A patient with a chromaffin cell tumor (pheochromocytoma) has chronically elevated blood epinephrine for years. Compared to a healthy person, how would you expect this patient's heart to respond to an additional epinephrine injection?
AA stronger-than-normal response, because the heart has been primed by years of epinephrine exposure
BA blunted response, because chronic epinephrine exposure causes downregulation (desensitization) of adrenergic receptors in cardiac muscle
CAn identical response, because receptor density is fixed by genetics and cannot change
DNo response at all, because the heart will have completely lost its adrenergic receptors
Chronic high hormone exposure triggers receptor downregulation — the cell internalizes and degrades surface receptors, reducing its sensitivity to further stimulation. This is an adaptive mechanism that prevents runaway stimulation, but it means the chronically stimulated heart has fewer functional β1 receptors than normal. A blunted (reduced) heart rate and contractility response to an epinephrine challenge is the expected outcome. This is mechanistically similar to drug tolerance, and it illustrates why hormone levels alone don't predict cellular response — you must also consider the receptor context the target cell has established.
Question 2 Multiple Choice
A researcher selectively blocks all β1 adrenergic receptors in a patient. Epinephrine is then administered at a physiological dose. Which outcome would you predict?
AAll epinephrine effects are eliminated, since β1 receptors mediate all catecholamine signaling
BCardiac effects (heart rate, contractility) are blocked, but bronchial smooth muscle (β2 receptors) and other tissues expressing different receptor subtypes would still respond to epinephrine
CEpinephrine's effects are unchanged, since the body compensates by upregulating α receptors immediately
DOnly inhibitory effects of epinephrine persist because β1 blockade unmasks α-adrenergic inhibitory pathways
Epinephrine binds multiple adrenergic receptor subtypes (α1, α2, β1, β2, β3), each expressed in different tissues. β1 receptors predominate in cardiac muscle; β2 receptors predominate in bronchial smooth muscle. Blocking β1 removes the cardiac response (reduced heart rate and contractility) but leaves β2-expressing tissues — bronchioles, uterus, skeletal muscle vasculature — fully responsive. This receptor-subtype specificity is why selective β1 blockers (metoprolol) can treat hypertension without causing bronchoconstriction, unlike non-selective β blockers. The hormone is the same; the receptor subtype determines the tissue-specific response.
Question 3 True / False
Steroid hormones act directly on DNA as transcription factors, producing changes in cell physiology more rapidly than peptide hormones, which should first activate G-protein cascades before affecting the cell.
TTrue
FFalse
Answer: False
The timescale relationship is the opposite of what intuition might suggest. Peptide hormones (epinephrine, insulin, glucagon) act in seconds to minutes because they work by modifying existing proteins through phosphorylation — a post-translational modification that doesn't require new protein synthesis. Steroid hormones act over hours to days because they change gene transcription, and the cell must then synthesize new protein before the effect is manifest. The 'faster pathway' (GPCR → second messenger → kinase → existing target protein) is faster precisely because it bypasses gene expression.
Question 4 True / False
The same blood concentration of a hormone can produce different — even opposite — physiological effects in different tissues, depending on which receptor subtype is expressed.
TTrue
FFalse
Answer: True
This is the central insight of receptor pharmacology applied to endocrinology. Epinephrine causes vasoconstriction in skin and gut (α1 receptors → IP3/Ca2+ cascade → smooth muscle contraction) but vasodilation in skeletal muscle (β2 receptors → cAMP → smooth muscle relaxation). Dopamine is excitatory in some pathways (D1/cAMP) and inhibitory in others (D2/Gi). The hormone circulates at the same concentration systemically; tissue-specific receptor subtype expression is what creates tissue-specific responses. This is also why pharmacology can selectively target organ systems — receptor subtype specificity is the lock, not the key.
Question 5 Short Answer
Why does knowing the blood concentration of a hormone alone not allow you to predict the cellular response, even if you know which target tissue you are considering?
Think about your answer, then reveal below.
Model answer: At least two additional factors mediate between hormone level and cellular response. First, receptor subtype: even within one tissue, different receptor subtypes couple to different second-messenger systems and produce different downstream effects. Second, receptor regulation: chronic high hormone levels cause downregulation (fewer receptors, blunted response), while chronic deficiency causes upregulation (more receptors, hypersensitivity). A cell with downregulated receptors will respond less to the same hormone concentration than a naive cell. Thus the effective signal is determined by hormone concentration × receptor density × receptor coupling efficiency — all three factors must be known.
This explains many clinical phenomena that otherwise seem paradoxical. Hypothyroid patients become hypersensitive to thyroid hormone when finally treated, because prolonged deficiency caused upregulation. Asthmatics who overuse β2 agonist inhalers see diminishing bronchodilation over time because of receptor downregulation. In each case, the hormone dose is the same but the receptor context has changed. The practical lesson is that endocrine physiology cannot be reduced to 'more hormone = more effect' — the receptor system is a dynamic gain control that adapts to the signal environment.