"Changes in the mammary function with increased number of lactations and aging"

众所周知，奶牛的产奶量随着年龄的增长和泌乳次数的增加而变化。对此已经给出了一些解释，但没有提供实验证据。因此，本研究旨在阐明乳腺功能随反复妊娠和衰老而变化的机制。在小鼠体内先后建立了4次泌乳。泌乳第1期乳腺分泌功能低于第2、3期，但乳腺细胞数量无显著差异。在第4期，产奶量随着乳腺细胞数量的减少和食物摄入量的减少而降低。不同年龄的小鼠以及相同年龄的已产和未产小鼠对给定剂量的雌激素和黄体酮的乳腺生长反应存在差异。此外，在这些生理条件下培养的分散的小鼠乳腺上皮细胞在含有或不含霍乱毒素和/或表皮生长因子的培养基中增殖不同。雌激素对不同年龄小鼠垂体和血清泌乳素水平的影响存在差异。在老年大鼠而非年轻大鼠中观察到垂体增殖的异质性，并且在年轻和年老大鼠中均被雌激素诱导。这些结果可能为揭示乳腺功能随年龄和泌乳提前变化的机制提供线索。

It is well known that the amount of milk produced by cows changes with advancing age and increased number of lactations. Some explanation has been given for this but no experimental evidence has been provided. Therefore, the present study was conducted to elucidate the mechanism by which mammary function changes with recurring pregnancy and aging. Four lactations were successively established in mice. Mammary secretory function was lower in the 1st than in the 2nd and 3rd lactations despite that no difference existed in mammary cell numbers. In the 4th lactation, milk yield reduces with decreased number of mammary cells and reduced food intake. Differences in mammary growth response to a given dose of estrogen and progesterone between mice at different ages and between parous and nuliparous mice at the same age were shown. Also, cultured dispersed mammary epithelial cells taken from mice under these physiological conditions proliferated differently in culture media with or without chorela toxin and/or epidermal growth factor. Pituitary and serum prolactin levels changed differently between different ages and between parous and nuliparous mice in response to estrogen. Heterogeneity in pituitary prolaction was observed in old but not young rats and was induced by estrogen administration both in young and old rats. These results may provide some clue to enlighten the mechanism of change in mammary function with aging and advancing lactations.