Induction of P5N-I enzyme activity occurs independently of the MAPK pathway. P5N-I activity of day-11 RBCs was determined before (t₀) and after a 16-hour incubation with 10 μM NE, in combination with 20 μM PD98059 (PD) and 5 μM SB203580 (SB). Shown are the mean enzyme activities ± SDs of 5 independent experiments.