Receptor Binding and Kinetics NS 1, Spring 2011 Judson Chandler Office: IOP Rm 469 Ext 2-5224

1. Receptor Binding and Kinetics NS 1, Spring 2011 Judson Chandler Office: IOP Rm 469 Ext 2-5224 chandj@musc.edu

2. Pharmacological parameters define receptors Vast majority present a special problem L R E Biological Response

3. What are the pharmacological properties of receptors? Biological Response Saturability Specificity Reversibility Additional receptors properties---Molecular Specific Binding vs non-specific binding Characterized by??

4. Ligand Receptor Interactions Law of Mass Action K1 K2 L + R LR Effect [L] [R] K2 Kd = = K1 [LR] where, K1 = association rate constant K2 = dissociation rate constant [L] = concentration of free ligand [R] = concentration of free receptor [LR] = concentration of bound receptor Kd = dissociation equilibrium constant

5. Since the total # of Receptors = [Rt] = [R] + [LR] rearrange [R] = [Rt] - [LR] [L] [R] Kd = combine equations [LR] [L] ([Rt] - [LR]) Kd = [LR] solve [L] [Rt] [LR] = [L] + Kd

6. So, since the fraction of receptors occupied (r) is [LR] bound r = = [Rt] total rearrange [LR] = r Rt [L] [Rt] combine [LR] = [L] + Kd [L][Rt] r Rt = [L] + Kd solve [L] r = [L] + Kd

7. [L] r = [L] + Kd Now, if the concentration of the ligand is varied, then a plot of r vs [L] will produce a hyperbola. This is a Langmuir plot

8. Sigmoid dose-response curve Why plot as a log?

9. Rosenthal-Scatchard Equation [L] r = [L] + Kd rearrange Kdr + r[L] = [L] rearrange r -r 1 = + [L] Kd Kd Scatchard plot

10. What if scatchard plot exhibit non-linearity? E bound or log log Emax - E Rt - bound where E = effect Hill plot

11. Before proceding, let’s define some terms: Potency = dependency of a drugs effect on it’s [ ] what is its relationship to binding affinity? Efficacy = degree of a response (i.e. maximal effect) also called Intrinsic Activity what is its relationship to binding affinity Full agonist = a drug that produces a maximal effect Partial agonist = a drug that does not ellicit an full response (even though all receptors are bound) Antagonist = a drug that has no efficacy

12. Competitive antagonist antagonism of the agonist response can be overcome by increasing the concentration of the agonist Non-competitive antagonist increasing agonist [ ] has no effect Effects on dose-response curves

13. If an agonist and an antagonist both bind to a receptor on the same site, how can one produce an effect and the other have no effect? Consider a receptor exist in two conformations: active (a) and inactive (i) Ri Ra DRi DRa Importance of affinity of a drug for the two conformations effect of a Full agonist vs a Partial agonist What if the drug has equal affinity for Ri and Ra? What if the drug has a preferrential affinity for Ri?

14. Let’s see how this would look on a plot

15. Recall the occupation theory of drug-receptor action: Response to a drug is directly proportional to the fraction of receptors occuppied. e.g. EC50 = Kd But …… what if EC50 is lower than the Kd Concept of Spare Receptors What can cause this? What if EC50 > Kd?

16. Concept of receptor desensitization subsequent response is decreased following exposure to an agonist Desensitization vs Down-regulation Homologous vs Heterologous desensitization R1 G E second messenger R2

17. Lastly…. Effect of GTP on displacement of binding by an agonist vs antagonist Propranolol = Beta receptor antagonist Isoproterenol = Beta receptor agonist 125I-IPIN = iodinated Beta receptor antagonist