Coexistence of delta sleep-inducing peptide and serotonin in midgut carcinoid tumour cells in vivo and in vitro.

By immunocytochemistry, delta sleep-inducing peptide (DSIP) was demonstrated to coexist with serotonin (5-HT) in a majority of midgut carcinoid tumour cells studied in biopsies and long-term cell cultures. Tumour cell colonies were characterized ultrastructurally and by confocal laser microscopy. The cultures produced several DSIP-like peptides chromatographically separated from culture media. DSIP has not yet proved to be a useful tumour marker clinically. Provocation with pentagastrin in patients with midgut carcinoid syndrome resulted in increased peripheral levels of 5HT, but not of DSIP.