Fig. 2.

Different sensitivity of three permeation characteristics of AMPA receptors to the relative abundance of GluR2.A, Oocytes isolated from a single frog were microinjected with GluR2 and GluR3 mRNAs at different molar ratios of GluR2 and GluR3. After the oocytes were cultured for 2–3 d, the following measurements were made from each cell:K_D(0)/[polyamine] as fit by the Woodhull equations,P_Ba/P_monovalentcalculated from reversal potential measurements in high Na⁺ and high Ba²⁺ medium, and the percentage block of kainate current at −70 mV elicited by 1 mm spermine. Data are expressed as a function of the GluR2 relative abundance, and the three measurements are scaled to permit comparisons. Each point represents the mean and SEM from four to six oocytes. B, Relation among the degree of block of kainate-evoked current by external spermine at −70 mV, the barium-to-monovalent permeability ratio, and the internal polyamine blocker affinity, as determined by the Woodhull model in oocytes injected with mixtures of GluR2 and 3 mRNAs. The ratio of GluR2 to GluR3 mRNAs was varied from 1:10 to 10:1 to produce receptors with a wide range of internal blocker affinity (n = 77 oocytes). C, Mixtures of GluR3(R612) and GluR3(Q612) were coinjected and a similar analysis performed as in A(n = 64 oocytes total). The dotted line, which represents the expected binomial abundance of receptors assembled with no GluR3(R) subunits, follows the equationf(x_o) = min + (max − min) · (1 − x)⁵, wheref(x_o) is the fraction of receptors without GluR3(R) subunits, as a function ofx = relative abundance of GluR3(R) protein in a functional receptor. Max and min represent the maximum and minimum GluR3(R)-dependent effect and were the only free variables.