Figure 4.

Effect of OCoA (10 μM) on [³H]GBC binding to SUR1 in the absence of MgATP. Specific binding (B_S), from two individual experiments, is shown as % B_MAX. The fit of B_S to the Law of Mass action (equation (3) in Methods) gave K_D values of 0.36 (0.32, 0.41)/1.5 (1.2,1.8) nM and B_Max values of 100 and 83±3% in the absence and presence of OCoA, respectively. 100% B_MAX was 3.6 pmol (mg protein)⁻¹. Nonspecific binding increased linearly with [³H]GBC concentration and was 20% of total binding at 7.5 nM [³H]GBC. Inset: Scatchard representation of the data (F denotes the free radioligand concentration, [³H]GBC_free). The control curve was fitted to a straight line with abscissa intercept 100 and gave a slope of −2.7±0.1 nM⁻¹; the curve in the presence of OCoA gave a slope of −0.59±0.05 nM⁻¹ and an abscissa intercept of 88±8%, which is slightly but significantly different from 100%.