Fura-2 signals evoked by kainate in leech glial cells in the presence of different divalent cations.

The glutamate-agonist kainate evokes Ca2+ transients in both neurones and glial cells. Owing to the membrane depolarization elicited by kainate, a Ca2+ influx could occur through voltage-gated Ca2+ channels or through the kainate-gated cation channels directly. We have measured ratio signals of the calcium indicator dye fura-2, injected into giant glial cells of the leech Hirudo medicinalis, as response to kainate (5-20 microM) in the presence of different divalent cations. The responses to kainate increased during the first 2-4 kainate applications, both in unclamped and in voltage-clamped cells. The fura-2 fluorescence ratio (F350/F380) still increased when Ca2+ was replaced by Ba2+ but was suppressed in Ca(2+)-free saline and in the presence of Ni2+ (2 mM). Co2+ and Mn2+ (2 mM) also reduced the kainate-induced fura-2 fluorescence signals, due to entry of these divalent cations into the cells and subsequent quenching the fluorescence of the intracellular dye. It is concluded that Ni2+ blocks the kainate-induced membrane depolarization and Ca2+ transient but apparently does not enter the cells, while Ba2+, Co2+, and Mn2+ appear to permeate the membrane, presumably through the kainate-gated channels.