Hypercalcemia in dogs with lymphosarcoma. Biochemical, ultrastructural, and histomorphometric investigations.

Dogs with lymphosarcoma and hypercalcemia had decreased trabecular bone volume and increased osteoclastic osteolysis, whereas dogs with lymphosarcoma that were normocalcemic did not have increased bone resorption. Increased osteoclastic resorption was present only in bone from hypercalcemic dogs that contained neoplastic tissue but not in bone free of tumors, suggesting that the factor(s) responsible for stimulating bone resorption were elaborated locally by the tumor tissue. Hypercalcemic dogs with lymphosarcoma had decreased concentrations of plasma immunoreactive parathyroid hormone and serum 1,25-(OH)2D compared with normocalcemic dogs with lymphosarcoma and control dogs with and without other neoplasms. Immunoreactive parathyroid hormone was not detected in lymphosarcoma tissue. The plasma concentration of 13,14-dihydro-15-keto-prostaglandin E2 (PGE2M) was increased approximately 2-fold in hypercalcemic dogs with lymphosarcoma as compared with other groups. Urine excretion of calcium, phosphorus, and hydroxyproline were increased in hypercalcemic dogs with lymphosarcoma. Ultrastructurally, lymphosarcomas were composed of tumor cells with large nuclei and a paucity of cytoplasmic organelles. Light and electron microscopic examination of parathyroid glands revealed inactive or atrophic chief cells in dogs with lymphosarcoma and hypercalcemia. The increased osteoclastic bone resorption in hypercalcemic dogs with lymphosarcoma was not mediated by increased circulating levels of immunoreactive parathyroid hormone and 1,25-(OH)2D but was dependent upon infiltration of bone marrow by neoplastic cells and, presumably, the local production of a bone resorption-stimulating factor.