L5 - Measurement of drug action at receptors

What is radioligand binding used to measure in pharmacology?

It measures the binding affinity of a drug for a receptor and determines the number of specific receptor binding sites in a tissue sample

. How is a drug labeled in a radioligand binding assay?

The drug is labeled with a radioactive isotope, such as 3H or 32P

. What does KD represent in receptor binding studies?

KD is the equilibrium dissociation constant representing the molar drug concentration required to occupy 50% of the total receptor population at equilibrium

. What is total binding in a radioligand binding assay?

Total binding is the combined binding of a radiolabelled drug to both specific receptor target sites and non-specific non-receptor proteins in tissue homogenate

. Why does a plot of total binding fail to reach a clear saturation plateau?

Because total binding includes unsaturable non-specific binding to non-receptor tissue proteins alongside specific receptor binding

. How is non-specific binding measured in a radioligand binding assay?

By incubating tissue samples with the radiolabelled drug in the presence of a large excess of unlabeled "cold" drug to occupy all specific receptor sites

. How is specific radioligand binding calculated?

Specific binding is calculated by subtracting non-specific binding from total binding

. What shape does a specific radioligand binding curve take?

It forms a saturation plot that plateaus (a rectangular hyperbola) as all specific receptor sites become occupied

. What two key parameters are determined from a specific radioligand binding saturation plot?

Bmax (the maximum density/number of specific receptors per tissue) and KD (the equilibrium dissociation constant/affinity of the drug)

. Can radioligand binding assays determine a drug's efficacy?

No, radioligand binding assays measure drug binding affinity and receptor number (Bmax), not receptor activation or efficacy

. What defines a reversible competitive antagonist?

An antagonist that binds reversibly to the agonist binding site with affinity but zero efficacy, producing a surmountable blockade

. What effect does a reversible competitive antagonist have on an agonist's log concentration-response curve?

It produces a parallel shift to the right without reducing the maximum response of the agonist

. What are three examples of clinically important reversible competitive antagonists?

Pancuronium, cetirizine, and propranolol

. What effect does an irreversible competitive antagonist have on an agonist's log concentration-response curve?

It produces a non-parallel rightward shift with a depressed maximum response, creating an insurmountable blockade

. What is the dose ratio in antagonist pharmacology?

The ratio of the agonist concentration required to produce a specific response in the presence of an antagonist to the agonist concentration producing the same response in its absence

. How does the dose ratio change with increasing concentration of a reversible competitive antagonist?

The dose ratio increases linearly with increasing antagonist concentration

. What is pA2?

The negative logarithm of the molar concentration of an antagonist that produces a dose ratio of 2

. If 6.3 x 10^-9 M of propranolol produces a dose ratio of 2, what is its pA2 value?

The log of 6.3 x 10^-9 is -8.2, making the pA2 (negative log) equal to 8.2

. What is the Schild equation used to calculate antagonist affinity?

log (dose ratio - 1) = log [antagonist concentration] - log KB

. What does KB represent in Schild analysis?

The equilibrium dissociation constant for a competitive antagonist acting at its receptor

. How is a Schild plot constructed?

By plotting Log (dose ratio - 1) on the Y-axis against Log [antagonist concentration] on the X-axis

. What is the expected gradient of a Schild plot for a reversible competitive antagonist?

A gradient of 1.0, typically falling in the range of 0.8 to 1.2

. What does a Schild plot gradient significantly different from 1.0 indicate?

It indicates that the antagonist is not a reversible competitive antagonist

. What does the X-axis intercept of a Schild plot represent?

The X-axis intercept occurs when Log (dose ratio - 1) = 0 (dose ratio = 2), which equals Log KB

. What is the mathematical relationship between pA2 and KB for a reversible competitive antagonist?

pA2 = pKB = -log KB

. If the Log KB of an antagonist from a Schild plot is -9.5, what is its pA2 value?

The pA2 is 9.5

. Is the pA2 value of an antagonist dependent on the agonist used in the experiment?

No, pA2 is an intrinsic property measuring antagonist affinity for the receptor and is independent of the agonist used

. How can receptor subtypes be characterized using antagonists?

Receptor subtypes are characterized and identified by comparing the pA2 values of different selective antagonists acting upon them

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