Mice were used 4 wk after last tamoxifen injection. == Normalizing T3levels in TR KO mice == TR KO mice and wild-type mice were fed for 5 wk with iodine-deficient, 0.15% 6-propyl-2-thiouracil food pellets (Harlan-Teklad, Madison, WI). in the AAC heart. These data suggest that TR in the coronary ECs regulates capillary density during cardiac development, and down-regulation of TR results in coronary microvascular rarefaction during pathological hypertrophy. The loss of small blood vessels in the heart with thickened wall is caused by a reduced number of thyroid receptor. Adequate capillary supply is important for tissue survival and proper function. Abnormalities of the microvasculature contribute to the pathogenesis of some forms of heart disease. In heart failure, myocardial hypertrophy and accelerated heart rhythm enhance oxygen demand, whereas supply is hampered by endothelial dysfunction (e.g.vascular rarefaction caused by capillary endothelium apoptosis and deficient growth of the microvasculature) and a thickened diffusion barrier (e.g.fibrosis, arterial hyperplasia, and myocyte hypertrophy). Increasing capillary density in the heart is therefore one of the potential therapeutic strategies to restore the impaired oxygen supply to the failing heart (1,2,3). Thyroid hormone (TH) treatment exerts beneficial effects in the cardiovascular system, such as lowering cholesterol levels and low-density lipoprotein levels and enhancing cardiac contractile function (4,5). Recently developed novel TH analogs lead to lipid lowering and XL-888 improvement in cardiac function (6,7). It also needs to be noted that TH results in an increased heart rate and arrhythmia (5,8,9). The actions of TH occur largely through its binding to the thyroid hormone receptor (TR), although rapid nongenomic effects have been described (10). Two genes, TR and TR (11), encode TRs and at least two TR isoforms and three TR isoforms have been identified. TR1, TR1, TR2, and TR3 isoforms bind to T3, whereas TR2 does not bind to T3and functions, at leastin vitro, as a TR1 and TR1 antagonist (12). Consistent evidence has shown that TR1 and TR1 Rabbit Polyclonal to VAV3 (phospho-Tyr173) expression in the heart is down-regulated in pathological hypertrophy (13,14,15); however, it is unclear whether TR regulates capillary density in pathological hypertrophy and which TR subtype controls angiogenesis. Vascular endothelial cells (ECs) express TRs and according to some reports TR is the predominant TR isoform (16,17). The effects of TH on microvascular formation, however, are still controversial. It has been demonstrated in some studies that XL-888 TH or TH analog have the capability to induce angiogenesis (18,19,20), whereas others showed that TH has no effect on angiogenesis (21,22). It is also unclear whether TH affects microvascular formation through its direct effect on ECs or indirect effect on surrounding cells (e.g.cardiac myocytes) to release proangiogenic factors. In this study, we focused on TR subtypes in ECs and investigated whether TRs regulate coronary microvascular formation during cardiac hypertrophy. == Materials and Methods == == Antibodies and reagents == M199, antibiotic reagents (Invitrogen Corp., Carlsbad, CA), anti-TR, antikinase insert domain protein receptor (KDR)/fetal liver kinase-1 (Flk1), antiplatelet/endothelial cell adhesion molecule 1 (PECAM), antiactin (Santa Cruz Biotechnology, Inc., Santa Cruz, CA), EC growth supplement, Matrigel (BD Biosciences, San Jose, CA), collagenase II (Worthington Biochemical Corp., Lakewood, NJ), dispase II (Roche Diagnostics North America, Indianapolis, IN) XL-888 were used in this study. XL-888 All other chemicals were from Sigma-Aldrich, Inc. (St. Louis, MO). == Animal preparation == Pressure overload was created in male mice (NIH Swiss, 6 wk old; Harlan Sprague Dawley, Inc. Indianapolis, IN) by ascending aortic constriction (AAC) as previously described (15,23). Mice were anesthetized with a mixture of ketamine (100 mg/kg, ip) and xylazine (5 mg/kg, ip) before surgery. All data were obtained from mice at 10 wk after AAC. T3administration was started at 8 wk after AAC and continued for 2 wk (3.5 ng/g body weight, ip daily). Plasma T3levels were 85.2 .