THE SODIUM-CALCIUM ANTIPORT OF HEART-MITOCHONDRIA IS NOT ELECTRONEUTRAL

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.