NSB depends on the extent of ligand depletion and cannot be calculated from apparent NSB. A. Total binding is shown when ligand depletion is negligible with increasing values of α ranging from 0 to 1 (integer log values of α from -7 to 0; legend for A-C in B). α = 0 is line with horizontal plateau at large concentration of [3H]EB. B and C. Similar to A except ligand depletion is substantial (R1T = 0.3 nM) or extreme (R1T = 300 nM). The effects of a particular α value on total binding depend on the extent of ligand depletion. D, E, F. NSB on expanded y-scales shows how increasing ligand depletion affects NSB (legend for α in F; integer log values from -7 to 0). G. Specific binding is not calculated correctly as the difference between total binding and apparent NSB when ligand depletion is significant. Solid black line: apparent NSB obtained without α4β2 nAChR. Lines for total binding at increasing concentrations (nM) of R1T and, therefore, increasing ligand depletion appear above apparent NSB. Corresponding lines for NSB when α = 0.1 are below apparent NSB. H. NSB (same code for lines as in G) and apparent NSB (solid line) from G are shown with an expanded y-scale. Binding in this figure was calculated with two sites modelfree.
Person and Wells BMC Biophysics 2011 4:19 doi:10.1186/2046-1682-4-19