Coronary vasodilatation is caused by ?
**Core Concept:** Coronary vasodilatation is the process by which the coronary arteries widen, allowing more blood to flow through them. This is an essential mechanism to ensure adequate oxygen and nutrient supply to the heart muscle. Coronary vasodilatation is a compensatory response to increased demand or reduced availability of oxygen and nutrients in the heart tissue.
**Why the Correct Answer is Right:** Coronary vasodilatation is primarily caused by the release of nitric oxide (NO), a potent vasodilator. NO is produced by endothelial cells lining the coronary arteries. Upon stimulation, endothelial cells release NO in response to various factors like increased oxygen demand, shear stress, or shear stress-induced nitric oxide synthase (eNOS) activation. NO binds to soluble guanylate cyclase (sGC) in smooth muscle cells, leading to the conversion of guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP). This conversion causes protein kinase G (PKG) activation, which in turn phosphorylates myosin light chains and causes smooth muscle relaxation, resulting in coronary vasodilatation.
**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because it does not address the specific mechanism of vasodilatation. Nitric oxide (NO) is a key player in the process, as explained above.
B. Angiotensin II is a potent vasoconstrictor, not a vasodilator. It acts on the smooth muscle cells of the coronary arteries, causing contraction and narrowing of these vessels.
C. Epinephrine is a vasoconstrictor, not a vasodilator. It stimulates beta-adrenergic receptors on smooth muscle cells, leading to increased intracellular cyclic adenosine monophosphate (cAMP) levels. cAMP activates protein kinase A (PKA), which phosphorylate myosin light chains causing smooth muscle contraction and vasoconstriction.
D. Serotonin is a vasoconstrictor, not a vasodilator. It stimulates alpha-adrenergic receptors on smooth muscle cells, leading to increased intracellular calcium ion concentration ([Ca^2+]) which activates calmodulin. Calmodulin binds to myosin light chain kinase (MLCK), causing myosin light chains phosphorylation and smooth muscle contraction, leading to vasoconstriction.
**Clinical Pearl:** In cases of ischemia or hypoxia, the heart releases endothelin-1 (ET-1), which is a potent vasoconstrictor acting on ET receptors on smooth muscle cells. This leads to increased [Ca^2+]i and contractility of smooth muscle cells, causing vasoconstriction. However, in response to hypoxia or ischemia, ET-1 is released as a counter-regulatory mechanism to increase coronary blood flow and improve oxygen supply to the heart muscle.
**Why the Correct Answer is Right:** In the context of the question, the correct answer is right because it explains the counter-regulatory mechanism of endothelin-1 (ET-1) release in response to hypoxia/ischemia and its effect on coronary blood flow.