COORDINATED CAPILLARY AND MYOCARDIAL GROWTH IN RESPONSE TO THYROXINE TREATMENT

Background: Thyroid hormone-induced cardiac hypertrophy is a model of enhanced physiological growth and angiogenesis. This study addressed t he growth and geometry of the capillary bed in relation to the develop ment of cardiac hypertrophy. Methods: Thyroxine was administered daily (0.2 mg/kg, s.c.) for 5 or 10 days to Fischer 344 rats. After obtaini ng ventricular function and hemodynamic data, the hearts were perfuse fixed, and specimens from the left ventricle (LV) were subjected to im age analysis to determine indices of capillary growth. Results: After 5 days of treatment, prior to cardiac enlargement, capillary length de nsity was significantly greater in the epimyocardium of the thyroxine rats than in the controls (saline injected). Most of the increase coul d be attributed to an increase in capillary numerical density, but som e enhancement in capillary profile axial ratio suggests that enhanced tortuosity or formation of oblique channels also occurred. After 10 da ys 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% du ring the first 5 days and by 9% during the next 5 days of treatment. C onclusions: These findings indicate that capillary angiogenesis preced es the development of ventricular enlargement due to thyroxine adminis tration. Therefore, angiogenesis in this model is not stimulated by th e presence of hypertrophy. (C) 1998 Wiley-Liss, Inc.