Thyroid hormone‐induced cardiac hypertrophy is a model of enhanced physiological growth and angiogenesis. This study addressed the growth and geometry of the capillary bed in relation to the development of cardiac hypertrophy.

Thyroxine was administered daily (0.2 mg/kg, s.c.) for 5 or 10 days to Fischer 344 rats. After obtaining ventricular function and hemodynamic data, the hearts were perfuse fixed, and specimens from the left ventricle (LV) were subjected to image analysis to determine indices of capillary growth.

After 5 days of treatment, prior to cardiac enlargement, capillary length density was significantly greater in the epimyocardium of the thyroxine rats than in the controls (saline injected). Most of the increase could be attributed to an increase in capillary numerical density, but some enhancement in capillary profile axial ratio suggests that enhanced tortuosity or formation of oblique channels also occurred. After 10 days of treatment, all capillary parameters (length, volume, and surface densities) were similar to the controls despite a 30% enlargement of the LV. We estimate that total LV capillary length increased by 14% during the first 5 days and by 9% during the next 5 days of treatment.

These findings indicate that capillary angiogenesis precedes the development of ventricular enlargement due to thyroxine administration. Therefore, angiogenesis in this model is not stimulated by the presence of hypertrophy. Anat. Rec. 251:44–49, 1998. © 1998 Wiley‐Liss, Inc.