Ca2+-dependent regulation of presynaptic stimulus-secretion coupling.
In the present study, we have investigated the role of Ca2+ in the coupling of membrane depolarization to neurotransmitter secretion. We have measured (a) intracellular free Ca2+ concentration ([Ca2+]i) changes, (b) rapid 45Ca2+ uptake, and (c) Ca2+-dependent and -independent release of endogenous glutamate (Glu) and gamma-aminobutyric acid (GABA) as a function of stimulus intensity by elevating the extracellular [K+] to different levels in purified nerve terminals (synaptosomes) from rat hippocampus. During stimulation, Percoll-purified synaptosomes show an increased 45Ca2+ uptake, an elevated [Ca2+]i, and a Ca2+-dependent as well as a Ca2+-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 Ca2+-dependent (exocytotic) release. Ca2+-dependent transmitter release as well as [Ca2+]i elevations show maximal stimulation at moderate depolarizations (30 mM K+). A correlation exists between Ca2+-dependent release of both Glu and GABA and elevation of [Ca2+]i. Ca2+-dependent release is maximally stimulated with an elevation of [Ca2+]i of 60% above steady-state levels, corresponding with an intracellular concentration of approximately 400 nM, whereas elevations to 350 nM are ineffective in stimulating Ca2+-dependent release of both Glu and GABA. In contrast, Ca2+-independent release of both Glu and GABA shows roughly a linear rise with stimulus intensity up to 50 mM K+. 45Ca2+ 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 mM K+.(ABSTRACT TRUNCATED AT 250 WORDS)
Verhage
M
M
Department of Experimental Zoology, University of Amsterdam, The Netherlands.
Besselsen
E
E
Lopes da Silva
F H
FH
Ghijsen
W E
WE
D000818:Animals; D001572:Benzofurans; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002132:Calcium Radioisotopes; D004987:Ethers / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000031:analogs & derivatives*; D005971:Glutamates / Q000378:metabolism*; D018698:Glutamic Acid; D006624:Hippocampus / Q000502:physiology*; D015759:Ionomycin; D007700:Kinetics; D008297:Male; D010477:Perfusion; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013574:Synaptosomes / Q000187:drug effects / Q000502:physiology*; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
D001572:Benzofurans; D002132:Calcium Radioisotopes; D004987:Ethers; D005456:Fluorescent Dyes; D005971:Glutamates; C049925:fura-2-am; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D015759:Ionomycin; D011188:Potassium; D002118:Calcium; D016257:Fura-2