Homomeric and heteromeric ion channels formed from the kainate-type subunits GluR6 and KA2 have very small, but different, unitary conductances. Howe, James R. Department of Pharmacology, Yale University School of Medicine, 333 Cedar Street, New Haven, CT 06520-8066, USA.
APStracts 3:0041N, 1996.
SUMMARY AND CONCLUSIONS
1.) Patch-clamp methods were used to study recombinant glutamate-receptor (GluR) ion channels expressed in human embryonic kidney cells (HEK 293) after transfection of the cells with cDNAs encoding kainate-type GluR subunits. Cells were transiently or stably transfected with either the fully edited (R) version of GluR6 or they were co-transfected with GluR6(R) and KA2. 2.) Concentration-response data were obtained for kainate and glutamate activation of homomeric GluR6(R) channels and fitted with Hill-type equations to give values for the agonist concentration at half-maximal activation ( EC 50 ), the Hill coefficient ( n H ), and the maximum current ( I max ). Analysis of results obtained in 7 cells with kainate gave mean values of 0.47 [mu] M and 1.47 for the EC 50 and n H , respectively. The corresponding values for glutamate were 32 [mu] M and 1.21 ( n = 5). The I max values obtained for kainate and glutamate in the same cells were similar, suggesting that both kainate and glutamate are full agonists at homomeric GluR6(R) channels. 3.) Spectral density analysis of current noise evoked by kainate and glutamate in GluR6(R)- expressing cells, or in outside-out patches from these cells, was used to obtain estimates of the apparent unitary conductance ( [gamma] noise ) of homomeric GluR6(R) channels. Results obtained with 10 [mu] M kainate in 15 cells gave a mean [gamma] noise value of 231 fS. The corresponding value from analysis of noise in outside-out patches was 259 fS ( n ? = 4). Spectral density analysis of glutamate-evoked noise in 6 cells gave a mean [gamma] noise value of 264 fS. 4.) Noise analysis of whole-cell currents recorded in GluR6(R)/KA2-expressing cells gave a mean [gamma] noise value of 572 +/- 40 fS ( n = 9). Unlike homomeric GluR6(R) channels, GluR6(R)/KA2 heteromers were activated by AMPA (0.25 to 5 mM). The mean EC 50 for kainate activation of GluR6(R)/KA2 channels was 1.62 [mu] M ( n = 6). 5.) The results indicate that both homomeric GluR6(R) and heteromeric GluR6(R)/KA2 channels have a unitary conductance in the femtosiemens range. The co-assembly of KA2 with GluR6(R) results in channels that have a different affinity for AMPA and kainate and a 2- to 3-fold larger unitary conductance.

Received 19 September 1995; accepted in final form 5 February 1996.
APS Manuscript Number J627-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 14 February 96