Competitive displacement curve

As noted previously, in all cases these various functions describe an inverse sigmoidal curve between the displacing ligand and the signal. Therefore, the mechanism of interaction cannot be determined from a single displacement curve. However, observation of a pattern of such curves obtained at different tracer ligand concentrations (range of [A ] values) may indicate whether the displacements are due to a competitive, noncompetitive, or allosteric mechanism. 

Competitive displacement for a range of [A ] values (Equation 4.8) yields the pattern of curves shown in Figure 4.6a. A useful way to quantify the displacement is to determine the concentration of displacing ligand that produces a diminution of the signal to 50% of the... 

Competitive, noncompetitive, and allosteric antagonism can be discerned from the pattern of multiple displacement curves. 

Aim To measure the affinity of a ligand by observing the inhibition it produces of a receptor-bound radioligand (or ligand that is traceable by other means). The object is to obtain an estimate of the equilibrium dissociation constant of the nonradioactive ligand receptor complex (alternately denoted KB or Kj. The pattern of displacement curves can also be used to determine whether or not the antagonism is competitive. 

Competition binding studies showing that when using compounds like jS-CCE (ethyl- S-carboline-3-carboxylate), which bind to the benzodiazepine receptor, the displacement curve for [ H]flunitrazepam was shallow in the hippocampus and... 

Competition curves for displacement of a radiolabelled ligand by a competitive inhibitor, (a) Typical displacement curves for two inhibitors having IC50 values of 10 nM and 1 xM. (b) Indirect logit-log plots of the data in (c) showing how IC50 values may be estimated (see O Section 2.5)... 

Rabbits were immunized with a conjugate of racemic cydazocine (Fig. 1 [la]) and competitive binding studies carried out with tritium-labeled d, i-cyclazocine (23). Various types of antibody mixtures were obtained from different animals. One animal immunized with the compound in the form [Ic] shown in Fig. 1 exhibited almost no affinity for the d isomer of cydazocine. In such an instance one would expect that the displacement curve for the d,l mixture would be displaced to higher concentrations than the displacement curve for the / isomer, and this indeed was observed. If the antibodies produced were completely nonenantioselective, then displacement curves for d, I, and the d,l mixture should be identical. This case was not observed. 

Fig. 3.3 Analysis of ligand-receptor interactions. (A) Scatchard plot in absence (control) or presence of two different displacers. X and Y are competitive and non-competitive displacers, respectively. (B) Displacement curve.

Studies reported by Peroutka in 1991 [12], carried out in bovine, porcine, guinea pig and humem caudate and cortex, also resulted in complex 5-CT displacement curves as well as monophasic sumatriptan competition curves which fail to fully displace the [ H]5-HT. This sumatriptan insensitive component. 

If the antagonism is insurmountable, then there are a number of molecular mechanisms possible. The next question to ask is if the maximal response to the agonist can be completely depressed to basal levels. If this is not the case, then there could be partial allosteric alteration of the signaling properties of the receptor. Alternatively, this could be due to a hemi-equilibrium condition that produces a partial shortfall to true competitive equilibrium, leading to incomplete depression of the maximal response but also antagonist concentration-related dextral displacement of the concentration response curve to the agonist (see Figure 10.19a). The model (see Section 10.6.5) used to fit these data is discussed in Section 6.5 and shown in... 

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