Mice received propranolol (Prop, 10 μg/g, 2 i.p. injections per day, P1–12). (A) The ejection fraction (EF) at P60 was not changed (n = 11 hearts for PBS, n = 6 hearts for Prop). (B) Diagram of experimental design. MI was induced at P60 and MRI and histology were performed 12 days after MI. (C) After MI, EF was higher in propranolol-primed mice. (D-G) The region of stretched-out myocardial wall is significantly smaller (D) and systolic myocardial thickening is greater (E), despite same scar size measured in vivo (F) and ex vivo (G, n = 5 hearts for PBS, n = 4 hearts for Prop). (H, I) Propranolol-primed hearts show a higher number of cardiomyocytes, determined by stereology (n = 5 hearts/group), after MI without change of heart weight (n = 5 hearts for PBS, n = 6 hearts for Prop) (I). Statistical significance was tested with t-test.