The pentagastrin test in the diagnosis of the carcinoid syndrome

Serotonin (5-HT) and substance P (SP) were assayed in peripheral blood in patients with known midgut carcinoids and hepatic metastases. All patients had supranormal basal levels of 5-HT and SP. The clinical and hormonal response was evaluated by two provocation tests, pentagastrin (PG) injection or calcium infusion. Pentagastrin caused flushing and gastrointestinal symptoms and elevated levels of circulating 5-HT, but not of SP. Pretreatment with a 5-HT2 receptor blocking agent (ketanserin) alleviated gastrointestinal symptoms but had no influence on either 5-HT release or PG-induced flushing. Calcium infusion induced carcinoid symptoms in only two of six patients, which were associated with elevated 5-HT levels (whereas elevated SP levels were seen in only one patient). We conclude that 5-HT is important for the development of gastrointestinal symptoms but not of flushing. Ketanserin may alleviate gastrointestinal symptoms but does not influence PG-induced release of 5-HT. Substance P and 5-HT do not seem to share a common release mechanism. It appears that PG testing is superior to calcium infusion as a provocative test in patients with the carcinoid syndrome.