The Metatarsal Cytochemical Bioassay of Parathyroid Hormone: Validation, Specificity, and Application to the Study of Pseudohypoparathyroidism Type I*

Abstract

The most sensitive method for assaying the bioactivity of PTH in unextracted plasma is the renal cytochemical bioassay. However, PTH acts on bone as well as kidney and clinical studies have suggested that the actions of circulating PTH level may be different at the two sites. We developed cytochemical bioassay for PTH based on the stimulation of glucose 6-phosphate dehydrogenase activity in the hypertrophic chondrocytes of the growth plate and the osteoblasts lining the metaphyseal trabeculae of rat metatarsal bones. The index of precision was 0.14 +/- 0.02 (SE) and the interassay variation was 31%. With this assay, plasma bioactive PTH levels in normal subjects and patients with primary hyperparathyroidism ranged from 0.5-18 ng/L and from 27-850 ng/L, respectively. Studies of patients with pseudohypoparathyroidism type I indicated that plasma PTH bioactivity in such patients is greater in the metatarsal bioassay than in the renal bioassay; no such differences were found in normal subjects or patients with primary hyperparathyroidism.

Skeletal responsiveness to parathyroid hormone in pseudohypoparathyroidism

“…It has been reported that plasma PTH bioactivity in patients with PHP type I was greater in the metatarsal bioassay than in the renal bioassay (46). Further, it was reported that no such differences were found in normal individuals or patients with primary hyperparathyroidism (46).…”

“…Further, it was reported that no such differences were found in normal individuals or patients with primary hyperparathyroidism (46). Bradbeer et al (46) suggested that PHP may be caused by a circulating inhibitor of PTH action that is less effective on the skeleton. This supports our hypothesis that bone is able to respond to circulating PTH in patients with PHP type I.…”

Studies on growth plate chondrocytes in situ: cell proliferation and differentiation

“…Previous examples of this approach include the highly sensitive cytochemical bioassays for polypeptide hormones, which for many years were the only means available for detecting normal circulating levels of these hormones. One recently developed assay was based on the effect of parathyroid hormone on hypertrophic growth plate chondrocytes (8).…”

Cytochemical bioassays: every dog has its day

“…67,68 The development of the bone CBA for PTH showed that the plasma from PHP patients generally had a higher level of PTH bioactivity compared to that measured in the renal CBA suggesting that the putative inhibitor was less active on the skeleton. 51 Although, further studies suggested that this inhibitor could be separated from intact PTH and had a molecular weight close to that for PTH 69 and that PTH missing the first two amino-terminal amino-acids acted as an antagonist of PTH bioactivity, 70 the concept of an inhibitor as part of the aetiology of PHP has been overshadowed by the extensive studies carried out on the genetic abnormalities associated with this disease. [71][72][73][74] Recently, concern has been raised regarding discrepancies between the circulating immuno-reactive level of PTH seen in secondary hyperparathyroidism resulting from renal failure and the apparent in vivo effects of PTH.…”