Ion channels as a pharmacotherapy target
Many drugs alter the permeability of cell membranes to ions, acting as blockers or activators of specific channels.
- Sodium channel blockers. They prevent the influx of sodium ions into the cell, which halts the process of membrane depolarization. This is the primary mechanism of action for local anesthetics. This group also includes antiarrhythmic agents (quinidine, lidocaine, procainamide) and antiepileptic drugs (phenytoin, carbamazepine), whose anticonvulsant activity is specifically related to the blockade of these structures.
- Calcium channel blockers. Calcium ions are critically important for muscle contraction, impulse generation in the sinus node, conduction of excitation, and platelet aggregation. Medications (nifedipine, verapamil) block slow calcium channels, preventing calcium from entering the cell. The result is relaxation of vascular smooth muscle, a decrease in heart rate, and suppression of atrioventricular conduction. They are used for hypertension, angina pectoris, and arrhythmias.
- Potassium channel modulators. Activators are actively used in this group (for example, minoxidil, diazoxide). They stimulate the efflux of potassium ions from the cell, causing membrane hyperpolarization. This leads to a decrease in vascular tone and a drop in blood pressure.
Enzymes and transport systems
Pharmacological action on enzymes generally consists in their inhibition (suppression of activity), which alters the concentration of important endogenous substances in the body.
- Monoamine oxidase (MAO) inhibitors. They block the enzyme that breaks down catecholamines. This leads to the accumulation of serotonin, dopamine, and norepinephrine in the central nervous system (used as antidepressants, e.g., nialamide).
- Nonsteroidal anti-inflammatory drugs (NSAIDs). Their target is the enzyme cyclooxygenase. Suppression of its activity reduces the synthesis of prostaglandins, providing a potent anti-inflammatory effect.
- Anticholinesterase agents. They block the enzyme acetylcholinesterase, halting the breakdown of acetylcholine. The accumulation of the neurotransmitter in the synaptic cleft increases the tone of skeletal muscles and smooth muscle organs (gastrointestinal tract, urinary bladder).
Transport systems also serve as important targets. Drugs bind to carrier proteins or ion pumps:
- Tricyclic antidepressants block carrier proteins of the presynaptic membrane, disrupting the reuptake of norepinephrine and serotonin.
- Cardiac glycosides block the ion pump (sodium-potassium ATPase) in cardiomyocytes, disrupting ion exchange across the membrane.
Structural proteins and extracellular mechanisms
Some drugs act on the cellular cytoskeleton. For example, the target for antitumor agents (vinca alkaloids) is the structural protein tubulin. The drugs bind to its monomers, preventing the assembly of microtubules. This produces an antimitotic effect—the malignant cell loses the ability to divide.
There is also chemical interaction, where the effect is achieved through direct reactions outside of cells rather than via receptors. A classic example is antacids. They simply chemically neutralize excess hydrochloric acid in the stomach, which helps with hyperacid gastritis and peptic ulcer disease.
Finally, a number of agents have no specific biological targets at all, but work exclusively based on their physicochemical properties. The osmotic diuretic mannitol increases osmotic pressure in the renal tubules. Water ceases to be reabsorbed back, is retained in the lumen of the tubules, and diuresis increases significantly.
Briefly: answers to topical questions
Which enzymes are targets for the action of drugs?
The targets for the action of drugs include human body enzymes as well as enzymes of disease pathogens. The sources list the following target enzymes:
- Monoamine oxidase (MAO) — responsible for the oxidative deamination of catecholamines; MAO inhibitors disrupt metabolism and cause the accumulation of norepinephrine, dopamine, and serotonin in the central nervous system.
- Cyclooxygenase (COX) — target of NSAIDs; inhibition reduces prostaglandin biosynthesis.
- Acetylcholinesterase — target of anticholinesterase agents; blockade of acetylcholine hydrolysis increases its concentration in the synaptic cleft.
- Phosphodiesterase-III (PDE-III) — target of milrinone; selective inhibition in cardiomyocytes leads to the accumulation of intracellular cAMP.
- Angiotensin-converting enzyme (ACE) — participates in the formation of angiotensin II in the vascular bed; ACE inhibition also prevents the breakdown of bradykinin.
- Catechol-O-methyltransferase (COMT) — an enzyme whose inhibitors are used to increase the effectiveness of levodopa.
- Dihydrofolate reductase — target of methotrexate; overexpression of this enzyme is a mechanism of resistance.
- $H^+/K^+$-ATPase — proton pump of gastric parietal cells.
- $Na^+$-$K^+$-ATPase — sodium-potassium pump of cardiomyocytes, blocked by cardiac glycosides.
- Reverse transcriptase, integrase, and protease — viral enzymes whose inhibition is the target of antiviral therapy.
Which drugs act through direct chemical reactions outside of cells?
The sources describe the following agents and substances that act through direct chemical reactions or chemical interaction outside of cells:
- Antacids — perform chemical neutralization of hydrochloric acid in the stomach.
- Antidotes in chemical antagonism — enter into a chemical reaction to form inactive compounds:
- sodium thiosulfate in heavy metal poisoning forms non-toxic sulfites;
- digibind binds cardiac glycosides into inactive complexes;
- protamine sulfate binds to heparin and neutralizes its anticoagulant effect.
- Protamine sulfate in severe bleeding during heparin therapy forms an insoluble complex with heparin, i.e., causes chemical neutralization.
- Potassium permanganate during gastric lavage in acute morphine poisoning causes chemical oxidation of morphine.
- Components of the «Carisolv» kit for chemomechanical treatment of carious cavities: gel No. 1 dissolves demineralized mineral components of carious dentin, and gel No. 2 with sodium hypochlorite dissolves exposed collagen fibers.
Which transport systems act as targets for drugs?
The targets for drugs are carrier proteins and ion pumps, the disruption of which alters the transport of substances across the membrane. The sources list the following transport systems:
- Presynaptic membrane carrier proteins — blocked by tricyclic antidepressants; this disrupts neuronal reuptake of norepinephrine and serotonin and increases their concentration in the synapse.
- Cardiomyocyte $Na^+$-$K^+$-ATPase — blocked by cardiac glycosides; transport of $Na^+$ out of the cell in exchange for $K^+$ is disrupted.
- NKCC2 — sodium-potassium-chloride cotransporter of the apical membrane of epithelial cells in the thick ascending limb of the loop of Henle; selective blockade inhibits the work of this carrier protein.
- NCC — sodium-chloride cotransporter of the distal convoluted tubule; its blockade disrupts the reabsorption of sodium and chloride ions.
- P-glycoprotein — a transporter protein whose activity can be altered by drugs or food components; this can change the plasma concentration of its substrates.
Which receptors are coupled to G-proteins?
G-protein-coupled receptors (metabotropic) are the most common type of receptors in the human body. The main representatives of this group include:
- Dopamine receptors.
- Serotonin receptors (certain subtypes).
- Opioid receptors.
- Histamine receptors.
- Receptors for most peptide hormones.
- $\beta_1$-Adrenoceptors are coupled to the $G_s$ protein, which activates the enzyme adenylyl cyclase.
What are the types of ion channels based on their opening mechanism?
The sources do not provide a complete classification of ion channels by their opening mechanism, but the following types are mentioned:
- Voltage-gated channels — for example, voltage-gated $Ca^{2+}$ channels, which open upon membrane depolarization; the opening of voltage-gated $Ca^{2+}$ channels in the $\beta$-cell is also described.
- ATP-sensitive channels — for example, ATP-sensitive $K^+$ channels of $\beta$-cells, the closure of which is associated with an increase in ATP concentration.
- Receptors directly coupled to ion channels, or ionotropic receptors — one of the types of membrane receptors.
- GABA_A receptor complex — associated with chloride channels; sodium thiopental leads to prolonged activation of chloride channels and the influx of chloride ions into the cell.
- Test items in the sources also mention ligand-gated calcium channels.
The effects of calcium channel blockers are easy to remember using the "Three V" rule: Vessels (relax), Vagal/heart rate (decreased), Velocity of conduction (decreased in the atrioventricular node).
Class and exam questions
How do drugs without specific targets work?
Their action is due exclusively to physicochemical properties. For example, the diuretic mannitol simply increases osmotic pressure in the kidneys, retaining water and increasing urine output without binding to receptors.
What is the mechanism of action of nonsteroidal anti-inflammatory drugs?
NSAIDs inhibit the enzyme cyclooxygenase (COX). This leads to a decrease in prostaglandin biosynthesis, ultimately resulting in an anti-inflammatory effect.
How do potassium channel openers work?
They open channels, promoting the efflux of potassium ions from the cell. This causes hyperpolarization of the cell membrane, a decrease in vascular smooth muscle tone, and a drop in blood pressure.
What else to study on this topic
- Mechanisms of depolarization, repolarization, and hyperpolarization of cell membranes.
- The role of calcium ions in muscle contraction and cell signaling.
- Oxidative deamination of catecholamines and the function of monoamine oxidase.
- Principles of neurotransmitter reuptake in synapses.
- Microtubule polymerization and the role of tubulin in cell mitosis.
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