Journal of Neuroscience, Vol 12, 2156-2164, Copyright © 1992 by Society for Neuroscience
Excitatory amino acid regulation of intracellular Ca2+ in isolated catfish cone horizontal cells measured under voltage- and concentration- clamp conditions
CP Linn and BN Christensen
Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77550.
[Ca2+]i was measured using fura-2-loaded isolated catfish horizontal cells
in the presence of L-glutamate and the glutamate analogs kainate (KA),
quisqualate (QA), and NMDA. Caffeine was used to release Ca2+ from
intracellular stores. Cell membrane potential was controlled with a voltage
clamp to prevent activation of voltage-dependent Ca2+ channels in the
presence of agonist. All excitatory amino acid agonists produced a rapid
and sustained rise in [Ca2+]i with the order of potency being QA greater
than Glu greater than KA greater than NMDA. The agonist-induced [Ca2+]i
increase was blocked in reduced [Ca2+]o and by
6-cyano-7-nitroquinoxaline-2,3-dione and 2-amino-5- phosphonopentanoate,
which are specific blockers for QA/KA and NMDA receptors, respectively. The
metabotropic receptor agonist trans-1- amino-1,3-cyclopentanedicarboxylic
acid (ACPD; 10-200 microM) had no effect on [Ca2+]i. Hill coefficients from
curves fitted to concentration-response data suggested an amplification of
the Ca2+ signal that was interpreted as calcium-induced calcium release
(CICR) from intracellular Ca2+ stores. Caffeine (10 mM) produced a rapid
transient rise in [Ca2+]i, confirming the existence of a Ca(2+)- sensitive
store. Following caffeine-induced depletion of Ca2+ from intracellular
stores, agonists were still able to produce increases in [Ca2+]i,
confirming Ca2+ influx through the agonist-gated channel. The
agonist-induced increase in [Ca2+]i was decreased following caffeine-
induced depletion, confirming a process of CICR. These results are
consistent with the hypothesis that excitatory amino acids can produce
direct modulation of [Ca2+]i by influx through the agonist-gated channel
and by CICR from intracellular stores.
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