Wall E H, Auchtung T L, Dahl G E, Ellis S E, McFadden T B
Lactation and Mammary Gland Biology Group, Department of Animal Science, University of Vermont, Burlington, 05405, USA.
J Dairy Sci. 2005 Jun;88(6):1994-2003. doi: 10.3168/jds.S0022-0302(05)72875-7.
Exposure to short day photoperiod (SD; 8 h light:16 h dark) during the dry period increased milk yield of cows in the subsequent lactation. We hypothesized that this effect is due to increased growth of mammary cells in response to enhanced prolactin signaling to influence the insulin-like growth factor (IGF) axis. Multiparous Holstein cows were dried off 60 d before parturition and assigned to long day photoperiod (LD; 16 h light:8 h dark) or SD during the dry period. Mammary biopsies were obtained at approximately -40, -20, -10 and +10 d relative to expected calving. Expression of IGF-I, IGF-II, and IGF binding protein-5 mRNA was assessed by real time reverse transcription-polymerase chain reaction. In SD cows, incorporation of [3H]-thymidine in vitro increased from -40 d to -20 d and was greater at -20 d than in LD cows. A later increase in proliferation was observed at -10 d in LD cows. For both groups, cell proliferation decreased during lactation. Analysis by terminal deoxynucleotidyl transferase dUTP nick end labeling revealed that the apoptotic index in mammary epithelial cells was less in SD cows than in LD cows. Expression of IGF-II mRNA increased during the dry period and into lactation and was greater in SD cows. Expression of IGF binding protein-5 mRNA increased during lactation, but was unaffected by day length. Expression of IGF-I did not differ over time or between treatments. We concluded that exposure to SD during the dry period enhanced mammary growth relative to LD, and this may be related to increased expression of IGF-II. Treatment differences in the temporal pattern of proliferation indicated the existence of a critical period wherein photoperiod affects mammary gland development during the dry period.
干奶期暴露于短日照光周期（SD；8小时光照：16小时黑暗）可提高奶牛随后泌乳期的产奶量。我们推测，这种效应是由于乳腺细胞生长增加，这是对催乳素信号增强的反应，从而影响胰岛素样生长因子（IGF）轴。经产荷斯坦奶牛在分娩前60天干奶，并在干奶期分配到长日照光周期（LD；16小时光照：8小时黑暗）或SD。在相对于预期产犊时间约-40、-20、-10和+10天时获取乳腺活检样本。通过实时逆转录-聚合酶链反应评估IGF-I、IGF-II和IGF结合蛋白-5 mRNA的表达。在SD奶牛中，体外[3H]-胸苷掺入量从-40天到-20天增加，且在-20天时高于LD奶牛。在LD奶牛中，-10天时观察到增殖的后期增加。对于两组，泌乳期间细胞增殖均减少。通过末端脱氧核苷酸转移酶dUTP缺口末端标记分析显示，SD奶牛乳腺上皮细胞的凋亡指数低于LD奶牛。IGF-II mRNA的表达在干奶期及进入泌乳期时增加，且在SD奶牛中更高。IGF结合蛋白-5 mRNA的表达在泌乳期间增加，但不受日长影响。IGF-I的表达在不同时间或处理之间没有差异。我们得出结论，相对于LD，干奶期暴露于SD可增强乳腺生长，这可能与IGF-II表达增加有关。增殖时间模式的处理差异表明存在一个关键时期，在此期间光周期影响干奶期乳腺的发育。
