NO⁻₂ and NO⁻₃ (NO_x ) accumulation in transfected U937 cells. (A) Undifferentiated U937 cells without or with L-NMA (1 mmol/L; n = 3). (B) PMA-differentiated U937 cells without or with L-NMA (500 μmol/L). Cell-free supernatant was collected at 24 hours for measurement of NO⁻₂ and NO⁻₃ (n = 5). (C) Effects of PMA and LPS on NO⁻₂ and NO⁻₃ accumulation at 24 hours (see Materials and Methods; n = 3). All data are means ± SE.