Early ultrastructural changes in the myocardium following thyroxine-induced hypertrophy

Nanoscale three-dimensional imaging of the human myocyte

The ventricular human myocyte is spatially organized for optimal ATP and Ca(2+) delivery to sarcomeric myosin and ionic pumps during every excitation-contraction cycle. Comprehension of three-dimensional geometry of the tightly packed ultrastructure has been derived from discontinuous two-dimensional images, but has never been precisely reconstructed or analyzed in human myocardium. Using a focused ion beam scanning electron microscope, we created nanoscale resolution serial images to quantify the three-dimensional ultrastructure of a human left ventricular myocyte. Transverse tubules (t-tubule), lipid droplets, A-bands, and mitochondria occupy 1.8, 1.9, 10.8, and 27.9% of the myocyte volume, respectively. The complex t-tubule system has a small tortuosity (1.04±0.01), and is composed of long transverse segments with diameters of 317±24nm and short branches. Our data indicates that lipid droplets located well beneath the sarcolemma are proximal to t-tubules, where 59% (13 of 22) of lipid droplet centroids are within 0.50μm of a t-tubule. This spatial association could have an important implication in the development and treatment of heart failure because it connects two independently known pathophysiological alterations, a substrate switch from fatty acids to glucose and t-tubular derangement.

Early cardiac hypertrophy induced by thyroxine is accompanied by an increase in VEGF-A expression but not by an increase in capillary density

Cardiac hypertrophy in response to hyperthyroidism is well known. However, the effects on cardiac microcirculation are still controversial in this model. The present study evaluated the effects of acute administration of two different thyroxine (T4) dose levels on the angiogenic response in the myocardium. Capillary density (CD), the CD to fiber density (FD) ratio (CD/FD), and intercapillary distance (ICD) were assessed, as was ventricle weight (VW) to body weight (BW) ratio (VW/BW). Collagen I and III messenger ribonucleic acid (mRNA) expression and VEGF-A expression were also determined by reverse transcriptase polymerase chain reaction (RT-PCR). Immunohistochemical detection of proliferating cell nuclear antigen (PCNA) expression in endothelial cell nuclei was also carried out. We simulated an acute hyperthyroidism situation in male Wistar rats by daily intraperitoneal injection of T4 (0.025 or 0.1 mg kg(-1) day(-1)) for 7 days. Hemodynamic parameters showed that T4 did not alter systolic blood pressure (SBP) but significantly increased heart rate (HR). Both T4 doses significantly increased VW. Morphologically, the higher T4 dose resulted in a 33% greater myocardial mass, which was not accompanied by alterations in collagen I and III mRNA expression. The CD and CD/FD parameters were significantly lower in the hyperthyroid rats treated with the higher dose than in the control animals, and PCNA-labeling analysis indicated total absence of marked capillary growth. However, although the acute treatment with T4 did not induce any alteration in capillary number and endothelial cell proliferation, the vascular endothelial growth factor (VEGF)-A mRNA and protein expression were significantly increased with the higher T4 dose. These data indicate that the cardiac hypertrophy induced by acute treatment with thyroid hormone precedes the angiogenic process, which probably occurs later.

Myocardial ultrastructure in cardiac hypertrophy induced by thyroid hormone?an acute study in rats

The early responses of the myocardium ultrastructure under thyroid dysfunction conditions, hemodynamic parameters, cardiac hypertrophy and ultrastructural evaluations were performed in hypothyroid and hyperthyroid rats submitted to different doses [T4-25 and T4-100; 0.025 mg and 0.1 mg kg(-1) body weight (BW).per day, respectively)]. All groups were treated for 7 days. The animals were sacrificed, the hearts were excised and weighed and the left ventricle tissue samples were processed for transmission election microscopy. Systolic blood pressure (SBP) was not altered by administration of T4. An increased heart rate and ratio of heart weight to body weight (HW/BW) were found in the hyperthyroid rats. However, the SBP and HW/BW decreased significantly in hypothyroid rats. No significant ultrastructural alterations were detected when the hypothyroid and T4-25 groups were compared with the control group. Alterations of cardiomyocytes nuclei of these groups were also not detected. Notably, disorganization of intercellular junctions was observed in many cardiomyocytes of T4-100 group. The present results indicate that in the early stages of hyperthyroidism, the cardiac hypertrophy development was mainly due to direct effects of thyroid hormone. Despite cardiac hypertrophy development, there is no ultrastructural evidence of myocardial degeneration.

Thyroid hormone and cardiac function in mice deficient in thyroid hormone receptor-alpha or -beta: an echocardiograph study

We investigated the effect of thyroid hormone (TH) receptor (TR)alpha and -beta isoforms in TH action in the heart. Noninvasive echocardiographic measurements were made in mice homozygous for disruption of TRalpha (TRalpha(0/0)) or TRbeta (TRbeta(-/-)). Mice were studied at baseline, 4 wk after TH deprivation (using a low-iodine diet containing propylthiouracil), and after 4-wk treatment with TH. Baseline heart rates (HR) were similar in wild-type (WT) and TRalpha(0/0) mice but were greater in TRbeta(-/-) mice. With TH deprivation, HR decreased 49% in WT and 37% in TRbeta(-/-) mice and decreased only 5% in TRalpha(0/0) mice from baseline, whereas HR increased in all genotypes with TH treatment. Cardiac output (CO) and cardiac index (CI) in WT mice decreased (-31 and -32%, respectively) with TH deprivation and increased (+69 and +35%, respectively) with TH treatment. The effects of CO and CI were blunted with TH withdrawal in both TRalpha(0/0) (+8 and -2%, respectively) and TRbeta(-/-) mice (-17 and -18%, respectively). Treatment with TH resulted in a 64% increase in LV mass in WT and a 44% increase in TRalpha(0/0) mice but only a 6% increase in TRbeta(-/-) mice (ANOVA P < 0.05). Taken together, these data suggest that TRalpha and TRbeta play different roles in the physiology of TH action on the heart.

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 the growth and geometry of the capillary bed in relation to the development of cardiac hypertrophy.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Early coronary angiogenesis in response to thyroxine: growth characteristics and upregulation of basic fibroblast growth factor

Although a substantial coronary angiogenesis occurs after thyroid hormone treatment, its regulation and relationship to cardiac hypertrophy are not understood. This study was designed to determine (1) the onset of capillary proliferation, (2) the sites of capillary proliferation, and (3) whether basic fibroblast growth factor (bFGF) upregulation occurs in response to thyroxine administration. Male Sprague-Dawley rats were injected daily with L-thyroxine (T4, 0.2 mg/kg s.c.). Bromodeoxyuridine labeling of capillary endothelial cells increased during the first 24 hours of treatment and peaked after 2 days of treatment. Northern blot analysis revealed a slight increase in bFGF mRNA during this period, followed by a doubling of expression by 48 hours, at which time bFGF protein was also increased. In situ hybridization, used to localize bFGF mRNA, showed an increase in transcripts within 24 hours after T4. This enhancement was uniform in the epimyocardium and endomyocardium. Histochemical analysis (double staining for alkaline phosphatase and dipeptidyl peptidase) of frozen sections, used to discriminate capillary profiles as arteriolar and venular, respectively, showed that growth occurred in the latter, since the percentage of capillary profiles positive for dipeptidyl peptidase was higher than the control value after 4 days of T4 administration. These data indicate that in the thyroxine model of cardiac hypertrophy (1) capillary DNA synthesis occurs after a single injection of thyroxine, (2) capillary growth coincides with an upregulation in bFGF mRNA and increase in bFGF protein, and (3) proliferation occurs in the venular capillaries.

Postischemic recovery of mechanical performance and energy metabolism in the presence of left ventricular hypertrophy. A 31P-MRS study

The present study was undertaken to define the effects of left ventricular hypertrophy on postischemic recovery of myocardial performance and high energy phosphate metabolism. Hemodynamics and 31P-magnetic resonance spectra were monitored simultaneously in the isolated Langendorff-perfused rat heart during 30 minutes of ischemia and 30 minutes of reperfusion. Left ventricular hypertrophy was produced by either suprarenal aortic constriction or chronic thyroxine administration. In chronic pressure overload hypertrophy, minimal coronary resistance was significantly higher (p less than 0.001) and the loss of purine nucleosides in the coronary effluent during early reperfusion significantly larger (p less than 0.001) compared with both normal hearts and thyroxine-induced hypertrophied hearts. Postischemic recovery of the baseline values for left ventricular developed pressure and phosphorylation potential was 43 +/- 4% and 82 +/- 4%, respectively, in chronic pressure overload hypertrophied hearts; 86 +/- 4% and 91 +/- 3%, respectively, in normal hearts (chronic pressure overload hypertrophy versus normal hearts, p less than 0.001 and p less than 0.05); and 100 +/- 4% and 98 +/- 2%, respectively, in thyroxine-induced hypertrophied hearts (normal hearts versus thyroxine-induced hypertrophied hearts, p less than 0.05 and p less than 0.05). Recovery after reperfusion was not related to intracellular pH, ATP, phosphocreatine, or inorganic phosphate levels during ischemia. Also, recovery was not related to developed pressure or oxygen consumption before ischemia. However, recovery was inversely related to coronary resistance and directly related to coronary flow before ischemia. Thus, functional and/or anatomic alterations of the coronary vascular bed and a greater loss of purine nucleosides during reperfusion are likely responsible for the attenuated compensatory response to ischemia and reperfusion in left ventricular hypertrophy induced by chronic pressure overload. On the other hand, the excess muscle mass per se does not seem to alter recovery, since thyroxine-induced myocardial hypertrophied hearts responded at least as well as normal hearts.

Myocardial characteristics of thyroxine stimulated hypertrophy. A structural and functional study

The effects of thyroxine-stimulated hypertrophy (TSH) were studied in the porcine left ventricular myocardium. Hypertrophy was produced in six adult pigs by administration of triiodothyronine (1 mg/kg; i.v.) for eight days. Six pigs served as controls. The degree of hypertrophy, determined by left ventricular-to-body weight ratio, was 47%. With hypertrophy there was a significant increase in heart rate, blood pressure and myocardial blood flows. Minimal coronary resistance measured during adenosine infusion was lower in the TSH group compared with the control group. Anatomic studies revealed a balanced proliferative response of mitochondria, myofibrils and the t-tubular system during TSH. Analysis of the microvasculature indicated that the capillary and arteriolar beds both experienced growth which paralleled myocyte growth during TSH. These results suggest that thyroxine administration promotes angiogenesis in the microvascular bed which provides a partial anatomic rationale for the lowered minimal coronary resistance.

Thyroxine-induced left ventricular hypertrophy in the rat. Anatomical and physiological evidence for angiogenesis

We examined anatomical and physiological responses of the left coronary vascular system to thyroxine-induced myocardial hypertrophy. Wistar-Kyoto rats (1 and 5 months old) were administered thyroxine (0.25 mg/kg per day) or the saline vehicle (sham-treated controls) for 2 months. At the ages of 3 and 7 months, each group of animals was used for one of three experimental protocols: determination of numerical capillary density in perfusion-fixed hearts, measurement of coronary reactive hyperemic responses following a 20-second coronary occlusion (peak-to-resting blood flow velocity) as an index of coronary reserve, and assessment of myocardial perfusion under resting conditions and during maximum coronary dilation (dipyridamole infusion) for the calculation of minimum coronary resistance per unit weight of the left ventricle or minimum coronary resistance of the total left ventricle. In both groups of thyroxine-treated animals, the left ventricular weight-to-body weight ratio increased by 35-40%. Capillary density of the 3- and 7-month-old Wistar Kyoto controls was 4467 +/- 352 (mean +/- SEM) and 4029 +/- 143 capillaries/mm2, respectively, but was increased significantly in the thyroxine-treated animals to 6052 +/- 409 capillaries/mm2 (3-month) and 4654 +/- 201 capillaries/mm2 (7-month). In both age control groups, the peak-to-resting blood flow velocity ratio was about 2.2. This index of coronary reserve was not changed in the thyroxine-treated animals. Myocardial perfusion measurements were limited to the 7-month-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural morphometric analysis of myocardium from dogs, rats, hamsters, mice, and from human hearts

Volume densities of mitochondria, myofibrils, and unspecified cytoplasm were measured by ultrastructural morphometry in myocardium from dogs, rats, hamsters, mice, and in biopsied tissue from human hearts. Human myocardium was composed of 23% mitochondria, 59% myofibrils, and 18% cytoplasm. Volume densities for mitochondria were 22% for dogs, 28% for rats and hamsters, and 32% for mice. Myofibrillar volume densities were highest in dogs with 63%, 57% for rats and hamsters, and 49% for mice. Differences were significant between all except man and dog, and rat and hamster. In an extensive analysis of canine myocardium, it could be shown that the quantitative composition of tissue from the left ventricular free wall (anterior, lateral, posterior) and the papillary muscles was identical. There were also no differences between subepi- and subendocardium as well as the midmyocardium. Volume densities from longitudinal sections were identical to those from transversal sections. Fixation with glutaraldehyde by perfusion or immersion provided identical results. There were no differences between volume densities in samples from the left ventricular free wall (anterior, lateral, and posterior) in rats, hamsters, and mice. It is concluded that each mammalian species is characterized by a very typical quantitative composition of the myocardium. The increase in mitochondrial volume correlated well with the increase in heart rate and oxygen consumption in smaller animals. These quantitative data are regarded as the morphological correlate of the differing functional capacity of hearts from different species.

Biochemical and morphological study of cardiac hypertrophy. Effects of thyroxine on enzyme activities in the rat myocardium

Experimental hyperthyroidism induced in rats by daily injections of 3,3',5,5'-tetraiode-L-thyroxine (0.5 mg/kg i.p.) for 14 days resulted in a significant increase in heart weight and heart weight/body weight ratio. Hemodynamic and morphological studies were performed in one group. Thyroxine-treated rats showed a characteristic cardiovascular hyperdynamic state, such as tachycardia and augmented rate of contraction, but no evidence of heart failure such as elevated end-diastolic pressures. The cardiac cells in hyperthyroid rats had a significantly larger diameter and more mitochondria than did those of the control rats. In another group the activities of cardiac enzymes involved in energy utilization and liberation were measured biochemically and compared with those of normal controls. Hyperthyroidism resulted in increased specific activity of cytochrome C oxidase and actomyosin ATPase in the myocardium. The specific activity of long-chain acyl-CoA synthetase, carnitine palmityl-transferase, carnitine acetyltransferase, malate dehydrogenase and citrate synthase showed a moderate to marked increment, whereas the specific activity of lactate dehydrogenase and pyruvate kinase remained at the control values. These results suggest that in hyperthyroid rat hearts the functions of both energy liberation and utilization systems are enhanced to meet the added workload. Moreover, the increased activity of the enzymes participating in fatty acid metabolism suggest that in thyroxine-induced hypertrophic and hyperdynamic rat hearts, fatty acids contribute more to the energy supply than do carbohydrates.