t-PA is released from a preformed, cycloheximide-resistant cellular pool.

Confluent HUVECs were pretreated for 20 hours with control medium (panel A) or sodium butyrate (3 mM) (panel B). The cells were then incubated for 4 hours in the absence (black bars) or presence (gray bars) of cycloheximide (5 μg/mL) to block protein synthesis, followed by stimulation for 30 minutes with histamine, forskolin, and epinephrine as in Figure 1. The supernatants were assayed for t-PA by ELISA. Sodium butyrate pretreatment caused a large increase in basal and stimulated t-PA release (compare scales in panels A and B). Cycloheximide decreased t-PA release from unstimulated cells but did not inhibit agonist-induced t-PA release. Results are the mean ± SEM of 4 independent experiments; *P < .05 by the paired Student t test, compared to CHX alone.