In view of the evidence that thyroid hormone administration has angiogenic 
effects on the hypertrophic myocardium, we tested the hypothesis that the 
capillary supply in the hypertrophic myocardium surviving infarction would be 
improved by administration of the thyroid hormone analog, diiodothyroproprionic 
acid (DITPA). We administered DITPA (MI-DITPA) or saline (MI-saline), s.c., to 
rats for 10 days following experimental infarction of the left ventricle (LV). 
Morphometric methods were used to assess capillarity and myocyte cross-sectional 
area in three regions of the left ventricle: (1) border (next to the scar of 
infarction); (2) adjacent (next to the border); and (3) remote (interventricular 
septum). Infarct size ranged from 20-85% of the LV free-wall, and both groups 
had similar mean infarct size. Capillary length density (LV) was significantly 
higher in the remote region of the treated group than in the MI-saline rats. LV 
in the border region, which experienced the most marked increase in cardiocyte 
cross-sectional area, was not significantly lower than in the other regions, 
indicating a more marked angiogenic response. In hearts with large infarcts (> 
or = 40%) LV in the border region was higher in the DITPA group than in the 
non-treated rats. In the MI-DITPA group, cardiocyte size in the border region 
was positively correlated with that of the other regions, which contrasts with 
the negative correlations noted for the MI-saline rats. These data suggest that 
DITPA therapy (1) may improve maximal perfusion potential of the hypertrophied 
myocardium surviving a myocardial infarction, and (2) is selectively effective 
in the border region of hearts with large infarcts.