Liquid scintillation fluid

In the present RIA system, a known amount of unlabeled aldosterone, an aliquot of plasma extract or medium samples, adjusted to a total volume of 0.3 ml by a buffer solution (1% BSA-borate buffer, pH 7.8), was incubated with 0.1 ml aldosterone antiserum, the anti-aldosterone antiserum No. 088, provided by the National Institute of Health, NIH, Bethesda, MD, USA, and diluted with 1% BSA borate buffer to 1∶3600 and 0.1 ml [³H]-aldosterone (50 Ci/ml, 1∶500 diluted, approximately 8,000 cpm, Amersham International plc., Bucks, UK) at 4°C for 24 h. Duplicate standard curves with five points ranging from 3 to 800 pg of aldosterone were included in each assay. An adequate amount (0.2 ml) of 0.5% dextran-coated charcoal (Sigma) was added and further incubated in an ice bath for 15 min. After incubation, the assay tubes were centrifuged at 1,000× g for 45 min. The supernatant was mixed with 3 ml liquid scintillation fluid (Beckman, Fullerton, CA, USA) before counting the radioactivity in an automatic beta counter (Wallac 1409, Pharmacia, Turku, Finland). The inhibition curves produced by ether-extracted rat plasma and the incubation medium of rat ZG cells were parallel to those given by unlabelled aldosterone.