Na+ entry via glutamate transporter activates the reverse Na+/Ca2+ exchange and triggers Ca(i)2+-induced Ca2+ release in rat cerebellar Type-1 astrocytes.
We have previously demonstrated that rat cerebellar Type-1 astrocytes express a very active genistein sensitive Na(+)/Ca(2+) exchanger, which accounts for most of the total plasma membrane Ca(2+) fluxes and for the clearance of loads induced by physiological agonists. In this work, we have explored the mechanism by which the reverse Na(+)/Ca(2+) exchange is involved in agonist-induced Ca(2+) signaling in rat cerebellar astrocytes. Microspectrofluorometric measurements of Cai(2+) with Fluo-3 demonstrate that the Cai(2+) signals associated long (> 20 s) periods of reverse operation of the Na(+)/Ca(2+) exchange are amplified by a mechanism compatible with calcium-calcium release, while those associated with short (< 20 s) pulses are not amplified. This was confirmed by pharmacological experiments using ryanodine receptors agonist (4-chloro-m-cresol) and the endoplasmic reticulum ATPase inhibitor (thapsigargin). Confocal microscopy demonstrates a high co-localization of immunofluorescent labeled Na(+)/Ca(2+) exchanger and RyRs. Low (< 50 micromol/L) or high (> 500 micromol/L) concentrations of L-glutamate (L-Glu) or L-aspartate causes a rise in which is completely blocked by the Na(+)/Ca(2+) exchange inhibitors KB-R7943 and SEA0400. The most important novel finding presented in this work is that L-Glu activates the reverse mode of the Na(+)/Ca(2+) exchange by inducing Na(+) entry through the electrogenic Na(+)-Glu-co-transporter and not through the ionophoric L-Glu receptors, as confirmed by pharmacological experiments with specific blockers of the ionophoric L-Glu receptors and the electrogenic Glu transporter.
Rojas
Héctor
H
Laboratorio de Fisiología Celular, Instituto Venezolano de Investigaciones Científicas, Caracas, Venezuela.
Colina
Claudia
C
Ramos
Magaly
M
Benaim
Gustavo
G
Jaffe
Erica H
EH
Caputo
Carlo
C
DiPolo
Reinaldo
R
D027322:Amino Acid Transport System X-AG / Q000502:physiology*; D000818:Animals; D001253:Astrocytes / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D020013:Calcium Signaling; D002412:Cations; D002531:Cerebellum / Q000378:metabolism*; D005455:Fluorescent Antibody Technique; D066298:In Vitro Techniques; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D051381:Rats; D017207:Rats, Sprague-Dawley; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D013050:Spectrometry, Fluorescence
D027322:Amino Acid Transport System X-AG; D002412:Cations; D019837:Ryanodine Receptor Calcium Release Channel; D019831:Sodium-Calcium Exchanger; D012964:Sodium; D002118:Calcium