MEASUREMENT OF N-TERMINAL-REGION 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 conc entrations in the milk of various mammals. However, little is known ab out its physiological role in this fluid. To obtain detailed profiles of PTHrP in milk, we measured the concentrations of PTHrP in human mil k by two different region-specific assays, PTHrP(1-87) (N-PTHrP) and P THrP(109-141) (C-PTHrP). We also examined the correlations between PTH rP 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+/-11.31nmol/l respectively on the 10t h day postpartum. N-PTHrP concentration remained steady until the 6th month postpartum when weaning starts, at which point it decreased slig htly. C-PTHrP levels changed in a similar way to N-PTHrP levels but we re 2- to 5-fold higher. Milk Ca concentration, and content, correlated with C-PTHrP concentration, and content (r=0.422 and r=0.769 respecti vely; P<0.0001) but not with N-PTHrP. N-PTHrP concentration in the mil k 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 PTH rP in human milk may play some role in the maintenance of lactation th rough the N-terminal region and in promoting Ca transfer into milk via the C-terminal region.