Hyperinsulinism in dogs with neoplasia of the pancreatic islets. A clinical, pathologic, and ultrastructural study.

Neoplasms of the pancreatic islets in 6 of 9 dogs were functionally active and were associated with a syndrome of hyperinsulinism. The serum concentration of immunoreactive insulin in a dog with beta-cell carcinoma was increased compared to normal dogs. Metastases in the liver and lymph nodes remained functionally active and were responsive to tolbutamide following excision of the primary neoplasm. The cells comprising functional islet-cell neoplasms had ultrastructural and histochemical characteristics of beta cells. At the level of ultrastructure the neoplastic cells had definitive evidence of secretory activity and the organelles concerned with protein synthesis were well developed. Secretory granules were numerous and had unique bar-shaped or round internal cores with a wide electron-lucid halo, characteristic of the insulin-secreting cell of the canine pancreas.