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SUMMARY
Foster litters were given to lactating rats whose own litters had been removed 3, 5 or 9 days previously. Lactation was restored in all groups by this re-suckling procedure within a relatively short period. The restoration of lactation became more difficult as the interval between the removal of the original litters and the application of the foster litters increased.
Changes in the nucleic acid content of the mammary gland after re-suckling were investigated in the rats whose litters were removed for 3 days before re-suckling. Both the DNAP content of the gland and its weight increased steadily in proportion to the length of the period of re-suckling, so that a fairly constant DNAP concentration was maintained in the tissue. The RNAP content and the RNAP:DNAP ratio, on the other hand, increased abruptly at about the time when milk secretion was initiated by re-suckling. The DNAP content continued to increase even after the onset of restored milk secretion. These results suggest that milk secretion after re-suckling began before full restoration of the tissue structure and that growth of the glandular tissue took place concurrently with milk secretion.
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SUMMARY
The effect of anaesthesia on lactation was studied in the lactating goat and the lactating rat.
Cyclopropane was used in the goat. All goats were milked once daily in the morning. The experimental and the control periods started on Monday and ended on Saturday; the control period was usually followed immediately by the experimental period. Average milk yields in eight experimental periods in three goats ranged from 91·4 to 108·3%, with a mean of 101·4±1·84% (s.e.) of those in the corresponding control periods.
The increase in intramammary pressure, which occurred 30–40 sec. after the start of milking of the contralateral teat in the normal lactating goat, was not observed during milking under anaesthesia.
Lactating rats were anaesthetized with sodium pentobarbitone. Six or eight pups were placed with the mother to suckle the teat for 2 hr. in the morning. This suckling regime was continued for 5 days. No increase in the body weight of litters was obtained when the litter suckled under anaesthesia. When oxytocin was injected into the anaesthetized mother, a gain in litter weight was observed during the experimental period, although it was smaller than in the controls.
In conclusion, a species specificity in the importance of the milk ejection response for the removal of milk has been demonstrated. The fact that milk secretion continued normally in the goat milked under the anaesthesia suggests that the secretion of the pituitary hormones concerned with the maintenance of milk secretion continues in the absence of the milking stimulus. In the rat, although milk secretion was maintained at a subnormal level by oxytocin replacement under anaesthesia, it was not possible to draw a final conclusion as to whether the suckling stimulus is dispensable for the maintenance of milk secretion.
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SUMMARY
In order to assess the role of the suckling stimulus in the regulatory mechanism of body weight and food consumption of lactating rats, changes in body weight and food intake under various conditions were investigated.
The results of the changes in food intake during the oestrous cycle, pregnancy and lactation, and after weaning were consistent with those reported by previous workers. The rate of increase in body weight was similar during dioestrus, early pregnancy, early lactation and after ovariectomy. Food consumption of mother rats was maintained at the lactating level for 24 hr. after the removal of suckling pups on the 12th day of lactation, then it rapidly decreased. Resumption of suckling by foster litters, 5 days after removal of the original litters, caused increase in body weight and food intake as well as the restoration of lactation in both normal and ovariectomized animals. The rate of increase in body weight arising from the resumption of suckling was similar to that observed in the ovariectomized rats that were not resuckled after removal of the litters. The increase in body weight in the resuckled rats was not necessarily associated with the enhanced food intake. In spite of vigorous suckling by pups, food intake of the resuckled animals did not show a progressive increase as in normal lactating mothers until the function of mammary glands was restored and sufficient milk was produced to increase the weight of the foster litters. After restoration of lactation food consumption of the resuckled mothers increased progressively, the increase being closely related to increased milk secretion.
The greater food intake in the resuckled rats than in the non-resuckled animals, even before restoration of milk secretion, seems to indicate a direct stimulating effect of the suckling stimulus on appetite; consumption of energy associated with milk secretion is also considered to be one of the main factors in inducing the enhanced food intake in lactating rats. A further possibility that the suckling stimulus participates in the regulation of body weight and food consumption in lactating animals by changing the ovarian function is discussed.
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SUMMARY
Changes in body weight and food consumption during lactation in rats nursing various sizes of litters were studied. The rate of increase in body weight of the mother rats during the experimental period (day 3–14 of lactation) was very similar in mothers with different numbers (2, 4, 8 and 12) of suckling pups/litter. It is suggested that the weight increase of adult female rats during lactation is related more closely to the alteration of ovarian function caused by the suckling stimulus than to the enhanced food intake during this period. Both the food intake of mother rats and the daily gain in weight of litters increased as the number of suckling pups/litter increased and as lactation advanced. Linear relationships were observed between the logarithm of the litter size and both the food intake of the mother rats and the weight gain of the litters. The presence of a close correlation between the food intake of mother rats and the quantity of milk produced by them is suggested.
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SUMMARY
In order to assess the stimulatory effect of prolactin on the secretion of progesterone from corpora lutea in lactating rats, ergocornine maleate (ECO; 1 mg/day), an inhibitor of prolactin secretion, was administered subcutaneously on days 6 and 7 of lactation in primiparous rats. By day 8 of lactation, the concentration of progesterone in ovarian venous blood fell to an undetectable level in the ECO-treated animals, while the concentration in the control animals was very high at this stage of lactation. The level of 20α-hydroxypregn-4-en3-one was significantly higher on day 8 of lactation in ECO-treated than in control rats. Lactational dioestrus was interrupted by treatment with ECO and vaginal oestrus appeared 3–4 days after the start of treatment. Administration of ECO caused deleterious depression of milk production and of food intake of mother rats. In the pair-fed control animals, lactation continued almost normally throughout the experimental period. Prolactin (1 mg/day) administered simultaneously with ECO increased progesterone to levels even higher than those in control rats and restored 20α-hydroxypregn-4-en-3-one levels to those of the controls. The effect of the drug on milk production was alleviated.
The results strongly suggest that prolactin is the most important factor in maintaining the function of corpora lutea in the lactating rat.
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SUMMARY
Changes in the rate of progesterone and 20α-hydroxypregn-4-en-3-one (20α-OH-P) secretion by the rat ovary during the course of lactation were investigated. The level of progesterone in the ovarian vein blood, which was very low on day 1 of lactation, increased progressively up to day 8 (355 μg./100 ml. blood), then declined to day 18 (45·5 μg./100 ml.) and day 21 of lactation. A high concentration of 20α-OH-P in blood was observed on day 1 and day 21 of lactation, but from day 4 to day 18 the secretion of this steroid was relatively low. The ratio of progesterone to 20α-OH-P exceeded unity from day 4 to day 16, and in the middle of lactation the concentration of progesterone in ovarian venous blood was about four times greater than that of 20α-OH-P. These changes in the secretory pattern of progestins during lactation are discussed in connexion with the effects of the suckling stimulus on prolactin and gonadotrophin secretion from the anterior pituitary gland.
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SUMMARY
The time of initiation of copious milk secretion in the mammary glands of pregnant rats and the effects of removal of ovaries and/or placentae with foetuses on lactogenesis were investigated by measuring the lactose content of the mammary glands.
In the normal pregnant rat, lactose was detected in the mammary gland on the 20th day of pregnancy, but not on the 15th. An abrupt increase in lactose content occurred on the 21st day of pregnancy.
Bilateral or unilateral ovariectomy, and Caesarean section, with or without ovariectomy, performed on the 18th or 19th day of pregnancy all resulted in a marked increase in the lactose content on the 2nd day after operation. Bilateral ovariectomy caused early delivery of the young and lactose content in the gland increased further after delivery. Foetuses in the unilaterally ovariectomized rats were retained normally to term.
Possible participation of placentae and foetuses in the mechanism of lactogenesis in pregnant animals by controlling the activity of the ovary is discussed.
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ABSTRACT
Effects of the suckling stimulus on the daily LH surge induced by chronic oestrogen treatment were examined in ovariectomized lactating rats. Wistar– Imamichi strain rats were kept under 14 h light:10 h darkness (lights on at 05.00 h). Litter size was adjusted to eight on day 1 (day 0 = day of parturition) and ovariectomy performed on day 2. Lactating rats deprived of their litters on day 0 served as non-lactating controls. Silicone elastomer tubing filled with oestradiol was implanted on day 6 or 15. Blood samples were collected through an indwelling cannula at 10.00 and 17.00 h on each day after implantation to detect daily LH surges. Daily LH surges occurred in the late afternoon in both lactating and non-lactating rats implanted with oestradiol on day 6 or 15. The amplitude of daily LH surges in lactating rats implanted on day 6 declined much more rapidly than in non-lactating rats implanted on day 6, but no significant difference was found in the profile of the LH surge between lactating and non-lactating rats implanted on day 15. Pituitary LH contents just before the daily LH surge (12.00–12.30 h) 4 days after implantation in lactating rats implanted with oestradiol on day 6 were significantly less than those in non-lactating rats implanted with oestradiol on day 6 or 15 and in lactating rats implanted on day 15. These results suggest that the mechanisms responsible for the oestradiol-induced LH surge were not impaired by the suckling stimulus, and that a rapid decline of the amplitude of LH surges observed in mid-lactation could be ascribed to the small amount of LH stored in the pituitary.
J. Endocr. (1988) 118, 311–316
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Faculty of Agriculture, Nagoya University, Nagoya 464, Japan
(Received 6 July 1976)
In 1965, Shinde, Ôta & Yokoyama reported that the initiation of lactose synthesis in the mammary gland could be advanced by ovariectomy and/or foeto-placentectomy performed on days 18 or 19 of pregnancy in the rat. The present experiments were designed to determine the stage of pregnancy at which the mammary gland was able to initiate lactose synthesis in response to ovariectomy. Corticosterone in plasma was also measured in relation to the changes around the time of lactogenesis reported previously (Ôta, Ôta & Yokoyama, 1974).
Primigravid Wistar-Imamichi strain rats, weighing between 200 and 280 g, were kept in a temperature- (23 ± 2°C) and light- (14h light: 10h darkness, lights on 05.00h) controlled animal room. The day on which sperm in the vagina or vaginal plugs were found was designated day 0 of pregnancy. Ovariectomy was performed between 17.30 and 18.00
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In order to study GH cell differentiation, we used the clonal cell lines called MtT/E and MtT/S cells, which were derived from a rat mammotrophic pituitary tumor. Although MtT/E cells are non-hormone-producing ones, Pit-1 protein is present in their nuclei, which suggests that MtT/E cells are progenitor cells of the Pit-1 cell lineage and have the potential to differentiate into hormone-producing cells. On the other hand, MtT/S cells produce GH; however, the responsiveness to GH-releasing hormone (GHRH) is weak and only a small number of secretory granules are present in their cytoplasm, which suggests that MtT/S cells are premature GH cells. In order to differentiate into GH cells from MtT/E cells as a progenitor cell, we examined several differentiation factors and found that retinoic acid (RA) induced the differentiation of MtT/E cells into GH-producing cells. RA-induced GH cells partially matured with the glucocorticoid treatment; however, the responsiveness to GHRH on GH secretion was incomplete. In order to elucidate the mechanism underlying full differentiation of GH cells, we used MtT/S cells. We treated MtT/S cells with glucocorticoid and found that they differentiated into mature GH cells with many secretory granules in their cytoplasm and they responded well to GHRH. These results suggested that MtT/E and MtT/S cells are progenitor or premature GH cells, and show different responses to differentiation factors. Our data also suggested that GH cells differentiate from their progenitor cells through multistep processes.
