Why Are Internal Mammary (Thoracic) Arteries Less Prone to Developing Atherosclerosis Compared to Coronary Arteries? Do Gut Microbiota Play a Role? A Narrative Review.

Atherosclerosis (AS), the leading cause of cardiovascular disease (CVD), is the thickening and stiffening of arterial walls, mainly of coronary arteries, the aorta, and the internal carotid artery. Blood flow is restricted by the deposit of lipid-rich macrophages (foam cells), calcium, fibrin, and cellular debris into plaques on the inner lining (tunica intima) of arterial walls. Damaged endothelia become inflamed and accumulate macrophages, monocytes, granulocytes, and dendritic cells, which intensifies plaque formation and increases the risk of myocardial infarction (MI) and thrombosis. Many of the anatomical and physiological abnormalities in arterial walls can be linked to colonic bacteria that produce inflammation-inducing metabolites, e.g., succinate, fumarate, fatty acids (FAs), reactive oxygen species (ROS), lipopolysaccharides (LPS), and trimethylamine-N-oxide (TMAO). TMAO triggers platelet formation, inhibits the synthesis of bile acids (BAs), accelerates the formation of aortic lesions, and upregulates the expression of membrane glycoprotein CD36 (also known as platelet glycoprotein 4) on the surface of platelets and epithelial cells. The ability of internal mammary arteries (IMAs) to produce higher levels of apolipoprotein C-III (apo-CIII) and paraoxonase (PON), compared to coronary arteries, prevents plaque buildup. The tunica intima of IMAs is rich in heparin sulfate and endothelial nitric oxide synthase (eNOS). Increased production of NO relaxes VSMCs and suppresses GTP cyclohydrolase (GTPCH), which lowers blood pressure. Higher levels of prostacyclin (PG12) produced by IMAs inhibit platelet aggregation. IMAs are structurally different from coronary arteries by having a thinner, non-fenestrated, tunica intima without a prominent internal elastic lamina. These characteristics render IMAs ideal conduits in coronary artery bypass graft (CABG) surgery. This review provides information that may explain why IMAs are less affected by inflammatory reactions and more resilient to plaque formation.