AJP - Cell Physiology, Vol 252, Issue 6 C604-C610, Copyright © 1987 by American Physiological Society
Strontium, barium, and manganese metabolism in isolated presynaptic nerve terminals
H. Rasgado-Flores, S. Sanchez-Armass, M. P. Blaustein and D. A. Nachshen
To gain insight into the mechanisms by which the divalent cations Sr, Ba,
and Mn affect neurotransmitter release from presynaptic nerve terminals, we
examined the sequestration of these cations, in comparison to Ca, by
mitochondrial and nonmitochondrial [presumably smooth endoplasmic reticulum
(SER)] organelles and the extrusion of these cations from isolated nerve
terminals. Sequestration was studied in synaptosomes made leaky to small
ions by treatment with saponin; efflux was examined in intact synaptosomes
that were preloaded with the divalent cations by incubation in depolarizing
(K rich) media. The selectivity sequence for ATP-dependent mitochondrial
uptake that we observed was Mn much greater than Ca greater than Sr much
greater than Ba, whereas that for the SER was Ca greater than or equal to
Mn greater than Sr much greater than Ba. When synaptosomes that were
preloaded with divalent cations were incubated in Na- and Ca-free media,
there was little efflux of Ca, Ba, Sr, or Mn. When the incubation was
carried out in media containing Na without Ca, there was substantial
stimulation of Ca and Sr efflux, but only slight stimulation of Ba or Mn
efflux. In Na-free media, the addition of 1 mM Ca promoted the efflux of
all four divalent cations, probably via Ca-divalent cation exchange. In
summary, the sequestration and extrusion data suggest that, with equal
loads, Mn will be buffered to the greatest extent, whereas Ba will be least
well buffered. These results may help to explain why Mn has a very
long-lasting effect on transmitter release, while the effect of Sr is much
briefer.
