Circulating parathyroid hormone concentrations in normal and vitamin D-deprived rat pups determined with an N-terminal-specific radioimmunoassay.

Plasma PTH concentrations were determined in rats with an antiserum to human PTH (1-34) that cross reacts with rat PTH in order to evaluate the effects of growth, vitamin D deficiency and dietary modulations of plasma calcium. Displacement curves for synthetic rat PTH (1-34) and diluted rat plasma were parallel with curves for the hPTH (1-34) standards and hPTH (1-84). Parathyroidectomy in 33-day-old rats resulted in a fall in iPTH from (mean +/- SE) 8.1 +/- 0.5 to 5.4 +/- 0.5 pg/ml in an assay in which the lowest detectable concentration was 3 pg/ml. Elevation of plasma Ca by 0.7 mg/dl by dietary calcium supplementation in 25-day-old pups led to a plasma iPTH level of 4.4 +/- 0.7 pg/ml compared with 9.9 +/- 0.7 pg/ml in controls. There were no significant changes in iPTH in control rats fed a complete diet containing 0.4% Ca and 0.4% P over the age range 14-56 days. Rat pups (-D) suckling vitamin D-deprived mothers had plasma Ca and iPTH values of 7.76 +/- 0.16 mg/dl and 173 +/- 27 pg/ml, respectively, at 25 days of age and 5.8 +/- 0.2 mg/dl and 677 +/- 85 pg/ml, respectively, at 56 days. The -D pups that had access to their mothers' calcium-supplemented diet (1.6% Ca and 1.4% P) had a mean plasma Ca value that was 0.5 mg/dl above that of the control group and a mean plasma iPTH level of 6.7 +/- 0.8 pg/ml. We suggest that the RIA for PTH measures mostly, if not entirely, bioactive PTH. We conclude that plasma iPTH (a) remains essentially unchanged in normal rats during the rapid growth period, (b) can increase up to 75-fold in hypocalcemic vitamin D-deficient rats, and (c) can be suppressed to barely detectable levels when plasma Ca is raised less than 1 mg/dl above the level of normal control rats.