Department of Animal Science, University of Vermont, Burlington 05405, USA.
J Dairy Sci. 2010 Sep;93(9):4062-70. doi: 10.3168/jds.2010-3162.
Frequent milking of dairy cows during early lactation elicits both an immediate increase in milk yield and a partial carryover effect that persists to the end of lactation. We hypothesized that the immediate response would be associated with a local increase in insulin-like growth factor (IGF)-I signaling and a consequent increase in mammary growth. Four multiparous cows were assigned at parturition to unilateral frequent milking [UFM; milking of the left udder half twice daily (2x; 0230 and 1430 h); milking of the right udder half 4 times daily (4x; 0230, 0530, 1430, and 1730 h)]. Mammary biopsies were obtained from both udder halves at 5 d in milk at 0530 h (immediately after 4x glands were milked). Incorporation of [3H]-thymidine into DNA and mammary cell apoptosis were not affected by UFM. Because biopsies were obtained when udder halves were at different postmilking intervals, our results reflected both the acute, transient mammary response to milking and the sustained mammary response to frequent milking treatment. We further hypothesized that the acute, transient response involves mechanisms distinct from those regulating the sustained response to frequent milking. To test that hypothesis, mammary biopsies were obtained from UFM cows (n=5) at 0500 h, when time postmilking was the same for both udder halves. Mammary cell apoptosis was not affected by UFM. Expression of genes involved in the IGF-I axis was analyzed to identify acute responses associated with milking, per se, versus sustained responses to frequent milking treatment. Removal of milk from 4x glands was associated with an acute increase in expression of IGF binding protein-1, -3, and -4 mRNA in 2x glands, whereas IGF-I expression was increased by frequent milking treatment. These effects, however, were significant only for expression of IGF binding protein-3. Expression of IGF-I receptor did not differ because of milking frequency but was higher in both udder halves immediately postmilking, indicating a systemic effect. We conclude that several genes of the IGF-I axis respond to milking, per se, or frequent milking treatment, via at least 3 distinct patterns. Increased milking frequency does not alter mammary cell proliferation or apoptosis at 5 d in milk; however, it may increase the bioavailability of IGF-I in the mammary gland. Moreover, the increase in local expression of IGF-I in 4x udder halves indicates a role for this gene in the immediate milk yield response to frequent milking during early lactation.
奶牛在泌乳早期频繁挤奶会立即增加产奶量，并产生部分持续到泌乳末期的持续效应。我们假设这种即时反应与胰岛素样生长因子（IGF）-I 信号的局部增加有关，并随之导致乳腺生长增加。在分娩时，将 4 头经产奶牛分配到单侧频繁挤奶[UFM；每天两次挤奶左乳房半部（2x；0230 和 1430 h）；每天 4 次挤奶右乳房半部（4x；0230、0530、1430 和 1730 h）]。在产后第 5 天的 0530 h（在 4x 腺体挤奶后立即）从两个乳房半部获得乳腺活检。[3H]-胸腺嘧啶核苷掺入 DNA 和乳腺细胞凋亡不受 UFM 影响。由于在获得活检时乳房半部处于不同的挤奶后间隔，因此我们的结果反映了挤奶对乳腺的急性、短暂反应和频繁挤奶处理对乳腺的持续反应。我们进一步假设，这种急性、短暂的反应涉及到与调节频繁挤奶处理的持续反应不同的机制。为了验证这一假设，从 UFM 奶牛（n=5）在 0500 h 获得乳腺活检，此时两个乳房半部的挤奶后时间相同。UFM 不影响乳腺细胞凋亡。分析与 IGF-I 轴相关的基因表达，以确定与挤奶本身相关的急性反应，而不是频繁挤奶处理的持续反应。从 4x 腺体中去除牛奶与 2x 腺体中 IGF 结合蛋白-1、-3 和-4 mRNA 的急性增加有关，而 IGF-I 表达则因频繁挤奶处理而增加。然而，这些影响仅对 IGF 结合蛋白-3 的表达有意义。由于挤奶频率而不是 IGF-I 受体表达没有差异，但在挤奶后立即在两个乳房半部更高，表明存在全身性效应。我们得出结论，IGF-I 轴的几个基因通过至少 3 种不同的模式对挤奶本身或频繁挤奶处理作出反应。增加挤奶频率不会改变产后 5 天的乳腺细胞增殖或凋亡；然而，它可能增加乳腺中 IGF-I 的生物利用度。此外，4x 乳房半部中 IGF-I 局部表达的增加表明该基因在泌乳早期频繁挤奶对产奶量的即时反应中起作用。
