Abstract
Primiparous Holstein heifers 180 or 260 days pregnant were studied for changes in serum and mammary tissue hormones and enzymatic changes in adipose and mammary tissue with induced lactation. Mammary and adipose tissue were collected on the day of caesarean section (3 and 14 wk prepartum) and while lactating (2 wk postsurgery). Mammary lipoprotein lipase (μmoles/hr/ mg protein) increased from .5 to 46.9 while adipose lipase decreased (12 to 8) with onset of lactation. Similarly, glyceride synthesis increased 8-fold in mammary tissue and decreased 6-fold in adipose tissue. Mammary dehydrogenase activity for isocitrate, glucose-6-phosphate, and 6-phosphogluconate increased 44, 18, and 40-fold with onset of lactation. Extractable protein for mammary tissues collected at surgery and 2 wk later were 130 and 62 mg/g tissue. Corresponding adipose tissues were 8 and 7. Serum prolactin and growth hormone measured 3 consecutive days before surgery were not different from postsurgical values (days 2 to 14 during lactation). Serum progestins dropped from 15.5ng/ml presurgery to .85ng/ml by day 1 postsurgery and remained low for the rest of the test period. Serum corticoids appeared to peak day 1 postsurgery, afterwards declining to presurgery amounts. Cortisol and progesterone in mammary tissue decreased with induction of lactation while corticosterone remained constant.
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