Suppression of parathyroid hormone inhibitory activity of plasma in pseudohypoparathyroidism type I by i.v. calcium.

The biological response, as measured in the cytochemical bioassay, to the high circulating levels of immunoreactive parathyroid hormone (iPTH) in patients with pseudohypoparathyroidism type I (PSPI) is affected by the presence of an endogenous inhibitor. This is evidenced by the reduced recovery of the biological activity of PTH added to the plasma. Here we have used i.v. calcium infusion to inhibit the release of PTH and studied endogenous PTH bioactivity and immunoreactivity as well as the recovery of the bioactivity of added PTH in a normal subject and two patients with PSPI. In the control subject both bioactive PTH (bioPTH) and iPTH fell in response to the increased levels of plasma calcium and the recovery of the biological activity of PTH added to the plasma remained unchanged. In the PSPI patients, although the level of iPTH fell, the level of bioPTH remained unchanged. However, the level of inhibitory activity of the plasma, as determined by the recovery of added PTH, was transiently normalized in response to the rise in plasma calcium. Thus i.v. calcium infusions not only suppress the release of endogenous iPTH but also the inhibitory activity present in the plasma of PSPI patients.