Preparation of serum oxytocin and arginine vasopressin prior to radioimmunoassay: simultaneous extraction and separation on C18 Sep-Pak cartridges.

Oxytocin (OT) and arginine vasopressin (AVP) are often secreted in response to the same stimuli. The hormones are seldom assayed together, however, because of labor intensive sample preparation and the duplicate volumes required. A method has been developed for the simultaneous extraction and separation of OT and AVP from a single serum sample. The method is suited for sample preparation prior to radioimmunoassay (RIA) and reduces sample volume and processing time by 50%. Serum, supplemented with labeled and unlabeled OT and AVP, was adsorbed onto C18 (octadecasilyl-silica, ODS) Sep-Pak cartridges. After washing with phosphosaline and 3% aqueous acetone, OT was eluted with 98% aqueous acetone followed by AVP with 80% acidified (0.02 mol/L HCl) acetone. The recoveries, determined by radioactivity and RIA measurements, were 86 +/- 3% (OT) and 71 +/- 7% (AVP). Cross contamination was less than 10%. Sep-Paks extracted up to 100 pg/mL of the hormones from 10 mL of serum. The method was employed to measure OT and AVP in the pregnant ewe. Both hormones were elevated during salt-loading and dehydration and were decreased by carotid infusions of ethanol.