Solid-phase radioimmunoassay and chemiluminoimmunoassay for somatostatin in unextracted rat plasma

Measurement of somatostatin (SS) in small microliter volumes of rat plasma obtained from the hypophysial portal circulation would be aided by a sensitive and robust technique not dependent on extraction. We have developed radioimmunoassay (RIA) and chemiluminoimmunoassay (CIA) for SS in unextracted rat plasma from a previously described solid-phase method in human plasma. Plasma SS was captured by primary antibody bound to second antibody coated polystyrene bead. After incubation the bead was washed and [125I]Tyr1-SS added. Following overnight incubation and washing, the bead was counted. The sensitivity for 50 microliters plasma was 11 pg/ml. Parallel displacement with abdominal portal and hypophysial portal plasma was demonstrated. For direct CIA, SS was labeled with acridinium and purified using 2 sequential gradient elutions on reversed-phase HPLC. Labeled SS performed satisfactorily in solution CIA, but did not bind well in solid-phase CIA. Hence, an indirect CIA using biotinylated SS and quantitation by acridinium labeled streptavidin was established with equivalent performance to solid-phase RIA. In summary, we have developed and validated sensitive and robust solid-phase RIA and indirect CIA for SS in unextracted rat plasma. This solid-phase method could serve as a universal system for the measurement of other neuropeptides in rat plasma.