To determine whether
platelet-activating factor (PAF)-induced release of
cyclooxygenase products might be dependent on
G proteins in vivo, we administered
pertussis toxin (PTX) (9.7-10.0 micrograms/kg iv) to conscious pigs approximately 48 h before bolus infusions of PAF (10 ng/kg). Autoradiography of
ADP-ribosylated lung
cell membrane proteins from PTX-treated pigs demonstrated marked reduction in the amount of radiolabel ([32P]
NAD) incorporated, indicating that PTX induced ADP-ribosylation of
G proteins in vivo. PAF, infused at hourly intervals from 0-4 h, caused increases in plasma concentrations of
thromboxane B2 (TxB2) concomitant with
pulmonary hypertension and vasoconstriction in anesthetized pigs. These physiological changes were blocked or markedly attenuated by
indomethacin, indicating they were dependent on
cyclooxygenase products. In PTX-treated pigs, the PAF-induced
pulmonary hypertension and vasoconstriction were modestly attenuated, whereas the increases in plasma TxB2 were markedly attenuated. PTX prevented PAF-induced aggregation of platelets in vivo as evidenced by blockade of
thrombocytopenia. However, in vitro, PAF-induced aggregation of platelets was independent of PTX. Moreover, incubation of platelet-rich plasma with 50 microM PAF failed to increase TxB2 levels. These findings suggested that a PTX-sensitive cell other than the platelet was responsible for triggering release of TxA2 and
thrombocytopenia in vivo. We conclude that PAF-induced release of TxA2, pulmonary vasoconstriction, and
thrombocytopenia in anesthetized pigs are dependent on a PTX-sensitive
G protein; however, the residual hemodynamic effects indicate involvement of a PTX-insensitive
G protein, or alternatively,
G protein independent pathways.