Plasma kinetics of exogenous bovine parathyroid hormone in calves.

The metabolism of purified bovine parathyroid hormone-(1-84) [bPTH-(1-84)] administered i.v. by constant infusions or rapid injections was investigated in 11 normocalcemic calves. Plasma kinetics of bPTH-(1-84) were derived with a radioimmunoassay system which recognised the intact hormone but no fragments. The metabolic clearance rates ranged from 9.1-22.9 ml/min/kg. The disappearance of the intact hormone and the reversible appearance of its NH2- and COOH-terminal fragments was investigated with additional and specific radioimmunoassay systems. At 2 min after the i.v. injection of bPTH-(1-84) a NH2-terminal fragment (M approximately 3500) was recognised and at 4 and 20 min COOH-terminal fragments (M approximately 7000 and 3500, respectively) were detected. The survival time in the circulation of the intact hormone and of its fragments was comparable. In conclusion, bovine bPTH-(1-84) and its NH2- and COOH-terminal fragments are metabolised in normocalcemic calves within minutes.