca2+] gradient. Under these conditions a gradient maintained by electroneutral 2Na+/Ca2+ antiport should be abolished because the mitochondrial Na+/H+ antiport keeps the [Na+] gradient equivalent to the [H+] gradient. The [Ca2+] gradient is abolished, however, when an uncoupler is added to dissipate delta psi or when the exogenous electroneutral antiport BrA23187 is added. In addition, [Ca2+] influx via the nNa+/Ca2+ antiport in nonrespiring mitochondria is enhanced when delta psi is abolished. These results are consistent with Ca2+ extrusion by an electrophoretic antiport that can respond to delta psi but not with an electroneutral antiport.

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Exchangeable intracellular Ca2+ as measured by 45Ca2+ uptake more than doubled when isolated adult rat ventricular cardiomyocytes were incubated 30 min with 8 microM cyclosporin; nevertheless the cells retained a normal rod-shaped morphology. High concentrations of ouabain caused a similar increase in 45Ca2+ uptake, but in this case the Ca2+ overload caused nearly all cells to hypercontract into a round disorganized form. The response to cyclosporin was concentration dependent with an apparent half-maximal effective concentration of 0.5 microM for enhancement of net 45Ca2+ accumulation. Verapamil (1 microM) could not inhibit this cyclosporin effect, but it was abolished by a 5-min preincubation with 12 microM crude ruthenium red. Cyclosporin also decreased the rate of 45Ca2+ efflux from prelabeled myocytes into Ca(2+)-containing and Ca(2+)-free media. These data are consistent with inhibition of mitochondrial 45Ca2+ efflux through the cyclosporin-sensitive mitochondrial inner membrane pore. It would appear that periodic transient increases in mitochondrial inner membrane permeability provide a pathway for mitochondrial Ca2+ extrusion under relatively normal conditions in isolated adult rat heart cells.

The metabolism and intracellular compartmentalization of adenine nucleotides in a preparation of adult rat heart myocytes showing good morphology, viability, and tolerance to calcium ion has been examined by high performance liquid chromatography. These myocytes contain an average of 23 nmol adenine nucleotide per milligram protein which is about 60% of the adenine nucleotide content of intact rat heart tissue. The loss of adenine nucleotide occurs during the incubation and washing steps that increase the yield of viable cells, rather than during the collagenase perfusion. An analysis of cellular compartments shows that the adenine nucleotide of the cell consists of 17 nmol adenine nucleotide in the cytosol, 5 nmol in the mitochondria, and 1.3 nmol adenosine diphosphate bound to myofibrils per milligram cell protein. Myocytes lose both adenosine triphosphate and adenine nucleotide when incubated anaerobically in the absence of glucose, and the lost adenine nucleotide can be accounted for as increased inosine, adenosine, and inosine monophosphate. Myocytes that contain less than 0.1 nmol of cytosol adenosine triphosphate per milligram cell protein maintain an intact sarcolemma, but are unable to carry out anaerobic glycolysis. Reoxygenation of anaerobic cells results in restoration of energy charge and a net resynthesis of about 2 nmol adenine nucleotide per milligram protein. Adenosine and inosine monophosphate decrease on reaeration of anaerobic cells, whereas inosine levels increase. When iodoacetate is added to block glycolysis, the decline in adenine nucleotide and production of inosine monophosphate are accelerated and there is no resynthesis of adenine nucleotide when anaerobic cells are reoxygenated . Large accumulations of inosine monophosphate are also seen in myocytes treated with an uncoupler of oxidative phosphorylation.

Net Ca2+ flux across the inner membrane of respiring heart mitochondria was evaluated under conditions in which virtually all Ca2+ movement can be attributed to the Na+/Ca2+ antiport. If this antiport promotes a passive electroneutral exchange of Ca2+ for 2Na+, the Ca2+ gradient should be equal to the square of the Na+ gradient at equilibrium. Because the mitochondrial Na+/H+ antiport equilibrates the Na+ and H+ gradients, the Ca2+ gradient should also equal the square of the H+ gradient. In a series of > 20 determinations at different matrix [Ca2+], different delta pH, and varying membrane potential, it was found that Ca2+ is transported out of the mitochondrion against gradients from 15- to 100-fold greater than the value predicted for passive electroneutral exchange. It is concluded that the observed gradients are too large to be sustained by passive Ca2+/2Na+ exchange. The observed gradients are compatible with an electrogenic Ca2+/3Na+ exchange. Alternatively another source of energy is available to support these gradients.