Morphological aspects of secretion in the glomus cell paraneurons of the carotid body: evidence for calcium-dependent exocytosis.

Carotid bodies were studied using thin section and freeze-fracture preparations following incubation in physiological media. Glomus cells incubated in medium containing either no calcium (Ca2+), 1.1 mM Ca2+, or 19 microM ionophore A23187 alone appeared normal with no evidence of exocytosis. However, cells incubated in the presence of Ca2+ and A23187 exhibited exocytosis, and a significant (P less than 0.01) decrease in the number of catecholamine-containing dense-core vesicles per micron2 of cytoplasm. Numerous dense-core vesicles were observed aligned along the glomus cell plasma membrane, and vesicles in close apposition to the membrane exhibited an electron-dense connecting material between the vesicle and the plasma membranes. Freeze-fracture replicas of the P-face of the glomus cell plasma membrane showed numerous 40-60 nm pits indicative of fusion sites between the vesicle and the plasma membranes. These results suggested that vesicle exocytosis was Ca2+-dependent and not restricted to specific active zones on the plasma membrane adjacent to afferent nerve endings. Therefore, catecholamine secretion and neuromodulation by these paraneurons may also occur by a mechanism of paracrine secretion.