In cultured rat vascular smooth muscle cells (VSMC),
inducible nitric oxide synthase (iNOS) expression evoked by
interleukin-1beta (IL-1beta) or
tumor necrosis factor-alpha was greatly enhanced in
hypoxia (2% O(2)), compared to in normoxia. In contrast, iNOS induction by
interferon-gamma,
lipopolysaccharide or their combination was barely influenced by
hypoxia. These results indicate that iNOS induction is regulated by
hypoxia in different manners, depending on the stimuli in VSMC.
Nitric oxide (NO) production in response to stimulation with
interferon-gamma plus
lipopolysaccharide was significantly decreased in
hypoxia, due to a decrease in the concentration of O(2) as a substrate. In contrast, the level of NO production in
hypoxia was almost the same as that in normoxia when the cells were stimulated by IL-1beta. In addition, cGMP increased in response to IL-1beta in
hypoxia to a level comparable to that in normoxia. Thus, it seems that the IL-1beta-induced expression of iNOS is up-regulated in
hypoxia to compensate for a decrease in the
enzyme activity due to the lower availability of O(2) as a substrate, and consequently a sufficient amount of NO is produced to elevate cGMP to an adequate level. In addition, the IL-1beta-induced synthesis of
tetrahydrobiopterin, a cofactor for iNOS, was also greatly stimulated by
hypoxia in VSMC.