Changes in parathyroid hormone-related protein concentrations in bovine milk from the early stage of lactation

The concentration of parathyroid hormone-related protein (PTHrP) in the blood of healthy animals is extremely low. However, milk contains a relatively large amount of PTHrP, and the changes in its levels in the early stages of lactation in cows remain unclear. To understand the characteristics of parturient changes in milk PTHrP content and the biological implications thereof, changes in milk PTHrP concentrations were measured at 7 time intervals between parturition and 21 days postpartum in 8 primiparous and 8 multiparous Holstein cows. Based on these results, milk samples were collected from 47 primiparous and 66 multiparous Holstein cows at 3 days postpartum to investigate the relationship between milk PTHrP concentration and the variables of cow age, milk yield, and milk calcium concentration. Milk PTHrP concentration in both parity groups was found to be lowest on the day of parturition (primiparous, 3.1±0.5 nM and multiparous, 1.6±0.3 nM) but to significantly increase on day 14 of lactation in primiparous cows (6.2±0.8 nM) and day 7 of lactation in multiparous cows (4.1±0.2 nM). Comparison of the 2 groups revealed that milk PTHrP concentrations in primiparous cows were higher than those of multiparous cows in the first 3 days of lactation. Although a significant negative relationship was found between milk PTHrP concentration and both age (r=−0.65) and milk calcium concentration (r=−0.19) at 3 days postpartum, no significant correlation was found between milk PTHrP concentration and milk yield. The study thus identified 3 unique characteristics of milk PTHrP concentration in the early stages of lactation: milk PTHrP concentration is higher in primiparous than multiparous cows, milk PTHrP concentration is lower in colostrum than later milk, and the difference in milk PTHrP concentration between primiparous and multiparous cows at 3 days postpartum is more strongly influenced by age than milk yield.