Parathyroid hormone-related protein and calcium homeostasis during the periparturient period of dairy cows

Plasma and milk concentrations of parathyroid hormone-related protein (PTHrP) at various stages of pregnancy and lactation were determined in thirty-nine 3- to 16-vear-old Brown Swiss and Red Holstein X Simmental dairy cows originating from 4 herds. Eighteen of the cows were separated into 2 groups: low-parity (LP, n = 8) cows if they were in their first or second pregnancy and high-parity (HP, n = 10) cows if they were in their third or greater pregnancy. Blood samples were collected from each cow on 1 occasion, 15 to 5 days before calving, and blood and milk samples were collected daily during 6 days after calving. Serum total and ionized calcium (Catot and Ca2+, respectively) and milk Catot concentrations were also quantified. A transient postpartum decrease of serum Catot and Ca2+ concentrations was observed, whereas milk Catot concentration was constant. Plasma concentration of PTHrP was detected in 11 of 21 cows by use of an immunoradiometric assay (range, 0.45 to 1.82 pmol/L). Daily mean (+/- SD) colostrum and milk PTHrP concentrations ranged from 3.25 (+/- 3.23) to 4.69 (+/- 1.36) nmol/L in LP cows and 2.74 (+/- 0.5) to 5.95 (+/- 0.33) nmol/L in HP cows. In all cows of the HP group and most cows of the LP group, milk PTHrP concentration was highest in the day-1 sample. Milk PTHrP concentration correlated positively with milk Catot concentration in HP cows (r = 0.5959, P < 0.0001). In contrast, there was a negative relation between milk PTHrP and milk Catot concentrations in LP cows (r = -0.3285, P < 0.02). Milk PTHrP concentration was not correlated with serum Ca2+ concentration at postpartum days 5 and 6, when serum Catot and Ca2+ concentrations had returned to prepartum values. Because correlation of the corresponding day, milk PTHrP concentration most likely is not a major determinant of Ca transport into milk and the PTHrP released into the blood stream is most likely not a major determinant of the endocrine regulation of serum Catot and Ca2+. Thus, although it is involved, PTHrP is not a major factor in the integrative endocrine, paracrine, and autocrine regulation of Ca homeostasis in lactating cows. It is hypothesized that Ca may be actively transported from blood into milk with a process modulated by PTHrP. These data suggest that PTHrP produced by the mammary gland is most likely not involved in the pathogenesis of parturient paresis (milk fever) in dairy cows.