Rat parathyroid hormone (rPTH) ELISAs specific for regions (2–7), (22–34) and (40–60) of the rat PTH structure: Influence of sex and age

Rat (r) PTH ELISAs were used to study the influence of age and sex on rPTH levels and circulating PTH molecular forms separated by HPLC. Standard curves and saturation analysis were undertaken to define epitopes. Rats were sacrificed at ∼27, 47 and 75 days. Relevant biochemical parameters and 25(OH) vitamin D were measured. Differences between sexes were analyzed by Kruskal–Wallis ANOVA, followed by Dunn’s test. Epitopes were localized in regions 2–7, 22–34 and 40–60 of rPTH structure for whole (W), total (T) and carboxyl (C) rPTH ELISAs. The W-rPTH assay only detected rPTH(1–84) and N-PTH in circulation while the T-PTH assay further detected large C-rPTH fragments. The C-rPTH assay detected all circulating rPTH molecular forms including smaller C-rPTH fragments. In both sexes, weight (p < 0.001), ionized calcium, creatinine, albumin and 25(OH)D values (p < 0.001) increased with age, while phosphate and alkaline phosphatase decreased (p < 0.001). In male rats, W-rPTH remained unchanged, while T-rPTH rose slightly (p < 0.05) and C-rPTH declined by half with time (p < 0.001). In female rats, W-rPTH (p < 0.05), T-rPTH (p < 0.001) and C-rPTH (p < 0.01) all increased in older animals. In both sexes, C-rPTH/W-rPTH and C-rPTH/T-rPTH ratios decreased between 25 and 47 days, to rise again between 47 and 75 days. The initial decrease may represent an adaptation to weaning and a change of diet between 25 and 47 days while the rise corresponds to higher calcium and 25(OH)D levels between 47 and 75 days. These changes were more pronounced in female rats, indicating an influence of sex on PTH molecular form secretion or metabolism.