Cell growth modulates nitric oxide synthase expression in fetal pulmonary artery endothelial cells

Nitric oxide (NO), produced by endothelial (e) nitric oxide synthase (NOS), is a critical mediator of vascular function and growth in the developing l ung. Pulmonary eNOS expression is diminished in conditions associated with altered pulmonary vascular development, suggesting that eNOS may be modulat ed by changes in pulmonary artery endothelial cell (PAEC) growth. We deter mined the effects of cell growth on eNOS expression in cultured ovine fetal PAEC studied at varying levels of confluence. NOS enzymatic activity was s ixfold greater in quiescent PAEC at 100% confluence compared with more rapi dly replicating cells at 50% confluence. To determine if there is a recipro cal effect of NO on PAEC growth, studies of NOS inhibition or the provision of exogenous NO from spermine NONOate were performed. Neither intervention had a discernable effect on PAEC growth. The influence of cell growth on N OS activity was unique to pulmonary endothelium, because varying confluence did not alter NOS activity in fetal systemic endothelial cells. The effect s of cell growth induced by serum stimulation were also evaluated, and NOS enzymatic activity was threefold greater in quiescent, serum-deprived cells compared with that in serum-stimulated cells. The increase in NOS activity observed at full confluence was accompanied by parallel increases in eNOS protein and mRNA expression. These findings indicate that eNOS gene express ion in fetal PAEC is upregulated during cell quiescence and downregulated d uring rapid cell growth. Furthermore, the interaction between cell growth a nd NO in the PAEC is unidirectional.