Letters to the Editor

SENSITIVE ASSAY FOR BIOLOGICALLY ACTIVE FRAGMENT OF HUMAN PARATHYROID HORMONE

The aim of our study was to evaluate the function and growth of parathyroid tissue autografted into the forearm of hemodialysis patients using several presently available methods. In a dynamic study, the secretory function of autografted tissue was evaluated in seven patients using either zero calcium dialysate or calcium infusion. In an additional prospective study, seven patients had repeated determinations of plasma immunoreactive parathyroid hormone (iPTH) concentration on samples from both forearms, a radionuclide evaluation of autograft function using thallium-201 chloride, and real time ultrasonography. Light microscopy analysis was performed in two patients. The dynamic study demonstrated that induction of hypecacemia was followed by an increase, and induction of hypercalcemia by a decrease, in circulating iPTH in both forearms using three different radioimmunoassays, similar to what has been reported for normal parathyroid tissue. A significant gradient (ie,>2.0) of plasma iPTH concentration in samples from both forearms was observed in only three out of the seven patients of the prospective study. Two of these patients disclosed an increased uptake of ²⁰¹TI chloride at the site of autografted tissue and had an echographically detectable mass. In both, hyperplastic parathyroid tissue was removed. At present, the remaining third patient does not have other features of recurrent hyperparathyroidism. In conclusion, autotransplanted parathyroid tissue of hemodialysis patients shows an adequate response to physiologic stimuli such as hypo- and hypercalcemia. Dynamic tests, therefore, appear to be a useful tool in the assessment of its function. In addition, radionuclide and echographic studies may be reliable adjuncts in the detection of marked parathyroid autograft hyperplasia.

In this work we have studied the action of the trihydroxylated vitamin D metabolite (1α, 24 R, 25(OH)₃ D₃) on parathyrin secretion by isolated parathyroid cells.

In contrast with the results obtained with mono and dihydroxylated vitamin D, lα, 24 R, 25(OH)₃D₃ increased the secretion of parathyrin.

The effective concentration of this metabolite was 1.54 × 10⁻⁹M for cells rat and 1.54 × 10⁻¹⁰M for human cells.

We have previously discussed the action of 1 α,25-(OH)₂D₃, (24R) 24,25-(OH)₂ D₃ and (25S) 25,26-(OH)₂D₃ on parathyrin secretion by isolated rat parathyroid cells. In this work, we have compared these effects with those obtained with 1 α -OH D₃, 25-OH D₃ and 1 α -OH D₂.

In decreasing order, the activities were : 1 α,25-(OH)₂D₃> 1 α -OH D₃ (24R) 24,25-(OH)₂D₃ > 25-OH D₃ > (25S) 25,26(OH)₂D₃> 1 α -OH D₂. The presence of two hydroxyl groups with one hydroxyl group in α position seems to have the higher activity to inhibit the parathyroid secretion. At least, the nature of the side chain conformation also plays a part upon the effect of PTH release.