nitric oxide synthase

Nitric oxide synthase exists in three isoforms with distinct expression patterns and regulation. eNOS (endothelial NOS, NOS3) is constitutively expressed in vascular endothelium, is calcium/calmodulin-dependent, and produces NO in response to shear stress and agonists like acetylcholine and bradykinin; this NO diffuses to adjacent smooth muscle, activates soluble guanylate cyclase, increases cGMP, and causes vasodilation—critical for maintaining vascular tone and blood pressure. Endothelial dysfunction with reduced eNOS activity contributes to atherosclerosis, hypertension, and erectile dysfunction (relevant to PDE5 inhibitor mechanism, e.g., sildenafil, which prevents cGMP breakdown). nNOS (neuronal NOS, NOS1) is also constitutive and calcium-dependent, found in central and peripheral neurons, functioning as a retrograde neurotransmitter involved in long-term potentiation, and mediating non-adrenergic non-cholinergic (NANC) neurotransmission in the GI tract (e.g., relaxation of the internal anal sphincter and lower esophageal sphincter); loss of nNOS-containing neurons in the esophagus is implicated in achalasia. iNOS (inducible NOS, NOS2) is calcium-independent and induced by cytokines (e.g., TNF-alpha, IFN-gamma) and endotoxin (LPS) primarily in macrophages and other immune cells; it produces large amounts of NO for microbicidal and tumoricidal activity but can also contribute to septic shock via excessive vasodilation and hypotension. NO itself is a short-lived free radical gas that diffuses freely across membranes, activates guanylate cyclase in target cells to increase cGMP, leading to smooth muscle relaxation (vasodilation, bronchodilation). Clinically relevant: nitroglycerin and other nitrates act as NO donors to treat angina; sepsis-related hypotension is partly due to iNOS-mediated NO overproduction; NOS inhibitors have been studied in septic shock (though with limited clinical success due to complications from impaired vasodilation elsewhere). Understanding NOS isoforms is essential for connecting pharmacology (nitrates, PDE5 inhibitors), physiology (vascular tone, GI motility), and pathology (sepsis, atherosclerosis, achalasia) on board exams.