Cytochemical bioassay of circulating concentrations of rat parathyroid hormone: application to a study of age and sex.

We have found that the cytochemical bioassay (CBA) method, originally developed to measure circulating levels of biologically active parathyroid hormone (bioPTH) in humans, also can measure endogenous concentrations of PTH in rats. Therefore, we have applied this assay method to examine the relationships between age and sex and the circulating levels of bioPTH, calcium, and phosphorus in Fischer rats. The concentration of bioPTH increased in both males and females from 5 to 15 months of age (p less than .001) with no significant sex-related difference. Mean bioPTH values ranged from 2.3 to 3.6 pg (human PTH equivalents)/ml; these values were much lower (1 to 2 orders-of-magnitude) than those reported by others who used radioimmunoassays for PTH that do not discriminate between biologically active and inert PTH fragments. The CBA values for bioPTH were also 1/4-1/5 those obtained in rat serum by newer, more sensitive methods, the N-terminal specific PTH radioimmunoassay and the bioassay based on stimulation of chick renal adenylate cyclase. Regression analysis did not reveal any significant correlation between serum calcium and age. However, serum inorganic phosphorus decreased significantly (p less than .001) from 5 to 15 months of age in both male and female rats and was much lower in females than in males (p less than .001). The fall in serum phosphorus was negatively correlated with the rise in bioPTH (p less than .001). This study, the first to our knowledge to use a CBA to detect physiological changes in the circulating levels of bioPTH in rats, demonstrates the usefulness of the assay in experiments in this species.