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This Article Full Text Full Text (PDF) All Versions of this Article: jas.2007-0726v1 86/5/1132    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Annen, E. L. Articles by Collier, R. J. PubMed PubMed Citation Articles by Annen, E. L. Articles by Collier, R. J.

Effect of continuous milking and prostaglandin E₂ on milk production and mammary epithelial cell turnover, ultrastructure, and gene expression¹

E. L. Annen^*,2, C. M. Stiening^*,3, B. A. Crooker, A. C. Fitzgerald^*,4 and R. J. Collier^*,5

^* Department of Animal Sciences, University of Arizona, Tucson 85721; and and Department of Animal Science, University of Minnesota, St. Paul 55108-6118

⁵ Corresponding author: rcollier{at}ag.arizona.edu.

Mammary epithelial cell (MEC) growth is reduced in continuously milked (CM) mammary glands, and administration of a mammogenic compound such as prostaglandin E₂ (PGE₂) at parturition might improve MEC growth in CM tissue. The objectives were to 1) compare MEC turnover, ultrastructure, and gene expression in CM and involuting mammary tissue, and 2) evaluate the effects of CM and intramammary infusion of PGE₂ on early lactation MEC turnover, ultrastructure, mammary gene expression, milk yield, and composition. First- and second-lactation cows (n = 8) were used in a half-udder model, in which one-half was dry for 60 d (CTL) and the other was CM. Udder halves (n = 16) were assigned to a postpartum (PP) treatment of PGE₂ (+PGE₂; 875 µg/10 mL of medium-chain triglyceride oil) or no PGE₂ (–PGE₂) treatment at parturition and at 72 h PP. Biopsies of CM and CTL quarters were obtained during milk stasis (MS) of the CTL half at 3 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 was 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) the expression of the proapoptotic genes Bcl-2-associated x protein (bax) and IGFBP5 and decreased (P < 0.05) -lactalbumin expression compared with CM tissue. In PP mammary 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. The 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 that 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