Microsphere measurement of myocardial capillary bed in hypoxic rats.

Rats exposed to hypobaric hypoxia for 24 h showed a significant increase in the number of perfused capillaries and in the number of radioactive microspheres trapped in the coronary circulation.

Cytophotometric analyses of myocardial RNA in rats exposed to altitude hypoxia.

Analytical cytophotometry of azure B-stained heart sections was employed to investigate the pattern of myocardial ribonucleic acid (RNA) alterations in rats exposed to acute (1-2 days) and prolonged periods (1-8 weeks) of hypoxia exposure (380 torr). Data support the existence of a slight transient drop in myocardial RNA on day one of exposure followed by a restoration to levels slightly elevated over controls during a 1- to 8-week exposure interval. Because of the high variability in RNA levels among myocytes (coefficient of variation, ca 40%), a shift in the proportion of low and high RNA containing segments of the myocyte population proved to be a more sensitive indicator of suppression or augmentation of RNA synthesis than the use of average RNA levels of the cell population analyzed. Microscopic analyses revealed the presence of compensatory vascular responses which could be effective in ameliorating the extent of tissue hypoxemia, i.e., capillary vasodilation on day 1 with a progressive increase in vascularization with prolonged exposure.

Myocardial Feulgen-DNA levels and capillary vascularization in hypoxia-exposed rats.

Male abino rats were exposed to simulated altitude of 18,000 ft (380 Torr) for 1 day to 8 wk. Histological sections of heart ventricles were stained by the Feulgen technique and amounts of Feulgen-DNA were determined cytophotometrically. Heart weights, relative to body weights, increased over controls at all exposure periods, as did myocardial capillary vascularization. Ninety percent of cardiac muscle nuclei were found to contain tetraploid amounts of DNA. All nuclei exhibited a high degree of chromatin dispersion in exposed and unexposed rats, which reflects a highly reactive metabolic state of cardiac cells in both groups. There was no evidence of increased ploidy or increased DNA synthesis associated with hypertrophy after acute or chronic hypoxia exposure. The data suggest that an initial event which could facilitate augmented protein synthesis during hypoxia acclimation appears to be an increased availability of oxygen and/or nutrients brought about by an expansion of the capillary bed and vascular proliferation.