Calcium Permeability of Non‐N‐Methyl‐D‐Aspartate Receptor Channels in Immature Cerebellar Purkinje Cells: Studies Using Fura‐2 Microfluorometry

: Using fura‐2 microfluorometry, I investigated the mechanism by which non‐N‐methyl‐d‐aspartate (NMDA) receptor agonists increase the cytosolic free calcium concentration ([Ca]in) in single cerebellar Purkinje cells isolated from 3–10‐day‐old rats. Kainate and α‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate dose‐dependently increased the cytosolic free Na+ concentration, which was measured using sodium‐binding benzofuran isophthalate microfluorometry, confirming the Na+ influx through ion channels linked to non‐NMDA receptors. The [Ca2+] increases induced by relatively lower concentrations of agonists were entirely dependent on external Ca2+ and were reduced by removal of external Na+ or by addition of a Ca2+ channel blocker, D600. The results indicate that the non‐NMDA agonist–induced [Ca]in increase was due mainly to Ca2+ influx through voltage‐dependent Ca2+ channels, which were activated by a massive Na+ influx. On the other hand, higher concentrations of agonists dose‐dependently increased [Ca]in under conditions in which activation of voltage‐dependent Ca2+ channels were blocked by a combination of Na+ removal with D600. These [Ca]in increases were Ca2+ dependent and little affected by adding a competitive NMDA antagonist. Non‐NMDA agonists also stimulated influxes of Mn2+ and Co2+, both of which can be monitored by quenching fura‐2 fluorescence under the same conditions. These results suggest that ion channels linked to non‐NMDA receptors on immature Purkinje cells are permeable to Ca2+, Mn2+, and Co2+.