Renal and hepatic extraction of carboxyl-terminal immunoreactive parathyrin in normal man

We measured the carboxyl-terminal immunoreactive parathyrin (parathyroid hormone) in samples from twenty catheterized patients with hypertension. The radioimmunoassay reacted equally well with intact parathyrin and carboxyl-terminal parathyrin fragments. The renal arterio-venous extraction ratio (extraction ratio = arterio-venous concentration difference divided by the arterial concentration) of endogenous carboxyl-terminal immunoreactive parathyrin equalled that of inulin, with mean values for both of 0.17. The carboxyl-terminal parathyrin fragments have about the same molecular weight as inulin, and they are extracted exclusively by glomerular filtration, whereas intact parathyrin is extracted by both peritubular uptake and glomerular filtration. The findings therefore agree with a high proportion of circulating carboxyl-terminal parathyrin fragments in normal man. The mean extraction ratio of endogenous carboxyl-terminal immunoreactive parathyrin across nine livers was 0.16. Some of the hepatic extraction might be due to cleavage and uptake of intact parathyrin, which constitutes about 15% of the circulating immunoreactive parathyrin, but the findings suggest that the liver also extracts and metabolizes the circulating carboxyl-terminal parathyrin fragments in normal man.