Dissociation between the Joro spider toxin sensitivity of recombinant a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors and their ability to increase intracellular calcium

We compared the toxin sensitivity, Ca²⁺ flux response and rectification properties of recombinant α-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptors obtained by transfecting human embryonic kidney (HEK) 293 cells with different ratios of GluR1 and GluR2 cDNAs (10:1 to 1:10). Simultaneous measurements of kainate-activated Ca²⁺ fluxes and inward currents, using fura-2 microfluorimetry under voltage clamp conditions, suggested the existence of GluR2 containing channels which are permeable to Ca²⁺ and insensitive to Joro spider toxin (JSTx). Imaging experiments showed that JSTx inhibition of the Ca²⁺ response induced by kainate was reduced by increasing the relative amount of GluR2. However, even at GluR1/GluR2(R) ratios of 1:1 and 1:4, cells were still able to flux Ca²⁺ when stimulated by kainate. GluR2 similarly inhibited the ability of JSTx to reduce kainate-evoked inward currents in whole cell patch-clamp experiments. Variations in the rectification properties of the AMPA currents, induced by changes in the cDNA ratio, were not always correlated with the changes in toxin sensitivity and [Ca²⁺](i) response. Thus, cells with almost linear I-V relationships were partially blocked by JSTx and still Ca²⁺ permeable. Our results indicate a dissociation between the toxin sensitivity and Ca²⁺ flux through GluR2 containing AMPA receptors and suggest that receptors with diverse Ca²⁺ permeabilities are generated by the expression of variable amounts of GluR2. Copyright (C) 1998 Elsevier Science Ltd.