Myocardial capillary density and muscle fibre size in rats born and raised at simulated high altitude.

Rats were born in a hypobaric chamber at a pressure of 500 mmHg and kept there at 460 mmHg for 5, 9 or 13 weeks. The myocardial capillary bed was labelled by perfusion with India ink. The density of myocardial capillaries and the area of cross-section of myofibres was estimated from histological sections. In the left ventricle there was a steady reduction in capillary density with increasing age which was similar in test and control rats. There was also an increase in myofibre size which was similar in test and controls. Hypoxia therefore failed to induce capillary proliferation or a reduction in myofibre diameter in the left ventricle and this is reflected in a constant capillary:fibre ratio. In the right ventricle, ventricular hypertrophy in the hypoxic rats induced a striking reduction in capillary density due to a two-fold increase in myofibre size. There was nevertheless some degree of capillary proliferation, probably a consequence of hypertrophy rather than the direct influence of hypoxia.