Background: As carcinoid tumors produce and secrete serotonin, various serotonin markers in blood, plasma and urine have been used as diagnostic tools, and quantification of the urinary excretion of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) is the method most frequently used.
Methods: [3H]lysergic acid diethylamide ([3H]LSD) binding to the platelet serotonin 5-HT2A receptor was investigated in nine patients with carcinoid tumors. The possible effect of serotonin-rich food on the receptor binding was also investigated.
Results: B(max) for [3H]LSD binding was significantly lower in the carcinoid group than in the control group (mean +/- SD: 17.6 +/- 1.3 vs. 23.9 +/- 5.2 fmol/mg protein; p = 0.007). Kd for [3H]LSD binding was significantly higher in the carcinoid group than in the control group (median: 1.14 vs. 0.71 nmol/L; p = 0.03). B(max) was inversely related to the urinary 5-HIAA excretion, but the correlation did not reach statistical significance (r(s) = -0.57; p = 0.14). Intake of five bananas per day for one week had no effect on B(max) or Kd in healthy volunteers.
Conclusions: The results are consistent with a down-regulation of the 5-HT2A receptor as a response to the high serotonin levels found in patients with carcinoid tumors. Intake of serotonin-rich food does not affect the receptor characteristics. Further studies are needed to determine whether the platelet 5-HT2A receptor status can be used as a supplement to urinary 5-HIAA and other biochemical variables in carcinoid tumors.
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