Development of a human stanniocalcin radioimmunoassay: serum and tissue hormone levels and pharmacokinetics in the rat

Stanniocalcin (STC) is a polypeptide hormone that was first discovered in fish and recently identified in humans and other mammals. In fish STC is produced by one gland, circulates freely in the blood and plays an integral role in mineral homeostasis. In mammals, STC is produced in a number of different tissues and serves a variety of different functions. In kidney, STC regulates phosphate reabsorption by proximal tubule cells, whereas in ovary it appears to be involved in steroid hormone synthesis. However there is no information on circulating levels of STC in mammals or the regulation of its secretion. In this report we have developed a radioimmunoassay (RIA) for human STC. The RIA was validated for measuring tissue hormone levels. However human and other mammalian sera were completely devoid of immunoreactive STC (irSTC). To explore the possibility that mammalian STC might have a short half-life pharmacokinetic analysis was carried out in rats. STC pharmacokinetics were best described by a two compartment model where the distribution phase (t1/2 α) equaled 1 min and the elimination phase (t1/2 β) was 60 min. However the STC in the elimination phase no longer crossreacted in the RIA indicating it had undergone substantial chemical modification, which could explain our inability to detect irSTC in mammalian sera. When we compared the pharmacokinetics of human and fish STC in mammalian and fish models the human hormone was always eliminated faster, indicating that human STC has unique structural properties. There also appears to be a unique clearance mechanism for STC in mammals. Hence there are major differences in the delivery and biology of mammalian STC. Unlike fishes, mammalian STC does not normally circulate in the blood and functions instead as a local mediator of cell function. Future studies will no doubt show that this has had important ramifications on function as well.