A Role for Divalent Cations in the Uptake of Noradrenaline by Synaptosomes

Abstract –The effects of divalent cations on the initial rates of noradrenaline uptake by synaptosomes were determined using Millipore filtration to terminate the reaction. The removal of either Ca2+ or Mg2+ from the incubation medium had no effect on uptake, but when both Ca2+ and Mg2+ were removed, uptake was reduced. Uptake was also diminished when Ca2+ was absent and 1 mm‐EGTA added to the medium. It appeared that Ca2+ was required for optimal uptake but that Mg2+ could partially substitute for Ca2+ in this regard. The reduction in the rate of uptake when both Ca2+0 and Mg2+ were absent could be rapidly and completely reversed by restoring Ca2+, Mg2+, or both Ca2+ and Mg2+ to the incubation medium. Of the divalent cations tested, Ca2+ and Mg2+, but not Mn2+, supported noradrenaline uptake. When the kinetics of uptake were examined, it was found that removing both Ca2+ and Mg2+ from the medium resulted in a reduction of the Vmax for noradrenaline uptake. It is apparent from these results that, in addition to facilitating the release of noradrenaline from noradrenergic terminals, Ca2+ may also play a role in the uptake of noradrenaline by presynaptic nerve‐endings in the CNS.