Substance P- and calcitonin gene-related peptide-like immunoreactivities in the human carotid body studied at light and electron microscopical level.

The distribution of substance P- (SP-) and calcitonin gene-related peptide-like immunoreactivities (CGRP-LI) was studied in the human carotid body at the light and electron microscopic level. Of the different compartments of the carotid body, the glomic lobules received the densest innervation by SP/CGRP double-labelled axons, followed by interlobular arteries and interlobular connective tissue. Ultrastructurally, preterminal SP/CGRP-LI axons are unmyelinated and measure 0.09-1.02 micron in diameter with a median diameter of 0.28 micron (calculated conduction velocity: 0.5 m/s). Within the glomic lobules, vesicle-containing varicosities are numerous around glomus cells, but only few of them directly contact (non-synaptically) glomus cells. The findings are consistent with the existence of two populations of SP/CGRP-LI C-fibers, one of them probably representing chemoreceptor C-fibers, the other belonging to the ubiquitous vascular sensory innervation. In addition to its localization in nerve fibers, CGRP-LI also occurs in dense core vesicles of a subpopulation of glomus cells. In this regard, the human carotid body is unique among mammalian species studied so far, which severely hampers the interpretation of pharmacological data obtained in laboratory animals in terms of human physiology.