Modulation of a voltage-gated calcium channel linked to activation of glutamate receptors and calcium-induced calcium release in the catfish retina

1. Catfish (Ictalurus punctatus) retinal cone horizontal cells contain an L -type calcium current that has been proposed to be involved in visual proce ssing. Here we report, on the modulation of this current by activation of g lutamate receptors and calcium-induced calcium release (CICR) from intracel lular calcium stores. 2. Fluorescence data obtained from isolated horizontal cells loaded with in do-1 provided evidence of calcium release from an intracellular calcium sto re sensitive to caffeine, calcium and ryanodine. In the presence of caffein e, ryanodine-sensitive stores released calcium in a transient manner. Relea se of calcium was blocked when cells were preincubated in BAPTA, in the pre sence of ruthenium red, or in low concentrations of ryanodine. 3. The release of calcium from ryanodine-sensitive stores directly correspo nded with a decrease of the voltage-gated L-type calcium current amplitude. Caffeine-induced modulation of the calcium current was reduced in the pres ence of ruthenium red. 4. Activation of ionotropic kainate receptors on catfish cone horizontal ce lls triggered CICR from ryanodine-sensitive stores and mimicked inhibition of the voltage-gated calcium current. Kainate-induced inhibition of the cal cium current was diminished when intracellular calcium stores were inhibite d with ruthenium red or depleted with ryanodine, or when calmodulin antagon ists or CaM kinase II inhibitors were present. 5. These results provide evidence that activation of an ionotropic glutamat e receptor on catfish cone horizontal cel Is is linked to calcium release f rom ryanodine-sensitive intracellular calcium stores and modulation of the L-type calcium current activity. Inhibition of this calcium current directl y or indirectly involves calmodulin and CaM kinase II and represents a poss ible mechanism used by horizontal cells to affect response properties of th ese cells.