REGULATION OF CA-2+ TRANSPORT IN BRAIN MITOCHONDRIA .1. THE MECHANISM OF SPERMINE ENHANCEMENT OF CA-2+ UPTAKE AND RETENTION

Spermine enhances electrogenic Ca2+ uptake and inhibits Na+-independent Ca2+ efflux in rat brain mitochondria. As a result, Ca2+ retention by brain mitochondria increases greatly and the external free Ca2+ level at steady-state can be lowered to physiologically relevant concentrations. The stimulation of Ca2+ uptake by spermine is more pronounced at low concentrations of Ca2+, effectively lowering the apparent Km for Ca2+ uptake from 3 μM to 1.5 μM. However, the apparent Vmax is also increased. At low Ca2+ concentrations, Ca2+ uptake is diffusion-limited. Spermine strongly inhibits Ca2+ binding to anionic phospholipids and it is suggested that this increases the rate of surface diffusion which reduces the apparent Km for uptake. The same effect could inhibit the Na+-independent efflux if the rate of efflux is limited by Ca2+ dissociation from the efflux carrier. In brain mitochondria (but not in liver) the spermine effect depends on the presence of ADP. In a medium that contains physiological concentrations of Pi, Mg+, K+, ADP and spermine, brain mitochondria sequester Ca2+ down to 0.1 μM and below, depending on the matrix Ca2+ load. Moreover, brain mitochondria under the same conditions buffer the external medium at 0.4 μM, a concentration at which the set point becomes independent of the matrix Ca2+ content. Thus, mitochondria appear to be capable of modulating calcium oscillations in brain cells. © 1990.