Parathyroid hormone-related protein (PTHRP) has been quantified by sensitive specific immunoassays in mammary venous blood and milk from 7 days before to 7 days after parturition in the goat. A significant venousarterial concentration gradient in plasma PTHRP 1–86 concentrations was demonstrated across the mammary gland, indicating that PTHRP enters the maternal circulation and may have a role in calcium homoeostasis during lactation. Significant and sustained increases in mammary venous and milk PTHRP 1–86 concentrations were found from 1 day before parturition to 7 days afterwards, with peak concentrations of 1·57 ± 0·58 pmol/l (plasma) and 8·69 ±2·95 nmol/l (milk) (mean ± s.e.m. ) occurring on day −1 and the day of parturition respectively. Estimates of the mammary output of PTHRP into plasma in four goats averaged 9% (range 1–25%) of that secreted into milk. Suppression of maternal prolactin concentrations by bromocriptine significantly reduced milk yield and the mammary venous PTHRP concentration, without affecting the concentration of PTHRP in milk. In conclusion, parturition in the goat is associated with a sustained increase in secretion of PTHRP into both plasma and milk; the former may be involved in maternal calcium homoeostasis, whereas the latter may have a role in the neonate.

Journal of Endocrinology (1992) 133, 87–93