Allosteric Activation of Brain Mitochondrial Ca2+ Uptake by Spermine and ty Ca2+: Brain Regional Differences

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Abstract
Analysis of the initial rates of 45Ca2+ uptake by rat brain mitochondria in Ca2+-1,2-bis(o-aminophenoxy)ethane-N,N,N'',N"-tetraacetic acid buffers indicated that nontelencephalic mitochondria exhibited both a much less pronounced stimulatory effect of spermine and significantly more hyperbolic kinetics of Ca2+ uptake than telencephalic mitochondria. Nontelencephalic mitochondria were also markedly less susceptible to a Ca2+-induced hysteretic allosteric activation of the Ca2+ uniporter. A new Ca2+ loading procedure, which strikingly illustrates differences in mitochondrial Ca2+ buffering characteristics, is also described. In this procedure, low concentrations of Ca2+ (1, 2, or 5 .mu.M) were repetitively added to mitochondria every 30 s while changes in free Ca2+ concentration were recorded. Spermine induced a marked attenuation of the rise in free Ca2+ level under these conditions. Steady-state rates of Ca2+ uptake were determined by a quantitative analysis of the buffering of repetitive Ca2+ additions, and, again, brain regional differences were qualitatively similar to those observed in the initial rate kinetics; Ca2+ uptake by nontelencephalic mitochondria in the steady state was markedly less responsive to stimulation by spermine and appeared to have a more hyperbolic dependence on Ca2+ in the absence of spermine. These results also suggested that there is a lag time in the activation of the uniporter by Ca2+, in addition to the hysteresis that has previously been observed in the deactivation of the uniporter. In summary, the present results indicate that the Ca2+ uniporter of nontelencephalic mitochondria is markedly less responsive to allosteric activation by both Ca2+ and spermine and that this is related to, but not entirely explained by, the more hyperbolic dependence of uptake on Ca2+ in nontelencephalic mitochondria. It is proposed that mitochondria buffering of intracellular Ca2+ and the modulation of this process by spermine differ in telencephalic and nontelencephalic regions of brain.