Functional and structural adaptation of the yak pulmonary circulation to residence at high altitude.

The high-altitude (HA) native yak (Bos grunniens) has successfully adapted to chronic hypoxia (CH) despite being in the same genus as domestic cows, which are known for their great hypoxic pulmonary vasoconstrictor responses (HPVRs), muscular pulmonary arteries, and development of severe pulmonary hypertension on exposure to CH. To determine possible mechanisms by which the pulmonary circulation may adapt to CH, yak pulmonary vascular reactivity to both vasoconstrictor and vasodilator stimuli and yak pulmonary artery structure were assessed. Hypoxia caused a small but significant HPVR, and norepinephrine infusion caused a greater rise in pulmonary arterial pressure (Ppa) than did hypoxia. Acetylcholine, an endothelium-dependent vasodilator, had no effect on Ppa but lowered pulmonary resistance (Rp) by causing an increase in cardiac output. Sodium nitroprusside, an endothelium-independent vasodilator, decreased both Ppa and Rp significantly. Yak small pulmonary arteries had a 4.1 +/- 0.1% medial thickness, with vessels < or = 100 microns devoid of smooth muscle. Yak pulmonary artery endothelial cells were much longer, wider, and rounder in appearance than those of domestic cows. Thus the yak has successfully adapted to HA conditions by maintaining both a blunted HPVR and thin-walled pulmonary vessels. Differences in both endothelial cell morphology and response to acetylcholine between the yak and those reported in the domestic cow suggest the adaptation to HA may include changes not only in the amount of pulmonary vascular smooth muscle but in endothelial cell function and structure as well.

[The functional morphology of the restructuring of the resistive vessels and capillaries of the lungs during adaptation to high altitude]. / Funktsional'naia morfologiia perestroiki sosudov soprotivleniia i kapilliarov legkikh v protsesse adaptatsii k usloviiam vysokogor'i a.

With the use of histologic, electron microscopic and morphometric methods the problems of adaptive morphofunctional rearrangement of lung vessels and capillaries in the process of adaptation of rabbits to high altitude are studied. The presence of secretory granules in the endothelial cells of lung arterioles and capillaries (which are likely to be morphological expression of local endocrine mechanisms of the vascular tone regulation) is established.

[The morphological changes in the pulmonary resistive vessels in the development of high-altitude pulmonary arterial hypertension]. / Morfologicheskie izmeneniia legochnykh sosudov soprotivleniia pri razvitii vysokogornoi legochnoi arterial'noi gipertenzii.

The structural alterations of pulmonary resistance vessels in rabbits during short-term adaptation to the high altitude were studied. As an example of ecological adaptation the structural alterations of pulmonary vessels in yaks living at altitudes above 3000 m were analyzed. Considerable structural alterations of endotheliocytes and blood cells were found that promote the appearance and development of high-altitude pulmonary hypertension. In yaks the adaptive alterations of pulmonary resistance vessels take place in the process of evolution. These alterations ensure, first, low resistance of pulmonary vessels when the volume of the vessel bed increases and, second, high functional activity of endotheliocytes. The changes are considered as a structural basis of ecological adaptation of pulmonary vessels providing a new level of homeostasis in pulmonary circulation.

[Functional morphology of resistant pulmonary vessels and capillaries in individual and species adaptation to high altitude]. / Funktsional'naia morfologiia sosudov soprotivleniia i kapilliarov legkikh pri vidovoi i individual'noi adaptatsii organizma k usloviiam vysokogor'ia.

Morphological and structural rearrangement of resistant pulmonary vessels and alveolar capillaries was assessed in lowland animals (rabbits) during high-altitude adaptation, in aboriginal high-altitude species (yaks, mountain goats) and on native highlanders. Structural adaptive developments in pulmonary vessels and capillaries of high-altitude animals contribute to maximal facilitation of gas diffusion. Similar adaptive changes in pulmonary resistant vessels and capillaries of laboratory animals and in native highlanders are associated with pathological alterations manifest in the elevation of pulmonary vascular resistance, right ventricular hypertrophy, increases in the thickness of the basal membrane of the air-blood barrier. In all the subjects studied the process of high-altitude adaptation is associated with hypertrophy of pulmonary endothelium. The intensification of pulmonary endothelium. The intensification of pulmonary endothelium metabolic activity may be directed at regulation of vascular tone.