How can I keep HEK cells alive while expressing NMDA receptors?

I am trying to express functional NMDA receptors in HEK293 line cells for single channel recording experiments.

The HEK cells are maintained in the standard way (Thomas & Smart 2005) and transfected with NR1 and NR2x subunit cDNAs and also GFP, using either lipofectamine or calcium phosphate precipitation. GFP expression suggests successful transfection, but cells exhibiting green fluorescence are uniformly swollen and dead and it's more or less impossible to obtain a successful patch. Untransfected cells seem to remain perfectly viable.

On the basis that the toxicity might be a consequence of Ca²⁺ influx through open NMDARs, I've tried including a cocktail of blockers in the growth medium, including AP5, kynurenic acid and Mg²⁺, but the transfected cells continue to die.

Can anyone suggest anything else I should be doing to keep the cells alive? Or am I just on a hiding to nothing? Other researchers seem to have managed this (eg Medina et al 1995, Vicini et al 1998) and do not seem to be doing anything substantially different, so I'm a bit at a loss.