Measurement of N- and C-terminal-region fragments of parathyroid hormone-related peptide in milk from lactating women and investigation of the relationship of their concentrations to calcium in milk.

Parathyroid hormone-related peptide (PTHrP) is found in very high concentrations in the milk of various mammals. However, little is known about its physiological role in this fluid. To obtain detailed profiles of PTHrP in milk, we measured the concentrations of PTHrP in human milk by two different region-specific assays, PTHrP(1-87) (N-PTHrP) and PTHrP(109-141) (C-PTHrP). We also examined the correlations between PTHrP and Ca concentrations in milk as well as the correlations between PTHrP and secreted milk volume. The levels of N-PTHrP and C-PTHrP were relatively low after delivery and gradually increased to 13.87 +/- 2.40 nmol/l (mean +/- S.E.M.) and 56.39 +/- 11.31 nmol/l respectively on the 10th day postpartum. N-PTHrP concentration remained steady until the 6th month postpartum when weaning starts, at which point it decreased slightly. C-PTHrP levels changed in a similar way to N-PTHrP levels but were 2- to 5-fold higher. Milk Ca concentration, and content, correlated with C-PTHrP concentration, and content (r = 0.422 and r = 0.769 respectively; P < 0.0001) but not with N-PTHrP. N-PTHrP concentration in the milk samples on the 4th day postpartum correlated with the volume of milk secreted during the 24 h before the samples were taken (r = 0.524, P < 0.01), but C-PTHrP concentration did not. These results suggest that PTHrP in human milk may play some role in the maintenance of lactation through the N-terminal region and in promoting Ca transfer into milk via the C-terminal region.