The principal findings of this study are: (a) the pulmonary artery pressure (Ppa), the ratio of right to left ventricular weight (RVW/LVW) and hematocrit (Ht) increased in accordance with the height of living altitude in all mammals, whereas in mammals well-adapted to high altitude, such as pika, blue-sheep and Yachi-nezumi, showed significantly lower values compared to other mammals; (b) when the mammals were exposed to simulated high altitude, hypoxic pulmonary vasoconstriction (HPV) in blue-sheep was significantly attenuated compared to pig; and (c) at the same altitude, a warm environment (by seasonal, latitudinal, and global warming effects) caused the decreasing RVW/LVW and Ht. These findings suggest that mammals well-adapted to high altitude such as pika, blue sheep and Yachi-nezumi have a potent adaptability to high altitude environments, having a small degree of right ventricular hypertrophy, low grade pulmonary hypertension and a small degree of Ht level at high altitude.

Generally, when animals are exposed to high altitudes, pulmonary hypertension and right ventricular hypertrophy are induced. As shown in Figure 17, the following two factors are considered to be possible causes of these phenomena: (1) constriction of the pulmonary artery because of hypoxia (hypoxic pulmonary vasoconstriction, HPV) and (2) an increase in the Ht as a result of an increase in red blood cells.

HPV, confirmed in 1946 by Von Euler [17], who used cats, is marked pulmonary hypertension resulting from the constriction of the pulmonary artery during hypoxic-air ventilation. Many reports have since appeared and

chronic exposure

Figure 17. Diagram of pulmonary hypertension and right ventricular hypertrophy at high altitude.

chronic exposure

Figure 17. Diagram of pulmonary hypertension and right ventricular hypertrophy at high altitude.

HPV was shown to be observed regardless of the animal species, induced also by migration of animals to high altitude regions as well as by exposure to low atmospheric pressure. Thus, the investigation of the pathophysiological mechanism(s) of HPV has progressed, however, its precise mechanism still remains unknown.

On the other hand, the increase in Ht because of an increase in red blood cells mentioned in (2) increases the blood viscosity and this increased viscosity is considered to substantially affect the pulmonary circulation, resulting in pulmonary hypertension. Sakai (1976) examined seasonal changes in the ventricle weight and RVW/LVW of Hime-nezumi, A. argenteus [11]. Total, left, right ventricle weight and RVW/LVW decrease in summer and increase in winter, according to the seasonal temperature changes (Figure 10). A significant negative correlation is observed between these factors and the environmental temperature. It is of interest that these seasonal changes in heart characteristics are related to blood characteristics. Sealander (1962) examined seasonal changes in blood characteristics in small wild mammals and revealed that the number of red blood cells, hemoglobin concentrations and hematocrit show high values in winter and low values in summer. Swigat (1965) dosed rats and mice with cobalt chloride and artificially induced polycythemia [16]. This experiment caused enlargements of the right ventricle. In a previous study, Sakai (1974) captured Hime-nezummi (A. argenteus) in southern warm regions and northern cold regions at the same time and examined the relationship between the heart size and hematocrit [10]. The study revealed that mice inhabiting warmer regions show significantly lower ventricle weight, RVW/LVW and hematocrit than those inhabiting cold regions. Also, a significant positive relationship was observed between the level of hematocrit and RVW/LVW (Figure 11). These reports suggest that hematocrit affects heart size, particularly in the right ventricle and the decline in RVW/LVW.

Decline in pulmonary artery pressure is predicted where reduction in right ventricle size and decline in RVW/LVW occur. Sakai (1984) examined the hemo-dynamics of lung circulation when hematocrit is gradually elevated by transfusion of red blood cells to sheep. Both systemic and pulmonary artery pressure rise with the increase in hematocrit, but the rise in the pulmonary artery was more remarkable. A higher hematocrit level resulted in higher pulmonary artery pressure and caused larger load on the right ventricle [12]. Therefore, in the present study, decline in the hematocrit and pulmonary artery pressure caused by the global warming is suggested to have resulted in the reduction in heart size, right ventricle weight and RVW/LVW level, which in turn caused reduction in the right ventricle size.

As observed above, the interaction between HPV and increase in blood viscosity because of an increase in the Ht appears to be certainly involved in pulmonary hypertension and right ventricular hypertrophy observed at high altitude (Figure 17).

Therefore, we believe that analysis of these two factors is a prerequisite for the elucidation of high-altitude pulmonary hypertension and right ventricular hypertrophy.

The increased pulmonary artery pressure or right ventricular hypertrophy at high altitude has been reported to vary widely among species and individuals of the same species, even when they are exposed to the same altitude [2,3,5,6,7,9]. Reeves et al.[9] reported species differences in the increase in pulmonary artery pressure resulting from chronic exposure to high altitudes; although pulmonary hypertension induced by exposure to high altitudes was remarkable in the cow and horse, it was minimal in the llama, dog, sheep and rabbit. It was also shown that there are two types of cow, i.e., the susceptive type, which shows a marked increase in pulmonary artery pressure when exposed to high altitude and the resistant type, which is less responsive to changes in altitude [9]. Genetic factors have been suggested to play an important role in terms of sensitivity to exposure to high altitude. Similar differences in the responsiveness to hypoxia are also noticed in humans; some individuals develop marked pulmonary hypertension but others do not [3,8].

When these observations are taken into together, a small degree of pulmonary hypertension or right ventricular hypertrophy at high altitude indicates better adaptability to it. Pika, which is an animal completely adapted to high altitude and has relatively low Ht level, no pulmonary hypertension or no right ventricular hypertrophy, is a good example [13,14]. Moreover, HPV is significantly lower in the pika than in the rat [2].

In conclusion, the pika, blue-sheep and Yachi-nezumi have developed almost the same physiological adaptation mechanism, i.e., attenuated HPV and deficient Ht increase for a high-altitude environment, as a result of their long history of habitation at high altitude, through a natural selection of better-adapted individuals. Genetic factors have been suggested to play important roles in this adaptability.

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