SINGLE-CHANNEL PROPERTIES OF CLONED NMDA RECEPTORS IN A HUMAN CELL-LINE - COMPARISON WITH RESULTS FROM XENOPUS-OOCYTES

1. Human embryonic kidney (HEK) 293 cells were transiently transfected with cDNAs encoding the NE1a-NMDA epsilon 1[NR2A] subunit combination of the NMDA receptor. Single channel behaviour was recorded from outside-out membrane patches, with the aim of comparing the results with those, recorded under the same conditions, from Xenopus oocytes injected with messenger RNA coding for the NR1a-NR2A combination. 2. Single channels in HEK 293 cells showed a main conductance level of 51.4+/-2.4 pS, compared with 50.1+/-1.4 pS for channels in oocytes. A subconductance level of 38.1+/-2.1 pS was found in HEK 293 cells, compared with 38.3+/-1.3 pS in oocytes. The frequencies of transitions between the shut and the two conductance levels were also very similar. 3. Distributions of shut times could be fitted with five exponential components. In HEK 293 cells the first three of these components had time constants of 39+/-4 mu s, 0.54+/-0.04 ms and 9.94+/-1.3 ms; in oocytes the values were 69+/-35 mu s, 0.54+/-0.15 ms and 6.53+/-4.6 ms, respectively. The relative areas of the components were also similar in the two systems. 4. The distribution of all apparent open times for the sublevels was fitted with two exponential components giving time constants of 0.18+/-0.02 ms and 1.31+/-0.17 ms (for HEK cells) or of 0.31+/-0.38 ms and 1.31+/-1.1 ms (for oocytes). The apparent open time distribution of the main level alone was fitted, in both cases, with one component, the time constants being 2.23+/-0.06 ms (HEK cells) and 2.2+/-0.33 ms (oocytes). 5. The measured values were very similar for the two different expression systems, and also to some native NMDA receptors. We therefore conclude that, at least as far as these properties are concerned, the NR1-NR2A subunit combination seems to behave in the same way no matter which of the expression systems is used.