Effect of continuous milking and prostaglandin E₂ on milk production and mammary epithelial cell turnover, ultrastructure, and gene expression

¹ Department of Animal Sciences, University of Arizona, Tucson, AZ 85721
² Department of Animal Science, University of Minnesota, St. Paul, MN 55108-6118

^* To whom correspondence should be addressed. E-mail: rcollier{at}ag.arizona.edu.

	Abstract

Mammary epithelial cell (MEC) growth is reduced in continuously milked (CM) glands, and administration of a mammogenic compound such as prostaglandin E₂ (PGE₂) at parturition might improve MEC growth in CM tissue. Objectives were 1) to compare MEC turnover, ultrastructure, and gene expression in CM and involuting tissue and 2) to evaluate the effects of CM and intramammary infusion (IMI) of PGE₂ on early lactation MEC turnover, ultrastructure, mammary gene expression, milk yield, and composition. First and second lactation cows (n = 8) were utilized in a half-udder model in which one half was dry 60 d (CTL) and the other CM. Udder halves (n = 16) were assigned to a postpartum (PP) treatment of PGE₂ (+PGE₂; 875 µg/10 mL medium chain triglyceride oil) or no PGE₂ (-PGE₂) treatment at parturition and 72 h PP. Biopsies of CM and CTL quarters were obtained during milk stasis (MS) of the CTL half at 3 d and 7 d after dry off of the CTL half (3d-MS; 7d-MS) and postpartum (PP) at 2 and 4 d (2d-PP; 4d-PP). Milk yield was reduced (P < 0.01) in CM udder halves compared with CTL halves (13.2 vs. 22.1 kg/d), but reductions were less in second lactation cows. The apoptotic index was greater (P < 0.05) in CTL glands than in CM glands (3d-MS, 0.52% vs. 0.11% and 7d-MS, 0.24 vs. 0.12, respectively). Proliferation of MEC was unchanged at 3d-MS, but increased (P = 0.01) in CTL halves at 7d-MS compared with CM halves (3.10% vs. 0.93%). At 2d-PP, MEC proliferation was increased (P = 0.05) in CM halves compared with CTL halves (1.3% vs. 0.6%), but was unaffected by PGE₂ (P > 0.2). Apoptosis was elevated in early lactation regardless of treatment. Ultrastructure was unchanged by dry period length or PGE₂. In prepartum tissue, involution in CTL halves increased (P < 0.05) expression of the pro-apoptotic genes Bcl-2 associated x protein (bax) and IGFBP5 and decreased (P < 0.05) -lactalbumin expression compared with CM tissue. In PP tissue, CTL halves expressed greater (P < 0.05) levels of ATP-binding cassette 1 (ABC1) and IGFBP5. Treatment with PGE₂ did not alter (P > 0.1) gene expression. Results confirm that CM reduced milk yield of cows with a mammary growth requirement. Reduced MEC turnover and milk yield were not alleviated by IMI of PGE₂, which indicates peripartum PGE₂ concentrations in CM glands are not limiting mammary growth or milk synthesis.

Key Words: apoptosis, dairy cow, dry period, mammary epithelial cell, proliferation, prostaglandin