Synaptosomal [Ca2+]i as influenced by Na+/Ca2+ exchange and K+ depolarization.
The modulation of the intrasynaptosomal concentration of Ca2+, [Ca2+]i, by Na+/Ca2+ exchange was studied using Indo-1 fluorescence. The electrochemical gradient of Na+ was manipulated by substituting Li+ or choline for Na+ in the external medium and, then, the influx of 45Ca2+ and the [Ca2+]i were measured. It was found that the increase in [Ca2+]i induced by K+ depolarization is lower if the value of [Ca2+]i has been previously raised by Na+/Ca2+ exchange, suggesting that Ca2+ entering by Na+/Ca2+ exchange reduces the Ca2+ entering by voltage-dependent calcium channels. Our results show that a value of [Ca2+]i of about 650 nM induced by Na+/Ca2+ exchange reduces by 50% the Ca2+ entering due to K+ depolarization and no Ca2+ enters through the channels if the [Ca2+]i is previously raised above about 800 nM. Furthermore, predepolarization of the synaptosomes in a Ca-free medium also inhibits by at least 40% the [Ca2+]i rise through Ca2+ channels. Thus, the results suggest that both predepolarization and [Ca2+]i rise due to Na+/Ca2+ exchange decrease the Ca2+ entering by voltage-sensitive Ca2+ channels. The Ca2+ entering by Na+/Ca2+ exchange might contribute to the regulation of neurotransmitter release. Our results also show that the presence of Li+ in the external medium decreases the buffering capacity of synaptosomes, probably by releasing Ca2+ from mitochondria by Li+/Ca2+ exchange.
Duarte
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CB
Department of Zoology, University of Coimbra, Portugal.
Carvalho
C A
CA
Ferreira
I L
IL
Carvalho
A P
AP
D000818:Animals; D002118:Calcium / Q000032:analysis / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism; D007211:Indoles / Q000737:chemistry; D008094:Lithium / Q000378:metabolism; D008564:Membrane Potentials; D008928:Mitochondria / Q000378:metabolism; D011188:Potassium / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D013574:Synaptosomes / Q000737:chemistry / Q000378:metabolism*
D015220:Calcium Channels; D002352:Carrier Proteins; D007211:Indoles; D019831:Sodium-Calcium Exchanger; D008094:Lithium; D012964:Sodium; C048960:indo-1; D011188:Potassium; D002118:Calcium