Hypoxia and pulmonary vascular endothelium.

The steady development of pulmonary hypertension in the rat between Days 1 and 14 of exposure to hypoxia is based largely on structural remodelling of the pulmonary arterial circulation. The changes induced by hypobaric hypoxia (380 Torr; PaO2 of 40 mmHg) are different in the pre-acinar muscular arteries from the intra-acinar (alveolar wall) arteries, including the effect on the endothelial cell. Quantitative analysis of normal lungs has revealed structural features peculiar to the pulmonary microcirculation. The precapillary arterial unit that lies within the alveolar region includes (i) a non-muscular region and (ii) a partially muscular region as well as (iii) a muscular region. The endothelial cells in these segments or 'compartments' appear different from each other in the normal animal and behave differently in disease. In the various segments the endothelial cell has different neighbours and there are differences in its boundary markers (the basement membrane and elastic lamina). In the normal precapillary arterial unit the neighbour may be a pericyte, intermediate cell or smooth muscle cell. The pericyte and intermediate cell are precursor smooth muscle cells that under the influence of hypoxia develop into mature muscle.