(A) Image of perfused and non-perfused capillaries in post-ischaemic left ventricle. Isolectin B4 labelling (white) defines positions of all vessels, while FITC-albumin labelling (green) shows vessels that are perfused. Bottom left capillary is completely non-perfused; top green capillary is fully perfused; lower green capillary is blocked halfway across the image. (B, C): NG2-labelling of pericytes (B) and merge (C) of the images (A) and (B) show pericyte processes constricting vessel at block site. (D–F) Another example set of images as in (A–C), showing two capillaries blocked near pericyte somata. (G) Normalised intensity of (background-subtracted) FITC-albumin (green) labelling along the centre of the capillary lumen across 20 block sites. (H) Cumulative probability distribution for the distance from capillary blockage sites to the nearest pericyte soma (black) and for the position expected (see Figure 3—figure supplement 1) if blocks occurred at positions independent of pericyte locations (significantly different, p=3.9×10−5). Comparing the experimental distribution with a theoretical distribution increasing linearly to one at a distance of 30 μm (see Image Analysis in Materials and methods) also showed a significant difference (p=7.6×10−8). (I) Ratio of capillary diameter at pericyte somata to the diameter at positions ~ 10 μm upstream after ischaemia, after ischaemia with adenosine (ado), and for sham-operated hearts (Con). (J) Diameter at pericyte somata after ischaemia, after ischaemia with adenosine, and for sham-operated hearts (Con) (all pericyte locations were measured, not just those associated with capillary blockages, for which the mean diameter after ischaemia was smaller: 3.19 ± 0.24 μm, n = 30). (K) Capillary blockage in an area of the heart with a neutrophil (labelled for neutrophil elastase, NE, bottom left) present outside the capillaries. (L) Pericyte (labelled with isolectin B4) near a blockage constricting a vessel with two red blood cells (RBCs, labelled for glycophorin A) trapped in the constriction. (M) Blockage-associated pericyte (labelled for NG2) that also labels for α-SMA. Numbers on bars are of pericytes. Data are mean ± s.e.m. P values in I-J are from Mann-Whitney tests and are corrected for multiple comparisons.