The effects of mutations on the concentration-effect relationship for potentiation.

<p>Data are shown for mutations of the tryptophan residue (<b>W</b>LAGMI; panels A and B) and the methionine residue (WLAG<b>M</b>I; panel C). ACh was applied at a concentration producing a response that produced a response of about 5 to 7% of maximal in the absence of steroid. The α subunit was expressed with β4 at an injection ratio of 20:1. The concentration-response relationship for potentiation was characterized by fitting a 3 parameter Hill equation to the ratios (Z[drug] = 1 + Zmax (1 / (1 + (EC₅₀/[drug])^nHill), where Z is the response to a concentration of ACh in the presence of drug relative to that in the absence, Zmax is the maximal effect, EC₅₀ is the concentration producing half-maximal potentiation, and nHill is the Hill coefficient. The fits did not include points that appeared to demonstrate block (e.g. data with 30 μM 17α-vinylestradiol). The data from each cell were fit separately, giving the following mean parameter estimates for βEST: α4: Zmax = 1.12 ± 0.12 (11 cells) and EC₅₀ = 4.92 ± 0.48 μM; for α4(F): 0.41 ± 0.06 (3) P = 0.0002 and 10.43 ± 2.00 μM P = 0.4; for α4(G): 0.56 ± 0.03 (4) P = 0.001 and 4.90 ± 0.38 μM P = 0.98; and for α4(MtoF): 1.00 ± 0.03 (8) P = 0.34 and 11.76 ± 0.39 μM P < 0.0001, with values given as mean ± SE (number of cells) and significance of difference to α4 by t-test. The best-fitting estimates for 17α-vinylestradiol are: α4: 1.93 ± 0.06 (3) and 3.84 ± 0.27 μM and α4(F): 1.41 ± 0.08 (4) P = 0.004 and 7.32 ± 0.68 μM P = 0.010. The points show mean ± SE while the lines show the relationship predicted by the mean fit parameters.