INTRODUCTION: Carcinoids are mainly found in the gastrointestinal (65%) and 
bronchopulmonary tract (25%). These neuroendocrine tumors secrete a wide range 
of bioactive peptides, including gastrin releasing peptide and neuromedin B, the 
mammalian analogs of bombesin. The purpose of this study was to investigate the 
quantity and localization of bombesin receptors in gastrointestinal and 
pulmonary carcinoids, and to reveal whether bombesin-like peptides (BLP) and 
their receptors are of any value in distinguishing pulmonary carcinoids from 
carcinoids of intestinal origin.
METHODS: Carcinoid tumors with pulmonary (no.=9) or intestinal (no.=15) 
localizations were analyzed by immunohistochemistry, autoradiography, and 
radioimmunoassay, to examine the presence of bombesin receptor subtypes and 
determine BLP levels in these tumors.
RESULTS: All 3 bombesin receptor subtypes (GRPR, NMBR, and BRS-3) were present 
on pulmonary and intestinal carcinoids by immunohistochemistry. In pulmonary 
carcinoids, low receptor ligand binding densities together with high and low BLP 
levels were found. Intestinal carcinoids showed predominantly high receptor 
ligand binding densities in combination with low BLP levels.
CONCLUSIONS: The expression of bombesin receptor subtypes is independent from 
the carcinoid tumor origin, and is therefore not recommended as a distinction 
marker, although carcinoids of pulmonary and intestinal origin possess different 
receptor binding affinities for bombesin and dissimilar BLP levels. The combined 
presence of bombesin and its receptors might suggest the presence of a paracrine 
or autocrine growth loop in carcinoids.