Vero E6 cells were treated with either 42 μM [3H]ETAR for 0, 4, 24, 48, and 72 hours (Panel A) or 8, 21, 42, 84, 210, or 420 μM [3H]ETAR for 24 hours (Panel B). Acid-soluble extracts of cell pellets were analyzed by strong anion exchange (SAX) HPLC to determine intracellular GTP and ATP concentrations. The intracellular GTP level for untreated cells was 204 pmole GTP per nmole ATP. Each number represents the mean ± standard deviation from 3 measurements.