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Background/aims: Serotonin is the diagnostic hallmark of midgut carcinoids, but the pathophysiology of spontaneous flushing is unknown. The aim of this study was to assess to what extent serotonin and catecholamine blood levels are related in time with spontaneous midgut carcinoid flush., Methods: Using specific radioenzymatic assays, we measured prospectively before, during, and after spontaneous flushing platelet-poor plasma and whole blood serotonin and plasma catecholamines and their metabolites in 10 patients with primary midgut carcinoids. Blood was drawn simultaneously from a forearm vein and an external jugular vein draining the flushing area., Results: During flushing, plasma serotonin and norepinephrine levels increased (P < 0.001) over preflush levels at both sampling sites. Intraflush serotonin and norepinephrine were twice as high (P < 0.01) in external jugular (9.57 +/- 1.40 ng/mL and 857 +/- 33 pg/mL, respectively) than in antecubital plasma (4.59 +/- 0.73 ng/mL and 471 +/- 26 pg/mL). Preflush and postflush levels were similar at both venous sites., Conclusions: Vein plasma serotonin and norepinephrine levels do increase during midgut carcinoid flush, especially in the flushing area. This may reflect a local release secondary to flush but also suggests a role for these bioamines in the pathogenesis of flushing.

Detailed studies of protein metabolism were undertaken in a patient with pellagra and hypoproteinemia associated with the carcinoid syndrome both before and after treatment. The synthesis of albumin improved from 82 mg per kg per day to 135 mg per kg per day with little change in the daily excretion of 5-hydroxyindole acetic acid. After treatment with nicotinamide the patient made good progress with a complete resolution of the signs of pellagra and protein malnutrition. These results support the hypothesis that a reduced availability of the essential amino acid L-tryptophan may limit the synthesis of albumin and nicotinic acid in patients with the carcinoid syndrome who become anoretic.

The effect of a 550-kcal mixed meal and of an intravenous injection of pentagastrin (0.06 microgram/kg body wt) upon peripheral blood serotonin concentrations has been compared in 10 carcinoid patients with hepatic metastases and healthy subjects. The fasting concentrations of blood serotonin in the patients (range 790-4500 ng/ml) were elevated compared with healthy subjects (range 71-310 ng/ml; n = 15). Urinary output of 5-hydroxyindole acetic acid was elevated in 8 patients but was in the healthy range for 2 patients. The healthy subjects (n = 9) responded to food with an increase in blood serotonin (maximum rise over mean basal of 32% +/- 4%) that was significant at 60, 75, 90, and 105 min postcibal. All carcinoid patients responded to food with a comparable (25% +/- 11% over basal) rise in serotonin but the pattern of release was erratic. All patients with tumor metastases exhibited symptoms of the carcinoid flush after eating, but there was no correlation between occurrence and severity of the flush and occurrence and magnitude of the rise in serotonin. Intravenous pentagastrin evoked a flush in all carcinoid patients, but there was no significant increase in peripheral blood serotonin either in the patients or in healthy subjects.