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Parathyroid Hormone-Related Peptide Content of Bovine Milk and Calf Blood Assessed by Radioimmunoassay and Bioassay^*

U.S. Department of Agriculture, Agricultural Research Service, National Animal Disease Center, Metabolic Diseases and Immunology Research Unit Ames, Iowa 50010;
INCStar Corp. Stillwater, Minnesota 55082;
the Departments of Pediatrics and Biochemistry and Molecular Biology, Medical University of South Carolina School of Medicine Charleston, South Carolina 294253

Address requests for reprints to: Dr. Jesse P. Goff, U.S. Department of Agriculture, Agricultural Research Center, National Animal Disease Center, P.O. Box 70, Ames, Iowa 50010.

Abstract

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 PTHrP could not be detected in any of the calf plasma samples. These results confirm the presence of biologically active PTHrP 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. (Endocrinology 129: 2815–2819, 1991)

Footnotes

^* No endorsements are herein implied.

Received July 22, 1991.