Figure 2. Thrombin-induced decrease of cAMP levels was insensitive to PTX and dependent on elevation of [Ca²⁺]_i.

A, a representative FRET experiment shows a thrombin-mediated increase in FRET despite PTX treatment to prevent G_i activation. R_iso and R_thrombin mark the minimal FRET ratio after isoproterenol stimulation and the FRET ratio about 30 s after thrombin application, respectively. B, the thrombin-induced increase of the ratiometric FRET signal, defined by R_thrombin–R_iso, of PTX-incubated cells was similar to that under control conditions (n= 9). However, R_thrombin–R_iso was reduced in PTX-treated HUVECs, that were measured in EGTA-buffered external solution without Ca²⁺ (**P < 0.01).