Determination of bioactive rat parathyroid hormone (PTH) concentrations in vivo and in vitro by a 2-site homologous immunoradiometric assay

Abstract

A new homologous 2-site assay for rat parathyroid hormone (IRMA), developed by Immutopics, Inc., has been evaluated and compared with a bone cell cAMP bioassay. Circulating PTH for adult rats assayed with this IRMA are in the range 10–15 pg/ml, and of the same order of magnitude as published values for biologically active PTH. The standard curve for the IRMA was linear over the range 3.4–240 pg/ml of rPTH 1–34, and serum samples diluted in parallel with the standard curve. The within-assay and between-assay coefficients of variation ranged from 5.2% ( n=18) to 7.6% ( n=24) and 8.3% ( n=16) to 26.4% ( n=10), respectively. Serum PTH values (mean±S.E.) for parathyroidectomized rats were 3.5±0.6 pg/ml ( n=18) versus 10.3±1.4 pg/ml ( n=16) for intact non-mated rats. Calcium injections suppressed circulating PTH by 50%. Lactating rats had serum PTH levels 5-fold higher and vitamin D deficient rats 60-fold higher than non-mated controls. PTH secreted from parathyroid cells in vitro was in the range 60–490 pg/ml as determined by the IRMA. These values represented 86.0±9.0% of the comparable bioassay values, indicating that the IRMA detects only bioactive PTH.