Sites of clearance of endogenous parathyroid hormone in the vitamin D-deficient dog.

The sites of clearance of endogenous parathyroid hormone (PTH) were studied in dogs who developed secondary hyperparathyroidism on a vitamin D-deficient diet. Simultaneous blood samples were obtained from the femoral artery, and the hepatic, renal, portal, and femoral veins. Radioimmunoassay of canine immunoreactive PTH (iPTH) in a heterologous bovine PTH (bPTH) system indicated that the kidney and liver extracted 49% and 46%, respectively, of the iPTH circulating through these organs. Characterization of the circulating iPTH in these animals was carried out by gel filtration and radioimmunoassay of the eluant fractions utilizing specific amino- and carboxyl-terminal antisera. The hormone in the peripheral circulation co-eluted with [125I]-iodobPTH and no fragments of iPTH were detected. Immunochemical differences between bovine and canine PTH were detected in the carboxyl-terminal region of the molecule. The results indicate that endogenous PTH in the vitamin D-deficient, hypocalcemic dog is cleared by the kidney and liver and that the predominant form of the hormone in this animal is similar to glandular bPTH 1-84.