Metabolism of intact parathyroid hormone in isolated perfused rat liver and kidney.

Metabolism of synthetic human parathyroid hormone (PTH) 2 X 10(-10) to 5 X 10(-9) M was studied in 16 isolated perfused rat kidneys and 12 isolated perfused rat livers. Organ clearances were measured by assays specific for intact PTH. Production of fragments was analyzed by high-performance liquid chromatography (HPLC) and radioimmunoassays specific for NH2-terminal, midmolecule, and COOH-terminal PTH. The livers cleared intact PTH and NH2-terminal immunoreactive PTH (iPTH) at the same rate. Midmolecule iPTH was cleared significantly (P less than 0.001) slower, as was COOH-terminal iPTH (P less than 0.005), and HPLC studies demonstrated production of midmolecule/COOH-terminal PTH fragments, while no NH2-terminal fragments were found. Clearance in the kidneys of intact PTH and of NH2-terminal, midmolecule, and COOH-terminal iPTH was not significantly different from clearance of inulin. No clearance of intact PTH was found in nonfiltering kidneys. HPLC studies did not demonstrate release of any PTH fragments from the kidneys. In conclusion, the liver was not selective for intact PTH, and differential hepatic clearance, possibly together with direct glandular secretion, may contribute to the predominance of COOH-terminal PTH fragments in plasma.