Somatostatin-immunoreactive nerve fibers in close association with capillaries in the small intestine.

Electron microscopic immunocytochemistry was used to identify somatostatin-immunoreactive nerve profiles around the capillary system of the small intestine in the rat and cat. The highest density of somatostatin-immunoreactive nerve profiles was seen in the mucosa, and 25-30% of all immunoreactive profiles were found immediately adjacent to the endothelial cells of capillary blood vessels. About half of the capillary endothelial cells were fenestrated. The majority of the immunoreactive nerve profiles were varicosities, containing synaptic vesicles. These observations suggest that somatostatin might be released from varicose nerve fibers and may contribute to the elevated levels of the peptide measured in portal blood as compared to peripheral blood.