Calcium binding sites in the vesicles of the carotid and aortic body chief cells.

Chief cells of the carotid and aortic body chemoreceptors possess numerous cytoplasmic dense-core vesicles which are known to contain primarily dopamine. Following fixation in solutions containing 50 mM CaCl2, a 20--30 nm electron-dense particle (EDP) is often observed eccentrically located in many of the vesicles. Approximately 44% of the carotid body and 16% of the aortic body vesicles contain an EDP. The EDP probably represents the Ca++ binding site critical to the stimulus-secretion coupling events culminating in exocytosis of these vesicles. The presence of Ca++ in the cytoplasmic vesicles was verified by electron probe X-ray microanalysis.