Book summary “Rang & Dale's pharmacology”: summary

Introduction

You often struggle to connect isolated biochemical facts with the clinical realities of patient treatment. Pharmacology bridges molecular drug targets and physiological systems to explain how therapeutic agents interact with the human body. Humphrey P. Rang and Dale structure this text to take you from foundational chemical principles up to specific organ systems and therapeutic applications. You will examine the mechanisms that govern drug action, movement, and interaction within living tissues.

• How do drugs bind to specific cellular targets?
• What determines the concentration of a drug in your bloodstream?
• Why do different organ systems respond uniquely to identical chemical agents?

1. General Principles of Drug Action

Drugs bind to specific cellular constituents to exert their pharmacological effects. Paul Ehrlich summed this up with the Latin phrase Corpora non agunt nisi fixata, meaning a drug will not work unless it is bound. You find that pharmacological effects generally require a non-uniform distribution where drug molecules bind to specific cellular targets.

Drugs bind to specific regulatory proteins to exert biological effects, governed by the Law of Mass Action and receptor occupancy.

One drop of a drug solution at 10 minus 10 mol/l concentration contains about 3 times 10 to the 9th drug molecules. Certain bacterial toxins like diphtheria toxin act with such precision that a single molecule taken up by a target cell is enough to kill it.

Protein Targets

Most drugs exert their primary physiological influence by binding to distinct regulatory molecules. You can classify these primary drug targets into four main groups:

• receptors;
• enzymes;
• carrier molecules;
• ion channels.

Biopharmaceuticals, osmotic agents, and direct DNA-binding drugs represent exceptions to this rule. Bisphosphonates used for osteoporosis bind directly to calcium salts in the bone matrix rather than protein, rendering the matrix toxic to osteoclasts like rat poison.

Receptor Interactions

Binding and activation are two separate steps in cellular pharmacology. Affinity is the tendency of a drug to bind to receptors, whereas efficacy is the tendency of the bound drug to activate the receptor and elicit a tissue response. Full agonists possess high efficacy to produce maximal responses. Partial agonists produce submaximal responses even at full occupancy, while antagonists have zero efficacy.

Phenylephrine acts as a full agonist causing maximal contraction of rabbit aorta strips, whereas other alpha-adrenoceptor agonists act as partial agonists producing submaximal responses.

The binding of drugs to receptors follows the Law of Mass Action. This generates the Hill-Langmuir equation for receptor occupancy at equilibrium. Plotting specific binding versus concentration yields a rectangular hyperbola, which converts to a symmetrical sigmoid curve on a logarithmic scale.

Binding of radioactive ligands like tritium-labeled cyanopindolol to cardiac cell membranes allows direct measurement of specific binding and calculation of the equilibrium constant and B max using Scatchard plots. Exactly 50% of binding sites are occupied when the drug concentration equals the equilibrium constant KA.

2. Receptor Families and Cellular Signaling

Receptors are classified into four major superfamilies that couple ligand binding to rapid ion fluxes, enzymatic cascades, or gene transcription.

Each superfamily operates on a distinct timescale. You can observe mechanisms ranging from millisecond ion flux to hours-long gene transcription modification. The human genome encodes 865 human GPCRs comprising 1.6% of the genome, at least 48 NR genes, and nearly 500 odorant receptors.

Receptor Superfamilies

• Type 1: Ligand-gated ion channels
• Type 2: G-protein-coupled receptors
• Type 3: kinase-linked and related receptors
• Type 4: nuclear receptors

Type 1 receptors, or ligand-gated ion channels, mediate the fastest synaptic events in the nervous system. They directly couple agonist binding to the opening of an ion pore within milliseconds. These ionotropic receptors are typically composed of a pentameric assembly of subunits possessing membrane-spanning alpha-helices. Each subunit has a molecular weight of 40-58 kDa and contains 20 membrane-spanning helices surrounding a pore.

Katz and Miledi used noise analysis to study acetylcholine action at the motor endplate. They calculated a single channel conductance of about 20 pS and a mean open time of 1 to 2 ms. This influx drives up to $10^7$ ions per second through a single channel.

Type 2 receptors, or G-protein-coupled receptors, constitute the largest family of membrane receptors. They feature a heptahelical structure with a single polypeptide chain of up to 1100 residues. The human genome contains 400 GPCRs excluding odorants.

The beta-adrenoceptor was the first GPCR to be cloned in 1986.

Type 3 receptors, or kinase-linked and related receptors, combine ligand binding with intrinsic or associated tyrosine kinase activity. Over 100 such receptors are cloned. Imatinib is a specific tyrosine kinase inhibitor introduced for treating chronic myeloid leukaemia.

Type 4 receptors, or nuclear receptors, are soluble intracellular proteins. They function as ligand-activated transcription factors to regulate gene expression and modulate lipid and hormonal signaling. At least 48 NR genes exist in man, and 10% of all prescription drugs are targeted by NRs.

Cellular Signaling Pathways

G-protein-coupled receptors regulate cellular function primarily through two major second messenger systems. These are the adenylyl cyclase/cAMP pathway and the phospholipase C/inositol phosphate pathway. There are 9 different molecular isoforms of adenylyl cyclase and 10 different mammalian PKC subtypes.

Prolonged agonist occupancy leads to desensitisation through receptor phosphorylation and internalisation. This protects cells from overstimulation. Phosphorylation occurs via second messenger kinases or GPCR kinases.

Non-receptorial ion channels form water-filled pores across cell membranes. They serve as critical drug targets governed by voltage changes or extracellular ligands. The human genome encodes more than 70 distinct potassium channel subtypes.

Receptor malfunction directly contributes to various human disease states. Autoantibodies can inactivate receptors or mimic agonists to cause hypersecretion disorders.

Myasthenia gravis is caused by autoantibodies that inactivate nicotinic acetylcholine receptors at the neuromuscular junction.

3. Pharmacokinetics and Drug Disposition

Drug disposition through absorption, distribution, metabolism, and excretion dictates therapeutic concentration profiles and elimination half-lives.

Drug disposition in the body is categorized into four stages known as ADME: absorption, distribution, metabolism, and excretion. Drug molecules move through the body via bulk flow in the bloodstream, lymphatics, or cerebrospinal fluid for long distances, and via diffusion molecule by molecule over short distances. Small molecules cross cell membranes by direct lipid diffusion, aqueous pores, membrane transporters, or pinocytosis. A drug's lipid solubility and ionization state governed by pH determine its ability to cross cell membranes and cause ion trapping. Non-polar molecules diffuse freely across lipid membranes based on their partition coefficient and permeability coefficient.

Weak acids and bases exist in equilibrium between charged and uncharged forms depending on local pH according to the Henderson-Hasselbalch equation, where only uncharged species readily cross membranes, leading to ion trapping in compartments of different pH. Aspirin has a pKa value of 3.5, whereas pethidine has a pKa value of 8.6. Specialized transmembrane transporters regulate drug movement across barriers, and genetic variations in these proteins alter drug response. Solute carrier (SLC) transporters facilitate passive movement or secondary active transport, while ATP-binding cassette (ABC) transporters act as active ATP-fueled pumps like P-glycoprotein. Approximately 300 human genes code transporters, including organic cation transporters (OCTs) and organic anion transporters (OATs).

ADME Processes

Genetic polymorphisms in these carriers, such as OCT1 and OCT2, cause individual variability in drug effectiveness and target-specific toxicity like cisplatin-induced nephrotoxicity at a 100 micromol/l concentration of cisplatin influencing OCT2 activity. Many drugs bind extensively to plasma proteins like albumin, but only the free unbound drug is pharmacologically active. Plasma albumin primarily binds acidic drugs, while basic drugs can bind to beta-globulin and acid glycoprotein, with normal albumin concentration in plasma at 0.6 mmol/l and an albumin level of 4 g/100 ml. Strongly bound drugs can exist as 1% or less of the drug in free aqueous solution. Sulfonamides occupy about 50% of plasma protein binding sites at therapeutic concentrations, allowing them to displace other substances such as bilirubin in premature babies.

Oral drug absorption depends on gastrointestinal motility, physicochemical properties, pharmaceutical formulation, and first-pass metabolism. Most oral drug absorption takes place in the small intestine via passive lipid diffusion or occasional carrier-mediated transport. Bioavailability measures the exact fraction of an administered dose that reaches the systemic circulation intact after surviving intestinal mucosal penetration and presystemic first-pass metabolism in the gut wall and liver. Typically, 75% of an orally given drug is absorbed in 1-3 hours, while acceptable bioequivalence ranges for AUC, Cmax, and tmax fall between 80% and 125%.

Pharmacokinetic Modeling

The apparent volume of distribution relates the total amount of drug in the body to its concentration in the plasma. Body water is divided into plasma, interstitial fluid, intracellular fluid, transcellular fluid, and fat compartments, representing 50% to 70% total body water as a percentage of body weight. The apparent volume of distribution can match specific fluid volumes for polar compounds confined to plasma or extracellular spaces, or greatly exceed total body volume for lipid-soluble drugs that sequester extensively into tissues and body fat. Drug metabolism proceeds through catabolic phase 1 reactions that functionalize molecules and synthetic phase 2 conjugation reactions that increase polarity for excretion. Phase 1 reactions involve oxidation, reduction, or hydrolysis, which often add a reactive group like a hydroxyl to serve as a point of attack for phase 2 conjugating systems.

The cytochrome P450 monooxygenase system embedded in the endoplasmic reticulum catalyzes the oxidative catabolism of lipophilic drugs. Cytochrome P450 enzymes are haem proteins requiring molecular oxygen, NADPH, and NADPH-P450 reductase to transfer an oxygen atom to the drug substrate to form a hydroxyl group, with 74 CYP gene families described in total. Renal elimination of drugs is driven by glomerular filtration, active tubular secretion, and passive diffusion across the tubular epithelium. Glomerular filtration allows free, non-protein-bound drug molecules to pass, while proximal tubular secretion uses carrier systems like organic anion transporters (OAT) and organic cation transporters (OCT) to actively clear drugs even if protein-bound. Up to 20 percent of renal plasma flow is filtered through the glomerulus, and the urine volume emerging is about 1 percent of the glomerular filtrate.

Total clearance is the fundamental pharmacokinetic parameter that quantifies the overall rate of drug elimination relative to its plasma concentration. Overall clearance represents the volume of plasma from which all drug molecules are removed per unit time by all routes combined, primarily renal and metabolic clearance. The single-compartment model treats the body as a single well-stirred volume where drug elimination follows exponential first-order kinetics. During chronic administration, a drug accumulates to its steady-state level over approximately three to five half-lives, yielding a 50 percent decrease in plasma concentration per one half-life.

4. The Autonomic Nervous System

Autonomic neurotransmission relies on acetylcholine and noradrenaline acting on distinct muscarinic, nicotinic, and adrenoceptor subtypes.

The autonomic nervous system consists of sympathetic, parasympathetic, and enteric divisions that regulate involuntary functions through a two-neuron efferent pathway. Autonomic pathways utilize preganglionic neurons originating in the central nervous system and postganglionic neurons residing in ganglia. The enteric division contains numerous intrinsic neurons within the gut wall capable of functioning independently. Pelvic nerves carry both sympathetic and parasympathetic fibres through pelvic ganglia to supply the bladder, rectum, and genitalia, including the nervi erigentes which evoke penile erection.

Acetylcholine and noradrenaline act as the primary neurotransmitters in the autonomic nervous system, adhering to specific distribution and receptor rules. All motor nerve fibres leaving the central nervous system and all postganglionic parasympathetic fibres release acetylcholine acting on nicotinic or muscarinic receptors. Most postganglionic sympathetic fibres release noradrenaline, with the prominent exception of sweat glands which use acetylcholine.

Cholinergic Transmission

Otto Loewi proved chemical transmission in 1921 using isolated frog hearts, demonstrating that vagus nerve stimulation releases a chemical substance that inhibits heart rate. Loewi stimulated the vagus nerve of a cannulated donor frog heart and transferred the perfusate fluid to a second recipient heart, observing an inhibitory effect identical to vagus stimulation. He later used fluorescence measurements to show sympathetic stimulation releases adrenaline.

Loewi was inspired by a dream in 1920, wrote notes he could not read in the morning, received the dream again the next night, and went to his laboratory at 3 a.m. to successfully perform the experiment.

Dale distinguished muscarinic actions that mimic parasympathetic stimulation and are blocked by atropine, and nicotinic actions that stimulate autonomic ganglia, voluntary muscle, and the adrenal medulla. Dale's experiment on a spinal cat demonstrated that acetylcholine causes a fall in blood pressure, followed by bradycardia, and after atropine, a large dose causes a rise in blood pressure due to sympathetic ganglia stimulation and adrenaline release. Nicotinic acetylcholine receptors are pentameric ligand-gated ion channels divided into muscle, ganglionic, and central nervous system types. The adult muscle receptor has the subunit composition (alpha1)2/beta1epsilondelta, while the main ganglionic subtype is (alpha3)2(beta2)3. Muscarinic receptors are G-protein-coupled receptors with five molecular subtypes that couple to different intracellular signaling pathways. M1, M3, and M5 activate the inositol phosphate pathway, whereas M2 and M4 inhibit adenylyl cyclase. Anticholinesterase drugs enhance cholinergic transmission by inhibiting enzymes that break down acetylcholine, prolonging its availability at synapses. In 1931, an estimated 20,000 Americans were affected by organophosphate contamination of fruit juice, and edrophonium is used diagnostically to test for myasthenia gravis.

Noradrenergic Transmission

Adrenoceptors are classified into alpha and beta types based on agonist potencies and selective antagonists, comprising nine distinct subtypes that act as G-protein-coupled receptors. Ahlquist postulated alpha and beta receptors based on agonist potencies. Subsequent work revealed two alpha subtypes and three beta subtypes. The biosynthesis of noradrenaline proceeds through a sequence from tyrosine to dopa, dopamine, and finally noradrenaline, with tyrosine hydroxylase serving as the rate-limiting step. L-tyrosine is converted to dopa by cytosolic tyrosine hydroxylase, which is inhibited by noradrenaline for moment-to-moment regulation. Dopa is converted to dopamine by dopa decarboxylase. Dopamine is then converted to noradrenaline by dopamine beta-hydroxylase inside synaptic vesicles.

The action of released noradrenaline is terminated primarily by reuptake via the noradrenaline transporter, followed by metabolic degradation via monoamine oxidase and catechol-O-methyl transferase. About 75% of released noradrenaline is recaptured by the plasma membrane noradrenaline transporter, while 25% is captured by extraneuronal cells. Intracellular monoamine oxidase and catechol-O-methyl transferase metabolize catecholamines into excretory products like vanillylmandelic acid. Adrenoceptor agonists produce diverse clinical effects by stimulating specific receptor subtypes, causing smooth muscle contraction via alpha receptors, smooth muscle relaxation via beta 2 receptors, and cardiac stimulation via beta 1 receptors. Alpha 1 stimulation causes potent vasoconstriction and raises arterial pressure. Beta 2 activation relaxes bronchial and uterine smooth muscle, making them useful in asthma and premature labor. Beta 1 stimulation increases heart rate and force of contraction. Beta-adrenoceptor antagonists reduce cardiac output, suppress renin release, and lower blood pressure, finding primary utility in cardiovascular disorders like hypertension, angina, and heart failure. Beta-blockers like propranolol and atenolol block cardiac stimulation and decrease oxygen consumption. While they effectively lower blood pressure and treat arrhythmias, hazards include bronchial constriction in asthmatics, cardiac depression, and masked symptoms of hypoglycemia.

Denervated tissues supplied by the autonomic nervous system develop adaptive supersensitivity to transmitter substances due to receptor proliferation and loss of clearance mechanisms. When a nerve is cut and its terminals degenerate, target organs like skeletal muscle and blood vessels respond much more strongly to applied transmitters. Contributing factors include receptor number increases, loss of neuronal uptake, and reduced sodium-potassium ATPase activity causing partial depolarization. Skeletal muscle acetylcholine receptors increase 20-fold or more after denervation and spread across the entire fiber surface instead of remaining localized to the endplate. Neurons frequently release multiple transmitters or co-transmitters concurrently, challenging classical single-transmitter doctrines. Non-adrenergic non-cholinergic transmission involves substances such as ATP, vasoactive intestinal peptide, nitric oxide, and neuropeptide Y. Co-transmission allows different mediators to exert fast, intermediate, or slow effects, or reach distant targets due to differential inactivation rates. In the guinea pig vas deferens, a single electrical stimulus causes a twin-peaked contraction where the early peak is abolished by the ATP antagonist suramin and the late peak by the alpha-1 adrenoceptor antagonist prazosin.

5. Cardiovascular and Renal Pharmacology

Cardiovascular and renal drugs regulate blood pressure, cardiac output, and fluid balance through targeted actions on ion channels, receptors, and transport enzymes.

Cardiovascular Agents

The cardiac action potential consists of five distinct phases driven by specialized ionic movements across the cell membrane. Phase 0 is rapid depolarization driven by inward sodium currents, reaching a threshold of about -60 mV. Phase 2 is a prolonged plateau caused by slow inward calcium currents through L-type channels and inward-going rectification reducing potassium conductance. Phase 4 is the pacemaker potential, a gradual diastolic depolarization driven by declining outward currents and increasing inward currents, including the funny current $I_f$.

Antidysrhythmic drugs are classified into four main groups based on their cellular electrophysiological mechanisms. Class I drugs block voltage-sensitive sodium channels and are subdivided into Ia, Ib, and Ic. Class II drugs are beta-adrenoceptor antagonists that suppress sympathetic-induced ectopic activity. Class III drugs prolong the action potential and effective refractory period by blocking potassium channels. Class IV drugs are calcium channel blockers that inhibit L-type channels in nodal tissue.

Organic nitrates relieve angina by releasing nitric oxide, which dilates veins and coronary arteries to reduce cardiac load and improve regional blood distribution. Lauder Brunton discovered the ability of organic nitrates to relieve angina in 1867. Sublingual glyceryl trinitrate has an effective duration of action of 30 min.

Calcium antagonists block voltage-gated L-type calcium channels, reducing calcium entry into cardiac and vascular smooth muscle cells. The plasma half-life of verapamil is 6-8 h.

The renin-angiotensin-aldosterone system controls fluid volume and vascular tone through a proteolytic cascade producing angiotensin II, a potent vasoconstrictor. Angiotensin II has 40 times the potency of noradrenaline in raising blood pressure.

Before 1950, no effective treatment for systemic hypertension existed. Essential hypertension is managed through lifestyle changes and staged drug regimens.

• Increase physical exercise
• Reduce dietary salt
• Reduce saturated fat
• Increase fruit and fibre intake
• Achieve weight reduction
• Reduce alcohol consumption

Heart failure involves inadequate cardiac output and counter-regulatory neurohormonal activation, treated with drugs that improve symptoms and prolong life. Without treatment, 50 percent of patients with the most severe grade of heart failure die within 6 months, and 50 percent of patients with mild or moderate heart failure die within 5 years.

• Administer loop diuretics for salt and water excretion
• Introduce ACE inhibitors or ARBs to reduce vascular resistance and cardiac afterload
• Add beta-adrenoceptor antagonists in low doses and increase slowly in stable patients
• Incorporate aldosterone antagonists such as spironolactone or eplerenone
• Utilize fixed-dose hydralazine and isosorbide dinitrate combination for indicated patient populations

Renal Diuretics

Loop diuretics inhibit the $Na^+/K^+/2Cl^-$ co-transporter in the thick ascending limb of the loop of Henle, causing copious urine production and substantial salt excretion. Furosemide and bumetanide excrete 15-25% of filtered $Na^+$, possess a 90 min plasma half-life, and maintain a 3-6 h duration of action.

Thiazide diuretics inhibit the $Na^+/Cl^-$ co-transport system in the distal convoluted tubule and are preferred for treating uncomplicated hypertension. Bendroflumethiazide has a time to maximum effect of 4-6 h and a duration of 8-12 h. In the ALLHAT 2002 trial, chlortalidone performed as well as newer antihypertensive drugs like ACE inhibitors and calcium antagonists.

Potassium-sparing diuretics act in the distal nephron to prevent potassium loss and improve survival in heart failure, though they carry a risk of hyperkalaemia. Spironolactone has a 10 min plasma half-life, while its active metabolite canrenone has a half-life of 16 h.

10 Key Ideas

1. Pharmacological effects require drug molecules to bind specific cellular targets, governed by the Law of Mass Action and receptor occupancy.
2. Receptors are classified into four major superfamilies that couple ligand binding to rapid ion fluxes, enzymatic cascades, or gene transcription.
3. Drug disposition through absorption, distribution, metabolism, and excretion dictates therapeutic concentration profiles and elimination half-lives.
4. Autonomic neurotransmission relies on acetylcholine and noradrenaline acting on distinct muscarinic, nicotinic, and adrenoceptor subtypes.
5. All motor nerve fibres leaving the central nervous system and all postganglionic parasympathetic fibres release acetylcholine acting on nicotinic or muscarinic receptors.
6. Adrenoceptors are classified into alpha and beta types based on agonist potencies and selective antagonists, comprising nine distinct subtypes that act as G-protein-coupled receptors.
7. Cardiovascular and renal drugs regulate blood pressure, cardiac output, and fluid balance through targeted actions on ion channels, receptors, and transport enzymes.
8. Antidysrhythmic drugs are classified into four main groups based on their cellular electrophysiological mechanisms.
9. Loop diuretics inhibit the $Na^+/K^+/2Cl^-$ co-transporter in the thick ascending limb of the loop of Henle, causing copious urine production and substantial salt excretion.
10. Potassium-sparing diuretics act in the distal nephron to prevent potassium loss and improve survival in heart failure, though they carry a risk of hyperkalaemia.