Ultrastructural changes in the ischemic zone bordering experimental infarcts in rat left ventricles.

Ultrastructural changes in myocardial cells from the ischemic border of infarcts (produced in rat left ventricles by ligating the anterior coronary artery in vivo) were examined 1 to 24 hours after ligation. Twenty-four hours after ligation, irreversibly injured cells showed a selective spreading of Z-band material over the I band; disappearance of M bands, prominent N bands, and disassembly of A bands were also noteworthy. Sixty minutes after ligation the cells of the ischemic border were ultrastructurally normal except for paradoxically relaxed sarcomeres, indicative of an inability to contract in response to the calcium influx produced by osmium tetroxide; progressive vacuolization of this zone was evident after 4 to 12 hours. Paradoxical relaxation may be an ultrastructural correlate of acute ischemic "pump failure".