1. Which of the following correctly matches each adrenergic receptor subtype with its G protein coupling and primary second messenger system?

• A) Alpha-1: Gs -> increased cAMP -> PKA activation -> smooth muscle relaxation; Alpha-2: Gi -> decreased cAMP -> reduced PKA -> inhibition of NE release; Beta-1: Gq -> IP3/DAG -> PKC activation -> increased heart rate; Beta-2: Gs -> increased cAMP -> PKA activation -> bronchodilation; Beta-3: Gi -> decreased cAMP -> reduced lipolysis in adipose tissue.
• B) Alpha-1: Gq -> PLC -> IP3 (releases SR calcium) + DAG (activates PKC) -> smooth muscle contraction, mydriasis, urinary sphincter contraction, glycogenolysis; Alpha-2: Gi -> inhibits adenylyl cyclase (reduces cAMP) + opens GIRK channels + closes N-type Ca2+ channels -> reduced NE release (presynaptic autoreceptor), reduced insulin secretion, CNS sedation, reduced sympathetic outflow; Beta-1: Gs -> adenylyl cyclase -> increased cAMP -> PKA -> phosphorylates L-type Ca2+ channels (inotropy), phosphorylates phospholamban (lusitropy/increased SERCA activity), and increases SA node If current (chronotropy); Beta-2: Gs -> increased cAMP -> PKA -> bronchial smooth muscle relaxation, uterine relaxation, vasodilation in skeletal muscle vasculature, glycogenolysis; Beta-3: Gs -> increased cAMP -> lipolysis in white adipose tissue and thermogenesis in brown adipose tissue.
• C) Alpha-1: Gq -> PLC -> IP3 + DAG -> IP3 raises intracellular calcium -> smooth muscle contraction and glandular secretion; Alpha-2: Gs -> increased cAMP -> PKA activation -> enhanced NE release from presynaptic terminals; Beta-1: Gs -> increased cAMP -> PKA phosphorylation of L-type calcium channels and phospholamban -> positive chronotropy and inotropy; Beta-2: Gq -> IP3/DAG -> bronchial smooth muscle contraction; Beta-3: Gs -> increased cAMP -> lipolysis in adipose tissue.
• D) Alpha-1: Gi -> decreased cAMP -> smooth muscle contraction through cAMP-independent calmodulin activation; Alpha-2: Gq -> PLC -> IP3/DAG -> presynaptic calcium release triggers enhanced ACh release at parasympathetic terminals; Beta-1: Gs -> increased cAMP -> PKA -> reduced heart rate through hyperpolarization of SA node via potassium channel phosphorylation; Beta-2: Gi -> decreased cAMP -> bronchial smooth muscle relaxation through cAMP-independent direct myosin dephosphorylation; Beta-3: Gq -> PKC -> lipolysis through PKC-mediated phosphorylation of hormone-sensitive lipase.
• E) Alpha-1, alpha-2, and beta-1 are all Gq-coupled receptors that signal through IP3/DAG pathways; the clinical distinction between alpha and beta receptors is based on tissue distribution rather than G protein coupling; beta-2 and beta-3 are both Gi-coupled receptors mediating inhibitory effects in bronchial and adipose tissue respectively; drugs classified as beta-agonists (salbutamol, salmeterol) are actually Gi antagonists that relieve tonic Gi-mediated bronchoconstriction.

2. The muscarinic acetylcholine receptors (M1-M5) are classified into two functional groups based on G protein coupling. Which of the following correctly identifies these two groups, their G protein couplings, and the clinically important tissue distributions of M1, M2, and M3?

• A) M1, M3, and M5 couple to Gq/11 (activating PLC -> IP3/DAG -> calcium release and PKC activation); M2 and M4 couple to Gi/Go (inhibiting adenylyl cyclase and activating GIRK channels); M1 is expressed predominantly in CNS neurons, gastric parietal cells, and autonomic ganglia (where it mediates the slow EPSP); M2 is expressed in cardiac SA and AV nodes and atrial myocardium (mediating vagal bradycardia and AV slowing) and on presynaptic parasympathetic terminals as an autoreceptor; M3 is expressed on smooth muscle (GI, bronchial, urinary bladder detrusor, vascular), exocrine glands (salivary, lacrimal, sweat, GI secretory cells), and vascular endothelium (mediating NO-dependent vasodilation).
• B) M1, M2, and M3 all couple to Gs (stimulating adenylyl cyclase) in parasympathetic effector organs; M4 and M5 couple to Gq in the CNS; the clinical effect of vagal bradycardia at the SA node is mediated by M2-Gs signaling that paradoxically reduces heart rate by activating a cAMP-independent protein phosphatase; atropine blocks all five muscarinic subtypes equally, which explains why it reverses bradycardia (M2 blockade), produces dry mouth (M3 blockade), and causes confusion in elderly patients (M1 CNS blockade).
• C) M1, M2, M3, M4, and M5 all couple to the same Gq/11 protein -- the clinical distinction between their effects (bradycardia for M2 vs. smooth muscle contraction for M3) reflects differences in downstream effector coupling at the IP3/calcium level rather than differences in G protein coupling; cardiac M2 receptors produce bradycardia through IP3-mediated calcium release that activates calcium-sensitive potassium channels, while smooth muscle M3 receptors produce contraction through IP3-mediated calcium release activating calmodulin-myosin light chain kinase.
• D) M1, M3, and M5 are Gq-coupled (PLC activation) while M2 and M4 are Gi-coupled (adenylyl cyclase inhibition and GIRK channel activation) -- the predominant clinically relevant subtypes are M1 (CNS cognition, gastric acid secretion via enteric ganglia, ganglionic slow EPSP), M2 (cardiac SA/AV node vagal slowing, cardiac atrial muscle -- NOT ventricular, presynaptic autoreceptor on cholinergic terminals), and M3 (smooth muscle contraction throughout GI/bronchial/urinary/vascular systems, all exocrine gland secretion, endothelial NO synthesis producing vasodilation); selective M3 antagonists (darifenacin, solifenacin) are used for overactive bladder with less cardiac M2-mediated side effects than non-selective antimuscarinics.
• E) All five muscarinic receptor subtypes couple exclusively to Gi/Go -- the distinction between M1/M3/M5 (excitatory effects) and M2/M4 (inhibitory effects) reflects whether the tissue expresses adenylyl cyclase (which is inhibited by Gi, producing an excitatory paradox through disinhibition of a tonic inhibitory cAMP-dependent pathway) or whether the tissue expresses GIRK channels (which are activated by Gi, producing direct inhibition); atropine produces both excitatory (M1/M3/M5 blockade removing paradoxical excitation) and inhibitory (M2/M4 blockade removing GIRK-mediated inhibition) effects.

3. Dopamine receptors are classified into D1-like (D1, D5) and D2-like (D2, D3, D4) families. Which of the following correctly identifies the G protein coupling of each family and the cardiovascular effects of dopamine at low versus high infusion doses?

• A) D1-like receptors (D1, D5) couple to Gi/Go, inhibiting adenylyl cyclase; D2-like receptors (D2, D3, D4) couple to Gs, stimulating adenylyl cyclase; at low doses (1-3 mcg/kg/min), dopamine activates D2 receptors in the renal vasculature, producing vasoconstriction and reducing renal perfusion -- the basis for low-dose dopamine's discredited use for renal protection; at high doses (greater than 10 mcg/kg/min), dopamine activates D1 receptors on the adrenal cortex, releasing aldosterone and producing sodium retention.
• B) D1-like receptors couple to Gs (increasing cAMP) and are expressed in the renal, mesenteric, and coronary vasculature where they mediate vasodilation; D2-like receptors couple to Gi (decreasing cAMP) and are expressed presynaptically where they reduce neurotransmitter release; at low dopamine doses (1-3 mcg/kg/min), D1 receptor activation produces renal and mesenteric vasodilation; at intermediate doses (3-10 mcg/kg/min), beta-1 receptor activation increases cardiac output; at high doses (greater than 10 mcg/kg/min), alpha-1 receptor activation produces vasoconstriction -- the dose-dependent receptor selectivity profile of dopamine has led to its use as a vasopressor in septic shock, though norepinephrine is now preferred as first-line.
• C) D1-like receptors (D1, D5) couple to Gs, stimulating adenylyl cyclase and increasing cAMP in renal tubular cells (increasing sodium excretion), vascular smooth muscle (producing vasodilation), and myocardial cells; D2-like receptors (D2, D3, D4) couple to Gi/Go, inhibiting adenylyl cyclase, activating GIRK channels, and inhibiting voltage-gated calcium channels -- D2 receptors on presynaptic dopaminergic and adrenergic terminals function as autoreceptors/heteroreceptors reducing neurotransmitter release; at dopamine doses of 1-3 mcg/kg/min, D1 and D2 receptor activation together produce renal vasodilation and natriuresis; at 3-10 mcg/kg/min, beta-1 receptor co-activation increases inotropy and chronotropy; above 10 mcg/kg/min, alpha-1 activation produces systemic vasoconstriction; the concept of reliable renal protection from low-dose dopamine has been definitively refuted by clinical trials.
• D) D1-like receptors couple to Gq (IP3/DAG pathway), mediating dopamine-induced renal vasoconstriction through IP3-mediated calcium release in renal vascular smooth muscle -- this explains why high-dose dopamine (above 10 mcg/kg/min) causes renal vasoconstriction through a D1 Gq mechanism before alpha-1 activation contributes; D2-like receptors couple to Gs, stimulating cAMP in the pituitary lactotrophs, which is why dopamine (and D2 agonists like bromocriptine) reduces prolactin secretion through a stimulatory rather than inhibitory signal.
• E) Dopamine at all clinical doses (1-20 mcg/kg/min) acts exclusively through alpha-1 and beta-1 adrenergic receptors -- dopamine receptors in the cardiovascular system are pharmacologically insignificant at clinically used doses; the dose-dependent cardiovascular profile of dopamine (vasodilation at low doses, inotropy at moderate doses, vasoconstriction at high doses) reflects purely the differential affinity of dopamine for beta-2 (lowest dose), beta-1 (intermediate dose), and alpha-1 (highest dose) adrenergic receptors.

4. Which of the following correctly identifies the receptor subtype responsible for vagally mediated bradycardia at the sinoatrial node, the ion channel effector it activates, and the mechanism by which atropine reverses this effect?

• A) Vagal bradycardia is mediated by M1 muscarinic receptors on SA node pacemaker cells -- M1 couples to Gq, generating IP3-mediated calcium release that activates a calcium-sensitive potassium channel (SK channel) producing hyperpolarization and slowing diastolic depolarization; atropine is a selective M1 antagonist that blocks this calcium-dependent potassium channel activation; the selectivity of atropine for cardiac M1 receptors over smooth muscle M3 receptors explains the relatively low dose required to reverse bradycardia compared to the dose needed to produce bronchodilation.
• B) Vagal bradycardia is mediated by M2 muscarinic receptors on SA node pacemaker cells and AV nodal cells -- M2 couples to Gi/Go, and the betagamma subunits of Gi/Go directly activate IKACh (the cardiac inwardly rectifying muscarinic potassium current, mediated by Kir3.1/Kir3.4 channels also called GIRK1/GIRK4); IKACh activation hyperpolarizes the pacemaker cell membrane, reducing the slope of spontaneous diastolic depolarization (phase 4) and increasing the threshold for action potential generation -- slowing the rate; simultaneously, M2-Gi reduces the funny current If (HCN channels) by reducing cAMP, further slowing pacemaker automaticity; atropine competitively blocks M2 receptors, removing Gi-mediated IKACh activation and cAMP reduction, restoring normal SA node automaticity and AV conduction.
• C) Vagal bradycardia is mediated by M3 muscarinic receptors on SA node pacemaker cells -- M3-Gq activation generates IP3 which releases calcium from the SR of pacemaker cells, and this calcium activates a hyperpolarizing calcium-sensitive potassium current (IK,Ca) that slows the pacemaker rate; the M3 receptor responsible for cardiac bradycardia is pharmacologically distinct from the M3 receptors responsible for smooth muscle contraction and glandular secretion, explaining why some muscarinic antagonists can selectively reduce bradycardia without affecting bronchomotor tone.
• D) Vagal bradycardia at the SA node is mediated by M2 muscarinic receptors -- Gi/Go betagamma subunit directly activates IKACh (GIRK1/GIRK4 channels), hyperpolarizing the pacemaker cell and reducing the slope of spontaneous diastolic depolarization; M2-Gi also reduces adenylyl cyclase activity, lowering cAMP and reducing HCN (If funny current) channel activity -- further slowing automaticity; at the AV node, M2 activation slows conduction velocity and can produce AV block; atropine blocks M2 receptors competitively, reversing IKACh activation and cAMP suppression; the low dose of atropine required for heart rate effects (0.5-1 mg IV) reflects the high vagal tone at the resting SA node -- with high endogenous ACh occupancy, even partial M2 receptor blockade produces significant heart rate increase.
• E) Vagal bradycardia is mediated by M4 muscarinic receptors on SA node cells -- M4 couples to Gi and activates the same GIRK channel as M2; the distinction between cardiac M2 and M4 is clinically important because cardiac glycoside toxicity (digoxin bradycardia) occurs through M4 receptor sensitization, and atropine is specifically an M4 antagonist used to reverse this sensitization; M2 receptors at the AV node are responsible for vagally mediated AV block and respond to different doses of atropine than M4-mediated SA node bradycardia.

5. Nicotinic receptors at the neuromuscular junction (NM) and autonomic ganglia (NN) are both ligand-gated ion channels activated by acetylcholine, but they are pharmacologically distinct. Which of the following correctly identifies the subunit compositions of NM and NN receptors and explains how this distinction is exploited by neuromuscular blocking drugs?

• A) NM receptors at the adult neuromuscular junction are composed of two alpha1 subunits, one beta1, one delta, and one epsilon (gamma in fetal/denervated muscle); NN receptors at autonomic ganglia are composed predominantly of alpha3 and beta4 subunits (with minor contributions from alpha5 and beta2); the subunit composition determines the drug-binding geometry of the ACh binding site and the channel pore properties; non-depolarizing NM blockers (rocuronium, vecuronium, cisatracurium) are designed to fit the NM receptor binding site (two alpha1 subunits providing the ACh binding pocket) with high selectivity; at clinical doses they do not significantly block ganglionic NN receptors; ganglionic blockers (hexamethonium, trimethaphan) are selective for NN-containing receptors; succinylcholine (depolarizing) is more selective for NM than NN at clinical doses but at high doses can stimulate ganglionic NN receptors.
• B) NM and NN receptors have identical subunit compositions (two alpha, one beta, one delta, one epsilon) but differ in the post-translational glycosylation of the alpha subunit -- NM alpha1 subunits are heavily glycosylated while NN alpha3 subunits are not; neuromuscular blockers (rocuronium, vecuronium) bind only to glycosylated alpha1 and cannot bind ungylcosylated alpha3, explaining their NM selectivity; the clinical significance is that patients with congenital glycosylation disorders may show reduced sensitivity to non-depolarizing NM blockers despite normal neuromuscular function.
• C) NM receptors have a subunit composition of alpha3-beta4 at the adult NMJ and alpha3-beta2 at the fetal NMJ; NN receptors at ganglia have a subunit composition of alpha1-beta1-delta-epsilon; neuromuscular blockers are selective for alpha3-containing receptors while ganglionic blockers are selective for alpha1-containing receptors -- this is the reverse of the commonly stated selectivity and reflects a recent revision of the conventional understanding based on cryo-EM structural data.
• D) NM and NN receptors both contain alpha subunits as their ACh-binding elements, but the alpha subunit identity differs: NM receptors use alpha1 (two copies per pentamer), while NN receptors use alpha3 (with beta4, and variably alpha5 or beta2 to complete the pentamer); the two alpha1 subunits in the NM receptor create a unique binding geometry for the bisquaternary or bulky quaternary ammonium structure of non-depolarizing blockers; the alpha3-containing NN receptor has a different pore geometry and ACh-binding site configuration that is not efficiently blocked by NM blockers at therapeutic concentrations -- this allows neuromuscular blockade without autonomic ganglionic blockade.
• E) NM receptors and NN receptors have identical subunit compositions but NM receptors are expressed at 10-fold higher density at the neuromuscular junction compared to NN receptor density at autonomic ganglia -- this density difference is the sole basis for the clinical selectivity of neuromuscular blockers for the NMJ over ganglia; at sufficiently high doses, all neuromuscular blocking agents inevitably produce ganglionic blockade because receptor occupancy eventually reaches the ganglionic NN receptors.

6. Which of the following correctly identifies the receptor mechanism by which beta-2 adrenergic agonists produce bronchodilation, and explains why these agents also produce hypokalemia as a clinically significant side effect?

• A) Beta-2 agonists (salbutamol, salmeterol, formoterol) activate Gs-coupled beta-2 receptors on bronchial smooth muscle, increasing cAMP and activating PKA -- PKA phosphorylates myosin light chain kinase (MLCK) inhibiting it, and phosphorylates large-conductance calcium-activated potassium channels (BKCa) opening them; the combination of MLCK inhibition (reducing cross-bridge formation) and BKCa opening (hyperpolarizing the smooth muscle cell) produces bronchodilation; hypokalemia results from beta-2 activation of the Na+/K+-ATPase pump (which is stimulated by cAMP-mediated PKA phosphorylation) in skeletal muscle, driving potassium from the extracellular space into skeletal muscle cells -- reducing plasma potassium; clinically this can worsen hypokalemia in patients on loop diuretics and precipitate arrhythmias in susceptible patients.
• B) Beta-2 agonists bind to Gq-coupled beta-2 receptors on bronchial smooth muscle, generating IP3-mediated calcium release from SR; paradoxically, this calcium release activates calcineurin (a phosphatase) rather than MLCK, dephosphorylating and inactivating myosin light chain, producing smooth muscle relaxation; hypokalemia occurs because IP3 also activates IP3-gated potassium channels in the bronchial smooth muscle cell membrane, causing potassium efflux into the airway lumen and absorption into the systemic circulation, falsely raising serum potassium levels rather than producing hypokalemia.
• C) Beta-2 agonists bind to receptors on mast cells and basophils, preventing histamine and leukotriene release -- bronchodilation is therefore indirect, mediated by suppression of bronchoconstrictor release rather than by direct smooth muscle receptor activation; hypokalemia results from beta-2 receptor activation on renal distal tubular cells, directly stimulating aldosterone-independent potassium secretion into the tubular lumen and promoting urinary potassium wasting.
• D) Beta-2 agonists produce bronchodilation through beta-2 receptor-mediated activation of phosphodiesterase-4 (PDE4), which degrades cAMP; the reduction in cAMP paradoxically produces bronchodilation because tonic cAMP in bronchial smooth muscle maintains an active MLCK conformation -- reducing cAMP releases MLCK from its active state and allows smooth muscle relaxation; hypokalemia results from beta-2 agonist-mediated inhibition of aldosterone secretion from the adrenal cortex (aldosterone normally drives potassium retention), allowing renal potassium wasting.
• E) Beta-2 agonists activate Gs-coupled receptors on bronchial smooth muscle -- increased cAMP activates PKA which phosphorylates and inhibits MLCK, reducing myosin phosphorylation and cross-bridge cycling; PKA also activates sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) increasing SR calcium uptake and reducing cytoplasmic calcium; and PKA opens BKCa channels producing hyperpolarization; the combined effect is smooth muscle relaxation and bronchodilation; hypokalemia results from beta-2 agonist activation of Na+/K+-ATPase in skeletal muscle (driven by PKA-mediated pump phosphorylation) causing redistribution of extracellular potassium into the intracellular compartment -- not potassium loss from the body; this transcellular shift can transiently lower serum potassium by 0.5-1 mEq/L and is additive with diuretic-induced potassium depletion.

7. Alpha-2 adrenergic receptors are expressed both presynaptically (as autoreceptors on adrenergic terminals) and postsynaptically (in multiple tissues). Which of the following correctly identifies three distinct postsynaptic alpha-2 receptor locations and the clinical effects produced by agonist activation at each site?

• A) Postsynaptic alpha-2 receptors are expressed only in the adrenal medulla (where they mediate epinephrine secretion), the renal tubule (where they mediate sodium reabsorption), and vascular endothelium (where they mediate NO-dependent vasodilation); clonidine's antihypertensive effect results from all three postsynaptic sites simultaneously: reduced adrenomedullary epinephrine secretion, renal sodium retention (paradoxically reducing blood volume), and endothelial NO-mediated vasodilation.
• B) All alpha-2 receptors are presynaptic autoreceptors -- there are no clinically significant postsynaptic alpha-2 receptors anywhere in the body; the antihypertensive effect of clonidine is mediated entirely by presynaptic alpha-2 autoreceptors on peripheral sympathetic terminals reducing NE release; the CNS sedation from clonidine is a pure side effect from non-specific CNS penetration rather than any specific postsynaptic central alpha-2 receptor activation; clinical drugs targeting postsynaptic alpha-2 receptors do not exist.
• C) Postsynaptic alpha-2 receptors in the brainstem NTS and RVLM: agonist activation reduces central sympathetic preganglionic outflow (mechanism of clonidine and methyldopa antihypertensive effect); Postsynaptic alpha-2 receptors on pancreatic beta cells: agonist activation inhibits insulin secretion by reducing cAMP-mediated exocytosis of insulin vesicles (clinically relevant in patients with diabetes on alpha-2 agonists); Postsynaptic alpha-2 receptors on platelets: agonist activation of platelet alpha-2A receptors promotes platelet aggregation by reducing cAMP in platelets (which normally inhibits aggregation); Postsynaptic alpha-2 receptors in the spinal cord dorsal horn: agonist activation produces analgesia by inhibiting presynaptic calcium channels on nociceptive afferent terminals and hyperpolarizing postsynaptic dorsal horn neurons (mechanism of intrathecal clonidine analgesia and the alpha-2 component of neuraxial anesthesia).
• D) Postsynaptic alpha-2 receptors in the locus coeruleus and brainstem: agonist activation produces sedation and anxiolysis (mechanism of dexmedetomidine's sedative effect and clonidine's CNS effects); Postsynaptic alpha-2A receptors in the prefrontal cortex: agonist activation strengthens PFC pyramidal cell firing by closing HCN channels (reducing the If current that reduces PFC network signal-to-noise) -- mechanism of guanfacine and clonidine benefit in ADHD and PTSD; Postsynaptic alpha-2 receptors on vascular smooth muscle: agonist activation (particularly at high plasma concentrations of alpha-2 agonists) produces vasoconstriction through Gi-mediated calcium sensitization -- explaining the transient paradoxical hypertension sometimes seen after IV clonidine bolus or during clonidine withdrawal.
• E) Postsynaptic alpha-2 receptors are expressed exclusively in the CNS -- peripheral alpha-2 receptors are entirely presynaptic autoreceptors with no postsynaptic function; the peripheral vascular effects of alpha-2 agonists (blood pressure reduction) are mediated entirely through the CNS via descending sympathetic pathways; direct peripheral administration of alpha-2 agonists (e.g., intra-arterial infusion) produces no local vascular effect because peripheral vascular smooth muscle does not express functional postsynaptic alpha-2 receptors.

8. The concept of receptor reserve (spare receptors) is important for understanding the pharmacology of partial agonists and the relationship between receptor occupancy and maximal tissue response. Which of the following correctly defines receptor reserve and explains its clinical significance for adrenergic pharmacology?

• A) Receptor reserve refers to the intracellular pool of unoccupied receptors held in reserve inside the cell membrane lipid bilayer -- these reserve receptors are mobilized to the cell surface when the primary surface receptors are occupied by agonist, maintaining constant receptor density regardless of agonist concentration; drugs that prevent receptor mobilization from the reserve pool (receptor reserve blockers) are used clinically to enhance the sensitivity of target tissues to endogenous catecholamines.
• B) Receptor reserve (spare receptors) exists when a tissue can achieve its maximal response (Emax) with less than 100% receptor occupancy -- if full agonist binding to only 10% of receptors produces the maximal contractile response, the remaining 90% are spare; receptor reserve amplifies tissue sensitivity (shifting the agonist concentration-response curve left relative to the binding curve) and provides a safety margin against receptor loss from disease or drug-induced downregulation; in tissues with high receptor reserve, a partial agonist (which produces submaximal activation per receptor) may still achieve full tissue Emax because even at full occupancy the partial agonist activates enough of the large receptor pool to produce the maximum response; in tissues with low or no receptor reserve, the same partial agonist cannot achieve Emax because it cannot produce sufficient signal even at 100% occupancy.
• C) Receptor reserve refers to the phenomenon where a drug binds to only a fraction of available receptors but occupies them with very high affinity -- drugs with receptor reserve have very high potency (low EC50) but very low efficacy (low Emax) because their tight binding prevents receptor conformational change; receptor reserve is the pharmacological basis for competitive antagonism: an antagonist with high receptor reserve outcompetes the endogenous agonist for binding without producing a biological response, lowering the effective concentration of agonist at the few receptors not occupied by the antagonist.
• D) Receptor reserve exists when the maximal tissue response can be achieved by occupying less than 100% of available receptors -- a consequence is that partial agonists (intrinsic activity between 0 and 1) in a tissue with high receptor reserve may behave as full agonists at low receptor densities but as antagonists in tissues with very high receptor density; this explains why pindolol (a beta-blocker with partial agonist activity, or intrinsic sympathomimetic activity ISA) produces less resting bradycardia than metoprolol (no ISA) -- at rest with low sympathetic tone, pindolol's partial beta-1 agonism provides some chronotropic support; under high sympathetic drive, pindolol competitively blocks beta-1 receptors occupied by the endogenous NE surge, providing beta-blockade when it is most needed.
• E) Receptor reserve is a purely theoretical concept with no clinical relevance -- in actual human physiology, all receptor systems operate at 100% occupancy under basal conditions (all receptors bound by endogenous agonist), and drug responses represent displacement of the endogenous agonist from fully occupied receptors rather than additional receptor binding; drugs cannot produce effects beyond the 100% baseline receptor occupancy, which is why the maximal tissue response to any drug equals the response to the endogenous agonist by definition.