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Ca 2+ ‐Dependent Regulation of Presynaptic Stimulus‐Secretion Coupling
Author(s) -
Verhage Matthijs,
Besselsen Elly,
Silvas Fernando H. Lopes,
Ghijsen Wim E. J. M.
Publication year - 1989
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1989.tb07413.x
Subject(s) - stimulation , extracellular , neurotransmitter , depolarization , endocrinology , chemistry , medicine , biophysics , glutamate receptor , synaptosome , biology , biochemistry , membrane , receptor
In the present study, we have investigated the role of Ca 2+ in the coupling of membrane depolarization to neurotransmitter secretion. We have measured (a) intracellular free Ca 2+ concentration ([Ca 2+ J i ) changes, (b) rapid 45 Ca 2+ uptake, and (c) Ca 2+ ‐dependent and ‐independent release of endogenous glutamate (Glu) and γ‐aminobutyric acid (GABA) as a function of stimulus intensity by elevating the extracellular [K + ] to different levels in purified ijierve terminals (synaptosomes) from rat hippocampus. Duriijg stimulation, Percoll‐purined synaptosomes show an increased 45 Ca 2+ uptake, an elevated [Ca 2+ ] i , and a Ca 2+ ‐dependejnt as well as a Ca 2+ ‐independent release of both Glu and GABA. With respect to both amino acids, synaptosomes respond on stimulation essentially in the same way, with maximally a fourfold increase in Ca 2+ ‐dependent (exocytotic) release. Ca 2+ ‐depen‐dent transmitter release as well as [Ca 2+ ]; elevations show maximal stimulation at moderate depolarizations (30 m M K + ). A correlation exists between Ca 2+ ‐dependent release of both Glu and GABA and elevation of [Ca 2+ ] i . C 2+ ‐dependent release is maximally stimulated with an elevation of [Ca 2+ ] I of 60% above steady‐state levels, corresponding with an intracellular concentration of ∼400 n M , whereas elevations to 350 n M are ineffective in stimulating Ca 2+ ‐dependent release of both Glu and GABA. In contrast, Ca 2+ ‐independent release of both Glu and GABA shows roughly a linear rise with stimulus intensity up to 50 m M K + . 45 Ca 2+ uptake on stimulation also shows a continuous increase with stimulus intensity, although the relationship appears to be biphasic, with a plateau between 20 and 40 m M K + . These findings indicate that Ca 2+ ‐dependent, exocytotic transmitter release is not simply enhanced by larger depolarizations of the plasma membrane and that a strong Ca 2+ ‐dependent regulatory mechanism exists in synaptosomes for the trigger of exocytosis, operating at a mean [Ca 2+ ] i of between 350 and 400 n M . Ca 2+ transport and buffering mechanisms possibly involved in this regulation and the role of the membrane potential are discussed.