Parathyroid hormone-related peptide content of bovine milk and calf blood assessed by radioimmunoassay and bioassay.

PTH-related peptide (PTH-rP) has recently been discovered to exist in high concentrations in milk. The development of a commercial RIA for PTH-rP has allowed us to extend these studies. We measured the PTH-rP content of milk from 42 Jersey cows from a single farm in various stages of lactation. Colostrum (first milk) contained 56 +/- 12 ng/ml immunoreactive PTH-rP (iPTH-rP). The iPTH-rP contents of milk 1, 2, 3, 5, 7, and 9 months into lactation were 77 +/- 19, 59 +/- 14, 57 +/- 10, 106 +/- 11, 119 +/- 16, and 168 +/- 17 ng/ml, respectively. Plasma was obtained from 7 Jersey calves at birth and at intervals after the ingestion of colostrum. No iPTH-rP was detected in the plasma at birth. Two hours after the ingestion of colostrum, the iPTH-rP content of plasma was 81 +/- 25 pg/ml. The plasma iPTH-rP concentration continued to increase to 384 +/- 84 pg/ml at 7 h and peaked at 444 +/- 84 pg/ml 12 h after birth. Two calves were sampled through the 60th hour after birth, at which time plasma iPTH-rP was 483 +/- 36 pg/ml. The biological activity of the PTH-rP in milk and plasma was assessed by its ability to stimulate cAMP accumulation in ROS 17/2.8 cells. The specificity of this response was determined by the ability of antiserum to PTH-rP to block the activity. The biological activity of the milk samples was between 31-95% of the activity suggested by immunoassay. Biologically active PTH-rP could not be detected in any of the calf plasma samples. These results confirm the presence of biologically active PTH-rP in milk and suggest that the iPTH-rP is capable of being absorbed. However, our results indicate that the biological activity of the PTH-rP is nearly completely absent once in the systemic circulation.