Expression of inducible nitric oxide synthase in human umbilical vein endothelial cells during primary culture.

The adaptive response of endothelial cells to stress may lead to the upregulation of nitric oxide (NO) production. Herein, we report inducible nitric oxide synthase (iNOS) induction in primary cultures of human umbilical vein endothelial cells (HUVEC). The enzyme expression was earlier observed in 12-h cultures, reaching maximal levels after 3 days and decreasing when cells become confluent. The time course of NO production by HUVEC paralleled iNOS expression during the whole culture period, indicating that enzyme was functionally active. Conversely, iNOS induction could not be further detected in HUVEC subcultures passed once from cells presenting maximal levels of iNOS expression in the primary culture. Induction of iNOS in HUVEC was not related to lipopolysaccharide contamination, since the enzyme expression was not affected in the presence of polymyxin B added to primary cultures. Further analysis showed that aminoguanidine, a specific iNOS inhibitor, did not affect cell proliferation, suggesting that the NO produced by HUVEC may not be directly related to cell growth. Platelet endothelial cell adhesion molecule-1 expression was upregulated during cell confluence, in contrast to the decrease of iNOS expression and activity. The data suggest that iNOS expression may be a molecular mechanism mediating the adaptive response of endothelial cells to culture environment.