Manabu Tsukitani

The effect of oxygen on the development of atherosclerosis in WHHL rabbits

The effect of hyperoxic or hypoxic inhalation on blood lipid levels and on the development of atherosclerosis was studied in young male WHHL rabbits. They were exposed to ordinary room air containing different concentrations of oxygen: 6 animals were exposed to 40% oxygen (hyperoxia group) or 5-10% oxygen (hypoxia group) for 5 h a day, 5 days a week for 8 weeks. Four control rabbits inhaled ordinary room air. The following results were obtained. (1) The severity of aortic lesions significantly decreased in the hyperoxia group and increased in the hypoxia group, when these two groups were compared. However, both hypoxic and hyperoxic groups did not statistically differ from the control. (2) Plasma cholesterol levels were not changed either by hyperoxic or hypoxic inhalation. (3) Plasma triglyceride levels were elevated only in the hypoxia group, with significant differences from the values in both control and hyperoxia groups. (4) There was no significant correlation between mean plasma lipid level and severity of aortic lesions. From these results, we conclude that hyperoxic or hypoxic inhalation respectively regresses or aggravates the development of atherosclerotic lesions, not by an indirect action on blood lipid concentrations but by a direct action on the vascular wall.

Effect of hypoxia on cholesterol accumulation in cultured rabbit aortic smooth muscle cells

We studied the effect of hypoxia on cholesterol accumulation in cultured rabbit aortic smooth muscle cells, which were incubated in a medium with normolipemic rabbit serum (NRS) or hyperlipemic rabbit serum (HRS). The cells were incubated in a humidified atmosphere of either 20% O2, 75% N2 and 5% CO2 (control cells) or 2% O2, 93% N2 and 5% CO2 (hypoxic cells). In a medium containing 20% NRS, the free cholesterol level of hypoxic cells was only a little higher than that of control cells, and there was no significant difference in esterified cholesterol content. On the other hand, in a medium containing 20% HRS, the free cholesterol level was slightly higher and the esterified cholesterol level was markedly higher in hypoxic cells compared with control cells. These results show that hypoxia promotes the accumulation of cholesterol, especially as ester, in smooth muscle cells cultured with hyperlipemic serum. These in vitro experiments indicate that hypoxia in the arterial wall associated with hyperlipidemia may play an important role in atherogenesis, although the precise mechanism remains unclear.

Effect of Tissue Hypoxia on the Development of Atherosclerosis

It has been suggested that hypoxia enhances the development of atherosclerosis. The aim of this study was to clarify the mechanisms. Experiments were designed to observe the effects of hypoxic and hyperoxic inhalation in WHHL rabbits and have revealed that atherosclerotic lesions in the hypoxia group were larger than those in the hyperoxia group. Since there was no fluctuation in plasma cholesterol level, the effect of hypoxia was probably a direct one. Cell culture studies were carried out, using rabbit aortic smooth muscle cells and fibroblasts, and the cells were incubated in the medium supplemented with hyperlipidemic serum. The esterified cholesterol contents in the cells under the hypoxic condition were 2–3 times higher than those under control conditions. Analysis of cholesterol metabolism revealed that hypoxia increased ACAT activity and suppressed cholesterol efflux.