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-year-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 (Ca(tot) and Ca2+, respectively) and milk Ca(tot) concentrations were also quantified. A transient postpartum decrease of serum Ca(tot) and Ca2+ concentrations was observed, whereas milk Ca(tot) 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 Ca(tot) concentration in HP cows (r = 0.5959, P < 0.0001). In contrast, there was a negative relation between milk PTHrP and milk Ca(tot) concentrations in LP cows (r = -0.3285, P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)