Abstract 226: Thyroid Hormone Stimulated Arteriolar Growth Along With Myocyte Hypertrophy Following Myocardial Infarction In Rats

Background Thyroid hormones (THs) have been shown to improve LV function and remodeling in animals post-MI and in the human setting, as well as induce capillary and arteriolar growth in the hypertrophic heart. Whether THs can improve arteriolar growth in the infarcted heart is unknown. Methods MI was produced by ligation of the LAD in female SD rats. Rats were divided into the following groups: (1) Sham MI, (2) MI, and (3) MI+T4 treatment (T4 pellet 3.3mg, 60 days release, implanted subcutaneously immediately following MI). Each group was further divided into two subgroups (n=9 per subgroup) for myocytes isolation and whole heart tissues collection eight weeks after surgery. Isolated myocyte shape and arteriolar density in the non-infarcted area were measured at terminal study. Results T4 treatment improved LV ±dp/dt, normalized TAU, and increased myocyte cross-sectional area without further increasing myocyte length in MI rats. T4 treatment tended to increase the total length of smaller arterioles (5 to 15 μm) proportional to LV weight increase. Conclusions These results suggest that long-term T4 treatment after MI has beneficial effects on myocyte and arteriolar remodeling in the non-infarcted area.

Effects of SEA0400 on Ouabain-induced Arrhythmias in Guinea Pigs

The sodium-calcium exchange (NCX) is one of the major regulators of intracellular Ca2+ concentration in cardiac myocytes. The effects of NCX blockers as antiarrhythmic agents are still controversial. We investigated antiarrhythmic effects of SEA0400 (SEA), a novel NCX inhibitor, on ouabain-induced arrhythmias in guinea pigs. In the whole animal arrhythmia model, we observed effects of SEA on the ouabain-induced arrhythmia using ECG recordings. In the isolated myocyte, we observed action potential configurations and oscillations due to calcium overload using the current clamp method. In the whole animal model, SEA at a dose range of 1-10 mg/kg (intravenous, bolus) suppressed ouabain-induced arrhythmias dose-dependently. In isolated ventricular myocytes, SEA (0.1-3 µM) suppressed ouabain-induced oscillatory activity observed between action potentials. SEA (0.1-3 µM) also suppressed ouabain-induced NCX current (INCX) that is also called transient inward current (ITI). Our results indicate that NCX is involved in arrhythmia and oscillatory activity induced by ouabain. The inhibition of these arrhythmias and oscillatory activity by SEA might result from the inhibition of NCX.Keywords: Na+-Ca2+ exchange (NCX); SEA0400; NCX inhibitors; current clamp; ECG.© 2012 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi: http://dx.doi.org/10.3329/jsr.v4i1.7722J. Sci. Res. 4 (1), 213-225 (2012)

A method to collect isolated myocytes and whole tissue from the same heart

A technique for isolation of cardiac myocytes and collection of whole heart tissue from individual hearts of adult rats is described in this study. After excision of the apical half of the left ventricle (LV) and cauterization of the cut edge, aortas were cannulated and high-quality isolated cardiac myocytes were collected after collagenase perfusion of the basal portion. Myocyte dimensions from the basal portion of cauterized and noncauterized hearts from matching rats were identical. Additionally, myocyte dimensions from the basal and apical halves of the LV were compared with the use of whole heart-isolated myocyte preps. No regional differences between basal and apical LV myocyte size were found. Therefore, this cauterization method can be used to collect isolated myocytes from the basal half and whole heart tissue from the apical half, with each half being representative of the other with respect to myocyte dimensions.

Treatment of subclinical hypothyroidism reverses ischemia and prevents myocyte loss and progressive LV dysfunction in hamsters with dilated cardiomyopathy

Growing evidence suggests that thyroid dysfunction may contribute to progression of cardiac disease to heart failure. We investigated the effects of a therapeutic dose of thyroid hormones (TH) on cardiomyopathic (CM) hamsters from 4 to 6 mo of age. CM hamsters had subclinical hypothyroidism (normal thyroxine, elevated TSH). Left ventricular (LV) function was determined by echocardiography and hemodynamics. Whole tissue pathology and isolated myocyte size and number were assessed. TH treatment prevented the decline in heart rate and rate of LV pressure increase and improved LV ejection fraction. The percentage of fibrosis/necrosis in untreated 4-mo-old CM (4CM; 15.5 ± 2.2%) and 6-mo-old CM (6CM; 21.5 ± 2.4%) hamsters was pronounced and was reversed in treated CM (TCM; 11.9 ± 0.9%) hamsters. Total ventricular myocyte number was the same between 4- and 6-mo-old controls but was reduced by 30% in 4CM and 43% in 6CM hamsters. TH treatment completely prevented further loss of myocytes in TCM hamsters. Compared with age-matched controls, resting and maximum coronary blood flow was impaired in 4CM and 6CM hamsters. Blood flow was completely normalized by TH treatment. We conclude that TH treatment of CM hamsters with subclinical hypothyroidism normalized impaired coronary blood flow, which prevented the decline in LV function and loss of myocytes.

Thyroid hormones induce unique and potentially beneficial changes in cardiac myocyte shape in hypertensive rats near heart failure

We examined the effects of thyroid hormones (THs) on left ventricular (LV) function and myocyte remodeling in rats with spontaneously hypertensive heart failure (SHHF). SHHF rats were treated with three different TH doses from 20–21 mo of age. In terminal experiments, LV function (as determined by echocardiography and catheterization) and isolated myocyte shape were examined in SHHF rat groups and age-matched Wistar-Furth control animals. Compared with Wistar-Furth rats, the ratio of α- to β-myosin was reduced in untreated SHHF rats. The α-to-β-myosin ratio increased in all TH groups, which suggests a reversal of the fetal gene program. Low-dose TH produced no changes in LV myocyte size or function, but high-dose TH produced signs of hyperthyroidism (e.g., increased heart weight, tachycardia). The chamber diameter-to-wall thickness ratio declined with increasing dose due to reduced chamber diameter and increased wall thickness. This resulted in a 38% reduction in LV systolic wall stress in the middle- and high-dose groups despite sustained hypertension. Isolated myocyte data indicated that chamber remodeling and reduced wall stress were due to a unique alteration in myocyte transverse shape (e.g., reduced major diameter and increased minor diameter). Based on our present understanding of ventricular remodeling and wall stress, we believe these changes are likely beneficial. Results suggest that TH may be an important regulator of myocyte transverse shape in heart disease.

Increased expression of cardiotrophin-1 during ventricular remodeling in hypertensive rats

Cardiotrophin-1 (CT-1) stimulates longitudinal myocardial cell hypertrophy. We examined the expression of CT-1, leukemia inhibitory factor (LIF), and gp130 by competitive RT-PCR and Western blotting in Dahl salt-sensitive (DS) rats with a high-salt diet, which showed a distinct transition from left ventricular hypertrophy (LVH) to congestive heart failure (CHF). The expression levels of CT-1 mRNA and protein were significantly increased at the CHF stage compared with the LVH stage and age-matched Dahl salt-resistant (DR) rats ( n = 6 for each group). mRNA expression of LIF was not changed in the left ventricle at any stage by RT-PCR. gp130 mRNA and protein levels of DS rats at 11 and 17 wk were significantly increased compared with age-matched DR rats. The isolated myocyte length of DS rats at 17 wk was the longest among the four groups of rats. The LV end-diastolic dimension (LVDd) of DS rats, determined by echocardiography, was significantly increased at the CHF stage. There was a significant correlation between the CT-1 protein level and LVDd. CT-1 may play a role in ventricular remodeling during transition from LVH to CHF in the rat hypertensive model.