Measurement Of Prostaglandins That Activate Platelet Adenylate Cyclase In Rabbit Arterial Blood; Studies On PGI2 And 6-KETO-PGE1
The increases in platelet cyclic [3H]AMP on addition of washed rabbit platelets containing 3H-labelled adenine nucleotides to fresh arterial blood samples mixed with 1 mM 3-isobuty1-1-methyl xanthine permitted assay of any activators of adenylate cyclase present. Blood PGI2 was calculated from the difference between the increases in cyclic [3H]AMP over 30 s in the presence and absence of antibody that bound PGI2 and the effects of PGI2 standards on cyclic [3H]AMP formation in preincubated blood. The PGI2 found in rabbit blood assayed within 2.5 min of arterial puncture (0.05 ± 0.01 pmol/ml in 10 males and 0.07 ± 0.02 pmol/ml in 8 females (means ± S.E.M.)) was far less than that needed to inhibit platelet function (>1 pmol/ml). The antibody used also bound 6-keto-PGE1, a metabolite of PGI2 that was one fifth as effective, both as an activator of rabbit platelet adenylate cyclase and as an inhibitor of platelet aggregation. PGI2 and 6-keto-PGE-1 were distinguished by their halflives in citrated blood at 37°C (10 and 44 min, respectively). Fresh arterial blood contained no assayable activity after incubation for 30 min, confirming that the material measured was PGI2.After an intravenous bolus injection of 1 nmol PGI2/kg, arterial PGI2 averaged 1.7 pmol/ml after 2 min (i.e. 90% had been removed) and returned to preinjection levels after 10 min. 6-Keto-PGE1, injected intravenously at 5nmol/kg, was cleared at a similar rate. After a bolus injection of 5 μg angiotensin II/kg, the PGI2-like material in arterial blood amounted to 7 pmol/ml after 2 min, but declined rapidly to control levels within 10-30 min. This PGI2-like material had the same half-life in vitro as authentic PGI2.The results show that though physiologically significant PGI2 does not normally circulate in rabbits, angiotensin II can raise blood PGI2 to inhibitory levels. Formation of 6-keto-PGE1 is unlikely to contribute to the effects of PGI2 on platelets in rabbits