Apply drug
Aim: Explore the Law of Mass Action.
The Law of Mass Action states that the rate of a reaction is proportional to the concentrations of the reactants. Test this by varying the drug concentration and observing how rapidly binding develops and what fraction of receptors becomes drug-bound: with more drug molecules available, binding occurs more rapidly and the equilibrium occupancy increases.
Note that you observe microscopic events at a limited number of individual receptors — an artificial situation in which binding and dissociation appear as random, discrete events rather than a smooth curve. Over time the time-averaged occupancy becomes a better estimate of the true steady state. The smooth ligand-binding curve for a very large number of receptors can be explored in the next exercise.
µM
The Law of Mass Action describes how the rate of a chemical reaction depends on the concentrations of the molecules involved. For a reversible binding reaction, increasing the concentration of the reactants increases the probability that they encounter one another and therefore increases the rate of complex formation, while the reverse reaction depends on the concentration of the complex that has already formed. In pharmacology, this principle provides a simple framework for describing the interaction between a drug and its molecular target, including ion channels:
$$ D + R \;\rightleftharpoons\; DR $$
where \(D\) is the free drug, \(R\) is the unoccupied receptor or ion channel, and \(DR\) is the drug-bound complex. Drug binding occurs with an association rate set by the association rate constant \(k_{on}\) and the concentrations of free drug and available target, while the complex dissociates at a rate set by the dissociation rate constant \(k_{off}\):
$$ \text{Association rate} = k_{on}[D][R] \qquad \text{Dissociation rate} = k_{off}[DR] $$
At equilibrium the rates of association and dissociation are equal, and for a simple 1:1 interaction the binding affinity is described by the equilibrium dissociation constant \(K_d = k_{off}/k_{on}\). A lower \(K_d\) indicates higher affinity, because less drug is needed to occupy the target; under these conditions \(K_d\) corresponds to the free drug concentration at which 50% of available targets are occupied.
From binding to ion-channel function
The same binding principles apply to drugs acting on both ligand-gated and voltage-gated ion channels, where binding may reduce, enhance, or otherwise modify channel activity. For voltage-gated channels, drug binding can additionally depend on the functional state of the channel — whether it is resting, open, or inactivated — so repeated activation can change the extent of drug binding, giving rise to state-dependent or use-dependent block.