A new radioimmunoassay for the midregion of canine parathyroid hormone.

A new radioimmunoassay for canine PTH employs a goat antiserum against native bovine and human PTHs, with either [125I]bPTH-(41-84) or [125I] [tyr43]-hPTH-(43-68) as Tracer. The assay is monospecific for the midregion (44-68) of PTH. Assay of 40-microliters aliquots allowed detection of PTH in 20 of 22 sera from normal dogs, with these normal iPTH levels inhibiting tracer binding by 19-68%. iPTH levels rose markedly after a decrease in serum calcium ion activity induced by intravenous injection of disodium EDTA; the rise could be detected at 3 min and showed two peaks, one at 3-6 min and one at 10-15 min. After parathyroidectomy, PTH immunoreactivity disappeared rapidly from serum. After nephrectomy, iPTH increased many-fold within 12 h. The assay has a similar specificity and sensitivity to our recently described midregion assay for human PTH and should prove useful in studies of normal parathyroid physiology.