Theophylline interferes with ligand binding in radioimmunoassays for somatostatin

Theophylline is often utilized as a secretagogue in studies on the in vitro release of somatostatin and other hormones. We report here on the effect of theophylline on ligand binding in somatostatin radioimmunoassays performed with three different antisera. For all three radioimmunoassays, tracer binding both in the presence and absence of unlabelled somatostatin was inhibited by the addition of theophylline to the RIA reaction mixture. This effect occurred at final assay concentrations of theophylline which may likely be encountered when assaying samples obtained in studies of theophylline-induced hormone release in vitro. The possible interfering effect of theophylline in radioimmunoassays should be considered whenever theophylline is used as a tool to study hormone release.