Exposure of human peripheral blood monocytes to the NO donor S-nitroso- N-acetyl-DL-penicillamine (SNAP) resulted in a rapid shift in cellular conformation of spontaneously activated cells from ameboid to round. The population of activated cells. ~7.1 ± 1.2%, was reduced 7-fold to 1.1 ± 0.4% following 0.5 h exposure to SNAP. Observation of monocytes for 6 h demonstrated a gradual release from NO inhibition initiating at 2.5 h following SNAP treatment and a period of hyperactivity that was maximal at ~5 h following SNAP exposure. During the rebound from the NO inhibition phase, there was a significant increase in the population of activated monocytes and an increased responsiveness to chemotactic agents such as IL-1, IL-8, and fMLP relative to that of cells treated with the chemotactic agents alone. Conformational changes induced by SNAP were associated with a reduction in F-actin and loss of filopodial extension. The loss and recovery of F-actin staining paralleled changes in cell activity, suggesting that NO may alter cellular activity by modulation of cytoskeletal actin. These data taken together suggest that inhibition of monocyte activity by NO results in an excitatory phase observed subsequent to release from NO inhibition and increased sensitivity to chemotactic agents. We propose that this rebound from NO inhibition may provide increased immunosurveillance to rectify immunological problems that have been encountered during the period of inhibition.
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