Measurement of 6-keto-prostaglandin E1 alpha in human plasma with radioimmunoassay: effect of prostacyclin infusion.

A radioimmunoassay for 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), a stable hydration product of prostacyclin (PGI2), is described. The method utilises plasma extraction with ethyl acetate and saturation analysis. Antiserum was produced in rabbits against 6-keto-PGF1 alpha-BSA conjugate, and its cross-reactions with 22 prostaglandins and related compounds were insignificant. The mean mass of added 6-keto-PGF1 alpha required to displace zero-point binding by 50 % was 55.6 +/- 5.1 pg (mean +/- SD, N = 10). The method blank was 7.8 +/- 2.4 pg/ml (N = 10), resulting in a detection limit of 12.6 pg/ml (mean blank + 2 SD). The intra-assay variation was between 4.1 - 8.2 % at concentrations between 150.3 pg/ml and 560.7 pg/ml (N = 10). The interassay variation was 12.4 % (N = 15). The recoveries of 100 pg/ml or 200 pg/ml added to the plasma samples (N = 10) were 90.2 % and 95.4 %, respectively. The 6-keto-PGF1 alpha concentrations in healthy subjects ranged from 92.7 to 394.5 pg/ml with a mean of 182.5 pg/ml in plasma. The presence of acetylsalicylic acid or indomethacin in the collection tubes caused no changes in plasma 6-keto-PGF1 alpha concentration. Intravenous infusion of PGI2 (1-8 ng/kg/min) into 6 healthy women caused linear increases in 6-keto-PGF1 alpha concentrations in plasma. After the discontinuation of PGI2 infusion, the 6-keto-PGF1 alpha levels decreased rapidly, the half-life being approximately 30 minutes.