Radioimmunoassay of bovine parathyroid hormone

A radioimmunoassay of bovine parathyroid hormone (PTH) was developed, using guinea pig antiserum against bovine PTH and highly purified 125I-labeled bovine PTH. The 125I-labeled PTH which had been purified by antiserum against PTH had higher sensitivity than that purified by the adsorption method with microfine precipitated silica Quso G 32. An equilibrium and an unequilibrium incubation methods were used comparatively. 125I-labeled PTH was incubated at the same time, with antiserum against PTH, and a sample in the former and added after preincubation in the latter, which gave better results than the former. By the unequilibrium incubation method, the plasma PTH level was measured in nonpregnant cows infused experimentally with ethylenediamine tetraacetic acid (EDTA) and untreated pregnant cows 2 to 30 days before parturition. As a result, plasma PTH levels were raised after EDTA infusion. The plasma PTH levels in cows of the late stage of pregnancy were 1.16 +/- 0.93 ng/ml. A reciprocal relationship was observed between plasma PTH and calcium levels. It was statistically significant with an inverse correlation coefficient of -0.661 (P less than 0.001).