Cellular hyperplasia and hypertrophy, capillary proliferation and myoglobin concentration in the heart of newborn and adult rats at high altitude.

Newborn rats and their mothers were subjected to a simulated altitude of 5000 m for 4-5 weeks. Weight, capillary density (CD), fiber cross-sectional area (AF) and capillary-to-fiber ratio (C/F) of right (RV) and left (LV) ventricles and myocardial myoglobin (Mb) concentration were measured weekly in the newborns and at the end of the high altitude sojourn in the adults. Results were compared to sea level controls. In the adults, adaptive changes were only observed in the right ventricle. In newborns both, RV and LV, exhibited significant alterations. After 2 weeks at 5000 m the ventricular weight increase was 223% (RV) and 40% (LV) in the newborns and 96% in the adults' RV. Whereas only fiber hypertrophy was detectable in the RV of the dams, cardiac weight increase of the acclimatized neonates resulted from both, hypertrophy and hyperplasia of the myocytes. Appropriate capillary proliferation kept CD constant. Cardiac Mb concentration did not change. We conclude, that capillary neoformation primarily counteracts the increase of the O2 diffusion distance due to fiber hypertrophy and/or hyperplasia.