(A) Fifteen samples were analyzed—five from non-failing donor hearts (#1–5); five from ischemic HF (#6–10), and five from idiopathic dilated HF patients (#11–15); serial sections from each sample were stained with oil red O (hematoxylin counterstain), AP activity, and alizarin red; microscopic fields containing arteries were identified and captured; arrows, small arteries. (B) Quantification of AP activity in each category of samples expressed as AP-positive area per high power field (HPF). (C) Representative consecutive sections from sample #5 (non-failing) stained for AP activity in the absence (left) or presence (right) of 12.5 mM of L-homoarginine, a specific TNAP inhibitor. (D) A photomerged overlay image of AP activity (white) and alizarin red staining (brown) from sample #15 (idiopathic dilated HF); (E) Top panels, AP activity (dark blue) in combination with α-smooth muscle actin (SMA) immunohistochemistry (brown); bottom panels, same vessels stained with SMA antibody (red) in combination with endothelial specific isolectin B4 (IB4, green); arrow, AP activity.