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Search for other papers by I. C. HART in
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Search for other papers by J. L. LINZELL in
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SUMMARY
Experiments to investigate the relative importance of the tactile, conditioned and possible metabolic components of the milking stimulus on the release of prolactin and growth hormone (GH) from the anterior pituitary of the goat are described. A comparison of the hormonal responses to milking the auto-transplanted mammary gland (i.e. denervated gland) with that obtained by milking the intact mammary gland of the same goat showed that the concentration of prolactin in the plasma increased only after milking the intact gland, whereas in two out of four goats an increase in plasma GH was detected several minutes after milking the transplanted gland. In the intact animal significantly more prolactin (P < 0·01) was released in response to milking both teats for 6 min as compared with that released by milking only one teat for the same time. No significant difference (P > 0·1) was found for GH. Similar quantities of prolactin and GH were released by goats milked when conscious or under anaesthesia. A comparison of the hormonal responses to teat stimulation in the same anaesthetized goats with and without the removal of milk from the mammary gland showed a significant reduction (P < 0·001) in the quantity of prolactin and GH released in the absence of milk removal. The significance of these results is discussed.
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Search for other papers by R. B. HEAP in
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Sites of production and uptake of progesterone were compared in acute experiments in two sheep and two goats 119–126 days pregnant. In the two goats progesterone was produced mainly by the ovaries (up to 10 mg./day, placenta 0 mg./day) whereas in the two sheep the placenta made the largest contribution (up to 14mg./day, ovaries about 2 mg./day). Adrenal production was less than 2% of the ovarian output except in one goat (20%). In four out of five foetuses studied, umbilical arterial concentrations of progesterone were higher than umbilical venous ones. Δ5-3β-Hydroxysteroid dehydrogenase was demonstrated histochemically in the foetal adrenals but not the ovaries.
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The infusion of vasopressin in amounts thought to be released during water deprivation had no effect on the rate of milk secretion or on milk composition. These results do not support the conclusions of Konar & Thomas (1970) that vasopressin at physiological levels affects the rate of milk secretion, since the doses they used were very much higher and probably outside the physiological range. The implications of these findings and their possible adaptive significance are discussed.
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Search for other papers by J. L. LINZELL in
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While it is known that efficient lactation depends upon many hormones it is not so well known whether all act directly on the mammary glands, and the effects of fluctuation in the circulating concentrations of hormones on intact animals have been little studied.
Recent work on the mechanism of secretion of the aqueous phase of milk has suggested the site of active monovalent ion transport (Linzell & Peaker, 1971a, b). It is therefore important to know if aldosterone affects these processes in normal animals.
Lactating Saanen or Saanen-Welsh cross-bred goats were used. They were milked every day at approximately 09.30 and 16.30 h and the milk yield of each gland and the time of milking recorded. In addition, milk samples were taken every day at the afternoon milking for 4 days before, and during treatment. The techniques and analytical procedures have been described previously by Linzell & Peaker (1971
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Search for other papers by J. L. LINZELL in
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SUMMARY
Changes in the composition of the aqueous phase of milk were found to precede oestrus by up to 4 days in five out of six goats. However, the composition had returned to normal by the time oestrous behaviour was first observed. The alteration in composition was characterized by increases in milk sodium and chloride concentrations, and decreases in potassium and lactose; fat, protein and immunoglobulin concentrations were unaffected. These changes were also observed in autotransplanted (i.e. denervated) glands.
Large doses of exogenous oestrogens (to match the secretion rate in late pregnancy) administered to goats in anoestrus resulted in a significant fall in milk yield, increases in milk fat, protein, immunoglobulin and sodium concentrations, and a decrease in potassium. Chloride and lactose concentrations were not significantly changed except in some goats when the milk yield was severely affected 4 days after oestrogens were given i.m.
Possible hormonal mechanisms and the nature of the changes in the secretory epithelium before oestrus and as a result of oestrogen treatment are discussed.
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Search for other papers by J. L. LINZELL in
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SUMMARY
Plasma progesterone concentration was measured in arterial, jugular, mammary venous and ovarian venous blood of 20 goats, together with measurements of mammary and ovarian blood flow.
The level in arterial plasma in conscious normal and in castrated males was less than 2·6 ng./ml.; in anoestrous and oestrous females it was 3·4 ± 0·8 (s.e.) ng./ml. In the luteal phase of the ovarian cycle the level rose to 10·7 ± 2·2 ng./ml. and during pregnancy it remained above 10 ng./ml. until near term. In a goat that aborted at 15 weeks of pregnancy no progesterone was detected 3 days before or at the time of abortion.
When an active corpus luteum was present in the ovaries, the mammary venous level of progesterone was lower than the arterial concentration in 38 out of 44 instances, the mean difference being 23 ± 4% of the arterial level. In two goats the concentration in jugular vein blood was 80 and 85% of the arterial level but the mammary venous levels were lower still (57 and 75 % of the arterial concentration, respectively). In three goats sampled repeatedly during the whole reproductive cycle, simultaneous measurement of mammary blood flow showed that during pregnancy the udder was taking up to 2–30 ng. progesterone/min./10 g. tissue from the plasma. In a fourth, much older goat (in early pregnancy, but not lactating), the uptake was consistently lower. In all four animals the arteriovenous difference was linearly related to the arterial plasma concentration (r = 0·84).
In ovarian venous blood, collected under spinal anaesthesia, the plasma concentration was not significantly different from that in peripheral blood during anoestrus and oestrus, but when active corpora lutea were present the concentration was 620–1600 ng./ml. Simultaneous measurement of ovarian venous outflow suggested that 9–10·5 mg. progesterone/day was being produced by the ovaries, and about 20 % of this was taken up by the udder.
Peripheral blood progesterone levels were unaffected by chasing the animal but were raised by spinal anaesthesia and were raised still further after laparotomy.
Search for other papers by J. Y. F. PATERSON in
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SUMMARY
In pregnant and in lactating goats the rate of cortisol secretion was measured by continuous intravenous infusion of tritium-labelled cortisol. Uptake of cortisol by the mammary gland was measured by simultaneous sampling of blood from the carotid artery and superficial epigastric vein, and measurement of mammary blood flow. In pregnant goats the rate of cortisol secretion was 9 μg/min but in preparturient and in lactating goats the rate was about 32 μg/min. There was a small but significant difference in cortisol concentration between the carotid artery and mammary vein. In pregnant goats the mammary uptake of cortisol was about 0·2 μg/min; this was increased to about 1·3 μg/min in preparturient and in lactating animals. Negligible quantities of cortisol were secreted in the milk.
Cortisol was administered by continuous infusion for 4 h into two lactating goats. Although there was some stimulation of milk secretion it seems likely that it resulted from a systemic rather than local action of cortisol.
Search for other papers by J. R. G. CHALLIS in
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SUMMARY
Since blood oestrogen levels are much higher during pregnancy in the goat than in the sheep a continuous isotope infusion technique was used to study oestrone metabolism in five experiments on two goats during late pregnancy and early lactation. The metabolic clearance rate was similar to that in the sheep in all experiments (mean value ± s.e.m., 4·02 ± 0·25 1/min); therefore the high blood oestrone concentrations in late pregnancy (851–2043 pg/ml) were due to high oestrone production rates (4·22–7·54 μg/min). There was some conversion of oestrone to oestradiol-17β by the mammary glands and other tissues (mean conversion ratio 17·1%) and a little (< 10%) binding of oestrone to erythrocytes. The uptake of oestrone by the mammary gland corresponded to < 3% of the total production rate. These results are compared with those obtained for other steroid hormones (progesterone, cortisol) in the goat and sheep.
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SUMMARY
Glucose entry rate was measured by primed infusion of [2-3H]glucose, and cortisol secretion rate by infusion of [1,2-3H2]cortisol, in two cows from 142 days before calving to day 287 of lactation. Mammary blood flow and the mammary uptake of glucose and cortisol were also measured.
In late pregnancy, cortisol secretion rate was 8·6 ± 3·17 (s.d.) μg/min and plasma cortisol concentration was 1·8 ± 0·52 μg/l. During parturition in one animal the secretion rate was 92 μg/min and plasma cortisol concentration was 15 μg/l. During lactation the secretion rate (26·4 ± 7·14 μg/min) and plasma cortisol concentration (5·6 ± 0·73 μg/l) were significantly greater than in dry cows. The mammary uptake of cortisol was 3 to 4% of the secretion rate in both dry and lactating cows.
Glucose entry rate was 5·77 ± 2·250 (s.d.) mg/min/kg0·75 in dry cows and there was no significant mammary uptake of glucose. During lactation the glucose entry increased to 9·45 ± 1·881 mg/min/kg0·75. Mammary uptake of glucose was 3·56 ± 1·949 mg/min/kg0·75. The non-mammary utilization of glucose, glucose entry less mammary uptake, was the same for dry and lactating cows.
There was a good correlation between glucose entry and milk yield, and between mammary uptake of glucose and milk yield. Since the mammary arterio-venous glucose concentration difference was relatively constant, it is suggested that the change in mammary blood flow may determine the change in glucose uptake and milk yield.
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Search for other papers by B. P. SETCHELL in
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SUMMARY
The effect of lactation on cardiac output, measured by Fegler's thermodilution technique, and on organ blood flow, measured by Sapirstein's indicator fractionation technique, has been investigated by studying three groups of rats: (1) on day 1 of lactation; (2) on day 12 of lactation, suckling a large litter; (3) on day 12, the young having been removed on day 1. Lactation was accompanied by a significant increase in the weight of the mammary glands, the liver and the small and large intestines and a decrease in the weight of the female reproductive tract and of the skin. Cardiac output in 12-day lactating animals was significantly higher than in non-lactating and in rats on day 1. The proportion of the cardiac output taken by the mammary glands, liver, gastrointestinal tract and their blood flow/g. tissue, increased. Skin blood flow increased, but not that of brain, heart or kidney.
