Uptake of dopamine into fractions of pig caudate nucleus homogenates

Summary

Homogenates of the caudate nucleus of the pig were submitted to differential centrifugation. The 20 000 g and 80 000 g fractions were isolated and a part of them was osmotically shocked. The highest dopamine content per mg protein was found in the intact 80 000 g fraction. Incubation experiments with the intact and the osmotically shocked fractions at 25°C revealed that the particles of the intact 20 000 g fraction took up dopamine; the influx of the amine was not enhanced by addition of ATP and magnesium to the incubation medium. On the other hand after osmotic shock the uptake of dopamine into the particles of this fraction was greatly enhanced by addition of ATP and magnesium. The uptake of dopamine into the particles of both intact and osmotically shocked 80 000 g fractions was likewise enhanced by ATP and magnesium. The uptake in all fractions was not influenced by ouabain. The influx of dopamine into the particles of the intact 80 000 g fraction was competitively inhibited by reserpine (K _i 0.96×10⁻⁸ M) and prenylamine (K _i 1.74×10⁻⁸ M). It is concluded that the intact 20,000 g fraction contains intact synaptosomes; the uptake of dopamine is independent of the presence of ATP and magnesium. The shocked 20 000 g fraction and the 80 000 g fractions contain synaptic vesicles; the uptake of dopamine into these vesicles is enhanced by ATP and magnesium.