Receptors

See also Enzyme Linked Receptors

Receptors are macromolecules with which endogenous or exogenous compounds interact to produce a characteristic biologic effect.

Law Mass Action provides that [L] X [R]/[LR] = K_off/K_on = K_d.  K_d is the concentration of ligand at equilibrium that causes 1/2 maximal saturation of receptors since when half the receptors are occupied by ligand, # free receptors = # bound receptors or [R] = [LR] and K_d = [L].

The law mass action (occupancy theory) provides that E = α E_max[L] / [L] + K_d where E is the effect produced, α is the intrinsic activity of the ligand (from 0 to 1),  and E_max is the maximal attainable effect.

Agonist are ligands (small molecules that bind receptors) that activate a receptor upon binding and are capable of eliciting the maximal attainable effect E_max and have intrinsic activity of 1.0. Partial agonists are ligands that produce only weak activation upon binding to a receptor and have intrinsic activity between 0 and 1. Antagonists are ligands that bind to a receptor but do not activate it and have an intrinsic activity of 0. Although antogonists have no intrinsic activity, they can block receptors from occupancy by either full or partial agonists.

Agonists differ in efficacy which denotes how large the maximum effect of a drug is relative to other drugs. More efficacious drugs are capable of producing greater effects at their maximal effective doses. Agonists also differ in potency which is the ability of a drug to cause a measured biology or functional change. The greater the potency, the less of the drug one needs to use.                         A         L                 N

For example, in  A, drugs L and N are more efficacious than                                 M

drug M which is a partial agonist. But drug L is more potent than                      log [Agonist]

any of the drugs where the Y axis is % of maximal effect.                      

A linear plat displays a rectangular hyperbola as in B.                    B                                   C

A semi log plot reveals a sigmoidal relationship                                                                                                         

between occupancy and response such that in the absence of (+) or       EC₅₀                                               EC₅₀ 

                                                                                                                                                                [Agonist]                                   Log [Agonist]

(-) cooperativity, 10-90% of response occurs over a 100 fold range of agonist concentration center around the EC₅₀ which is the concentration of drug that produces 1/2 of the drug's maximal response. Often there is amplification between occupancy and response such that the EC₅₀ lies

to the left of the K_d for receptor occupancy. This often occurs when the receptor can      D

exist in more than one state which can be altered by the presence of an allosteric

 effector.                                                                                                                                                                       EC₅₀    K_d 

The affinity of a ligand for its receptor is defined as 1/K_d. A high affinity for reversible ligands usually refers to K_ds in the nM range or lower.

Experimental Protocols: The following approaches are commonly used:

• Saturation binding experiments measure the extent of binding in presence of different concentrations of radioligand and are used to determine B_max and K_d.

       The data is analyzed by Rosenthal-Scatchard Plot. Specific binding is simply the total radiolagand bound minus non-specific binding to things like the tube.                              

B_max is where the line crosses the X axis.     

Slope =  -1/K_D                                                                               Specific Binding                                                                                                     

All of the terms can be determined from the formula: B=FB_max/K_d+F where "B" = the concentration of receptor that has radioligand bound to it, "F" or "free" is the concentration of ligand that is not bound to the receptor and B_max is the total amount of receptor present (maximal # binding sites).                                                                      

• Kinetic experiments are used to determine the rate constants K_on and K_off which permit a calculation of the K_d.
• Competitive binding experiments are used to determine the affinity of the receptor for the competitor.

Receptors are expressed in all types of cell membranes. Cell surface receptors are typically transmembrane proteins because they bind signal molecules in the extracellular space and generate different intracellular signals on the opposite side of the plasma membrane. There are 3 major categories of PM receptors:

(1) Neurotransmitter-activated receptors which include 2 classes:

• G-protein coupled receptors (GPCRs);
• ligand-gated ion channels.

(2) tyrosine kinase receptors

(3) Nuclear/hormone/steroid receptors

Receptor Regulation

(1) Sensitization/Up Regulation:  is a process whereby a cell becomes more responsive to a given concentration of a compound. This can occur in the following ways:

• Increase in functionality: deals with increased efficiency of the receptors. For example, a receptor might increase functionality upon phosphorylation.
• Increase in the # of receptors: This can occur by 1) increasing the expression of receptors, 2) clustering the receptors where they need to be, 3) recruitment of receptors from the cytoplasm to the PM, 4) decreased turnover of receptors or 5) induction which is de novo synthesis of novel receptors via transcriptional activation. An example of de novo induction are B bradykinin receptors  which are GPCRs that are detectable only in tissues that have been exposed to inflammatory stimuli or in tissues from individuals with inflammatory diseases like arthritis. One can in fact test for whether someone has some of these diseases by testing for the presence of these receptors.

(2) Desensitization/Down Regulation: is a process whereby a cell becomes less responsive to a given concentration of a compound.

Desensitization typically involves the following:

• phosphorylation of receptors
• internalization of plasma membrane associated receptors
• decreased mRNA and protein synthesis

If the desensitization is receptor specific in that a decrease in responsiveness involves receptors for only one type of agonist, then it is referred to as homologous desensitization.

If the decrease represents a generalized loss that involves receptors for more than one type of agonist, it is called heterologous desensitization.