Correct Answer: D. Bronchodilation
Endothelin-1 (ET-1) is a potent vasoconstrictor peptide produced by endothelial cells that exerts multiple systemic effects through ETA and ETB receptors. The question tests knowledge of ET-1's established physiological actions. Bronchodilation is NOT an action of endothelin-1; in fact, ET-1 causes bronchoconstriction via ETB receptors on airway smooth muscle. This is the discriminating fact that makes option D the correct answer. ET-1 is well-established to increase cardiac contractility (inotropic effect via ETA receptors), decrease glomerular filtration rate through renal vasoconstriction and mesangial contraction, and cause systemic vasoconstriction—all hallmark actions. In Indian clinical practice, endothelin antagonists are being explored in pulmonary hypertension and diabetic nephropathy management, where ET-1 overproduction drives both vasoconstriction and GFR decline. The bronchoconstrictor property of ET-1 is particularly relevant in asthma and ARDS pathophysiology, making bronchodilation a clear false statement about ET-1 biology.
Why the other options are wrong
A. Has inotropic effect — This is TRUE and therefore wrong for an 'except' question. Endothelin-1 binds ETA receptors on cardiac myocytes and increases intracellular calcium, producing a positive inotropic effect. This is a well-established action and a common trap—students may confuse ET-1's cardiac effects with vasodilation, but ET-1 is a potent cardiac stimulant used as a marker in heart failure pathophysiology. B. Decreased GFR — This is TRUE and therefore wrong for an 'except' question. Endothelin-1 causes renal vasoconstriction (especially of afferent arterioles) and mesangial contraction, both of which reduce glomerular filtration pressure and GFR. This is a key mechanism in diabetic nephropathy and acute kidney injury, making it a definitive ET-1 action in Indian nephrology practice. C. Vasoconstriction — This is TRUE and therefore wrong for an 'except' question. Endothelin-1 is one of the most potent vasoconstrictors known, acting via ETA receptors on vascular smooth muscle. Systemic vasoconstriction is ET-1's signature effect and is central to its role in hypertension, pulmonary hypertension, and endothelial dysfunction in Indian populations with metabolic syndrome.
High-Yield Facts
- Endothelin-1 causes bronchoconstriction (not bronchodilation) via ETB receptors on airway smooth muscle—this is the key discriminator in this question.
- ETA receptor activation mediates vasoconstriction, positive inotropy, and mitogenic effects; ETB receptor activation mediates vasoconstriction, bronchoconstriction, and endothelin clearance.
- Endothelin-1 decreases GFR through afferent arteriolar vasoconstriction and mesangial contraction—a critical mechanism in diabetic nephropathy and acute kidney injury.
- Endothelin antagonists (e.g., bosentan, ambrisentan) are used in pulmonary arterial hypertension management in Indian tertiary centers.
- ET-1 is elevated in sepsis, acute respiratory distress syndrome (ARDS), and endothelial dysfunction, contributing to both bronchoconstriction and systemic hypotension paradoxically.
Mnemonics
ET-1 Actions: VIC-B Vasoconstriction, Inotropic effect (positive), Constricts GFR (decreases it), Bronchoconstriction (NOT bronchodilation). Use this to remember all true ET-1 actions and the false one. ETA vs ETB Receptor Outcomes ETA: Vasoconstriction, Inotropic effect, Mitogenic. ETB: Bronchoconstriction, Vasoconstriction, Endothelin clearance. Both cause constriction; ETB is the bronchoconstrictor.
NBE Trap
NBE pairs endothelin-1 with vasodilation or bronchodilation to trap students who confuse it with nitric oxide or prostaglandins. The 'except' format amplifies this trap by asking students to identify the ONE false action among four statements.
Clinical Pearl
In Indian patients with pulmonary hypertension or diabetic nephropathy, endothelin-1 overproduction drives both pulmonary vasoconstriction and GFR decline. Recognizing ET-1 as a bronchoconstrictor (not dilator) is critical when interpreting why endothelin antagonists improve both renal and pulmonary outcomes in these populations.
_Reference: Guyton & Hall Physiology Ch. 19 (Vascular Function); Harrison Ch. 237 (Pulmonary Hypertension); KD Tripathi Pharmacology Ch. 11 (Endothelin Antagonists)_