Substance P and 5-HT in granules isolated from an intestinal argentaffin carcinoid.

Intestinal argentaffin carcinoids, thought to originate from enterochromaffin cells, occasionally contain large amounts of substance P-like immunoreactivity in addition to 5-HT. The cytoplasmic granules of one such tumour were isolated. The granules, which in the electron microscope were shown to be argentaffin, contained both substance P-like immunoreactivity and 5-HT. The results support the view that substance P is localized in a population of enterochromaffin cells where it is stored in the cytoplasmic granules together with 5-HT.