Transplantation-induced atrophy of normal and hypertrophic rat hearts: effect on cardiac myocytes and capillaries.

Changes in tissue structure of hearts undergoing atrophy following heterotopic isotransplantation were studied. Both normal and hypertrophic hearts were used, originating from male and female rats. Aortic constriction produced in newborn rats, resulted in an 86 and 155% increase of left ventricular mass in male and female rats, respectively. On day 50, control and experimental animals were killed, half of their hearts were analysed morphometrically, while remaining hearts were transplanted into the abdominal cavity of recipient rats. Transplantation resulted in significantly decreased cardiac mass in control hearts (approximately 50% decrease compared to values at transplantation), and an even more pronounced decrease in hypertrophic hearts. Cardiac hypertrophy was characterized by significant decreases in capillary and myocyte densities. While myocyte density simply reflected changes in cell size, evidence for additional capillary growth was found (the aggregate length of capillaries per left ventricle increased by 57-88%). Cardiac atrophy resulted in increased capillary density, despite evidence of some capillary involution in transplanted hearts (aggregate length of capillaries decreased by 30-35% and 52-64% for transplants of normal and of hypertrophic hearts, respectively). Myocyte density increased due to a proportional decrease in the size of cardiac myocytes. In transplanted hearts, an increasing number of myocytes containing nuclei located close to the nearest capillary, suggests that changes in myocyte size are not symmetrical. The present study demonstrated the remarkable plasticity of the heart in neonatal animals; they were capable of large increases and decreases in cardiac mass within a few weeks, with more pronounced changes in female rats. These changes were accompanied by changes in myocardial structure, indicating some capillary growth in cardiac hypertrophy, and involution of coronary capillaries in cardiac atrophy.

Early and late effect of neonatal hypo- and hyperthyroidism on coronary capillary geometry and long-term heart function in rat.

OBJECTIVE: The aim of the present study was two-fold: (1) to examine the effect of hyper- and hypothyroidism on the developing coronary capillary network in neonatal rats, and (2) to determine in adult rats that had re-established euthyroid status whether long-term changes in capillary geometry or cardiac function had been induced by either neonatal thyroid condition. METHOD: Two-day-old rats were treated every other day for 12 or 28 days with either 3,3'5-triiodo-l-thyronine or 0.05% 6-n-propylthiouracil. After this time, treatment was stopped and in two-thirds of the rats morphometric examination of capillary geometry and immunohistochemical detection of proliferating cell nuclear antigen (PCNA) expression in endothelial cell nuclei were conducted. Remaining rats were weaned and grew to 80 days of age, at which time persistent changes in capillary geometry, PCNA expression, and cardiac function were assessed. RESULTS: Neonatal hyperthyroidism induced cardiomegaly (P < 0.01), whereas neonatal hypothyroidism attenuated cardiac growth (P < 0.01). Capillary numerical density, capillary segment lengths and PCNA-labelling analysis indicated marked capillary growth in hyperthyroid rats (P < 0.05), but attenuated capillary growth in hypothyroid rats. The elicited capillary growth response appeared to be more dependent on altered tissue maturation than on cardiac growth rate. After discontinuing treatment both neonatal thyroid conditions induced a deficit in left ventricular growth (P < 0.01). Furthermore, neonatal hyperthyroidism appeared to inhibit subsequent capillary growth in distal regions of the capillary bed in addition to inducing lasting positive chronotropic and inotropic effects on cardiac function (P < 0.05). Neonatal hypothyroidism did not produce any lasting changes in capillarization or in cardiac function. CONCLUSIONS: Results suggest that neonatal thyroid status influences early growth and development of the coronary capillary network, possibly by regulating tissue maturation, as well as inducing lasting effects on subsequent cardiac and capillary growth and heart function.

Short- and long-term effects of neonatal hypo- and hyperthyroidism on coronary arterioles in rat.

Neonatal hypo- and hyperthyroid effects on coronary arteriolar geometry were examined in newborn male Sprague-Dawley rats treated for 12 or 28 days with either triiodothyronine or propylthiouracil. Long-term effects were assessed in weaned rats 52 days after stopping treatment. Influence of both neonatal conditions was more pronounced after 28 days. Neonatal hyperthyroidism induced cardiac hypertrophy; neonatal hypothyroidism attenuated cardiac growth. Hyperthyroid rats had similar arteriolar and capillary numerical densities and arteriolar length density but significantly greater (P < 0.05) total arteriolar length than control. Hypothyroid rats had similar arteriolar numerical and length densities, greater capillary numerical density (P < 0.05), but markedly lower total arteriolar length (P < 0.01) than control. Results suggest that neonatal hyperthyroidism stimulates arteriolar and capillary growth, whereas neonatal hypothyroidism attenuates arteriolar but not capillary growth. After cessation of treatment, total arteriolar length in previously hyperthyroid rats did not change despite increased cardiac mass, whereas previously hypothyroid rats demonstrated marked increases in both cardiac mass and total arteriolar length (P < 0.01). These results indicate a lasting inhibitory effect of early hyperthyroidism on subsequent arteriolar growth.

Proliferating cell nuclear antigen (PCNA) detection of cellular proliferation in hypothyroid and hyperthyroid rat hearts.

Immunocytochemical techniques examining the expression of cell proliferation-related markers such as proliferating cell nuclear antigen (PCNA), may be employed to provide visual and quantitative evidence of cell proliferation. The efficacy of this method in frozen samples was tested on sections of 2-day-old rat heart. Mouse monoclonal anti-PCNA and goat anti-mouse IgG2a peroxidase-conjugated antibodies were applied to tissue cross-sections. PCNA positive (PCNA+) cells were evident in these sections as intensely stained, dark brown nuclei. This method was then applied to the hearts of four groups of adult rats (1) 1 month hyperthyroid, (2) 1 month hypothyroid, (3) 6 days hyperthyroid following 1 month hypothyroidism, or (4) euthyroid, in order to assess the ability of identifying and quantifying PCNA+cell types (myocyte, endothelial, or remaining, "other" cells). Serial cross-sections were prepared, half of the sections processed using the PCNA technique, while corresponding sections were stained with hematoxylin and eosin. Identification of PCNA+cell types was made by comparison of PCNA with H&E sections. PCNA labeling indices for a given cell type were expressed as the proportion of PCNA+nuclei per 1000 nuclei. No PCNA+myocyte nuclei were observed in any of the adult hearts. Only hypothyroid/hyperthyroid demonstrated significantly higher PCNA labeling of endothelial and "other" nuclei compared to euthyroid. This finding agrees with previous morphometric data which suggested capillary growth in these hearts. Results demonstrate that quantitation of PCNA expression on frozen heart tissue is possible, and can be used to provide direct, discernible evidence for cell proliferation.

Geometry of coronary capillaries in hyperthyroid and hypothyroid rat heart.

Coronary capillary geometry was studied in male rats treated with 3,3',5-triiodo-L-thyronine (T3; Hyper), 6-N-propyl-2-thiouracil (PTU; Hypo), or a sequence of PTU and T3 (Hypo/Hyper). Ventricular mass and heart-to-body mass ratios revealed myocardial hypertrophy in Hyper, atrophy in Hypo, and a return of ventricular mass to control (Con) values in Hypo/Hyper rats. From cross-sectional analysis, capillary densities for Hyper and Hypo/Hyper were comparable with Con, despite increased left ventricular mass. Hypo rats demonstrated increased capillary density. In Hyper and Hypo rats, tissue area surrounding individual capillaries (capillary domain) decreased, compared with Con, for capillaries close to the feeding arteriole. In Hyper and Hypo/Hyper, capillaries distal to the feeding arteriole had similar domain areas as Con; in Hypo, this area was smaller. From longitudinal analysis, capillary segment lengths were significantly shorter in all groups compared with Con. Our data suggest that while hypothyroidism induced myocardial atrophy and hyperthyroidism induced myocardial hypertrophy, both thyroid states stimulated capillary proliferation.

In-series fiber architecture in long human muscles.

The fiber architecture of adult human sartorius and gracilis muscles was examined using a combination of fiber microdissections and histological methods. Intact fibers were dissected from fascicles of muscle strips that were digested in nitric acid. All of these fibers terminate intrafascicularly by tapering to a fine strand at one or both ends. They measure 4-20 cm after correction for shrinkage. Systematic dissections of 1 cm long blocks sampled at intervals along the muscle length suggest that tapered fiber endings occur at all locations along the muscle but are most common centrally; here they accounted for up to 14% of dissected fibers in each block. Transverse sections of muscle confirm that fiber profiles with small diameters occur at all levels of the muscle but are especially common in sections more than 5 cm from its origin or insertion. The architectural arrangement demonstrated here suggests that long human muscles, like muscles in other species, are composed of relatively short, in-series fibers. This has many implications for the neural activation and force-developing behavior of these muscles that must be considered when paralyzed muscles are reanimated using electrical stimulation. Further, it may predispose long muscles to certain types of neuromuscular damage and dysfunction.