PARATHYROID HORMONE-RELATED PROTEIN - A REGULATED CALCIUM-MOBILIZING PRODUCT OF THE MAMMARY-GLAND

Parathyroid hormone-related protein shares similarities in sequence an d function with the endocrine hormone, parathyroid hormone. However, u nlike parathyroid hormone, a product of the parathyroid glands, parath yroid hormone-related protein has a wide distribution in tissues, incl uding the mammary gland. Although during pregnancy the expression of p arathyroid hormone-related protein in the mammary gland is low, follow ing birth, protein levels rise sharply in the gland in response to ele vations in serum prolactin. Large amounts of parathyroid hormone-relat ed protein are secreted into milk, suggesting a possible role in the n eonate. Transient phosphaturia and elevations of parathyroid hormone-r elated protein in mammary vein plasma support a possible endocrine fun ction for parathyroid hormone-related protein during lactation. Recent evidence suggests a local function for parathyroid hormone-related pr otein in the lactating mammary gland, and evidence exists that parathy roid hormone-related protein stimulates calcium secretion by the goat mammary gland. Parathyroid hormone-related protein, a putative vasodil ator, is produced by the external nutrient vasculature of the mammary gland, and levels within this tissue are regulated during lactation. I nfusion of parathyroid hormone-related protein into the ovine mammary artery increases gland blood flow, suggesting a role for the protein i n modulation of mammary gland hemodynamics. Regulation of parathyroid hormone-related protein synthesis by the lactating gland, together wit h the protein's actions on regional blood flow and calcium secretion, support an important function in the mammary gland during lactogenesis .