The magnitude and mechanisms of the uptake of milk precursors by the mammary gland

Effect of rice bran as a replacement for oat grain in energy and nitrogen balance, methane emissions, and milk performance of Murciano-Granadina goats

The objective of this experiment was to study the effects of substituting oat grain with rice bran on energy, nitrogen and carbon balance, methane emissions, and milk performance in dairy goats. Ten Murciano-Granadina dairy goats in late lactation (46.1 ± 3.07 kg) were assigned to 2 treatments in a crossover design, where each goat received both treatments in 2 periods. One group of 5 goats was fed a mixed ration with 379 g of oat grain/kg of dry matter (O diet) and the other group of 5 goats was fed a diet that replaced oat grain with 379 g/kg dry matter of rice bran (RB diet). Diets were formulated to be isoenergetic and isoproteic, so bypass fat was added to reach the same amount of energy in both diets. The goats were allocated to individual metabolism cages. After 14 d of adaptation, feed intake, total fecal and urine outputs, and milk yield were recorded daily over a 5-d period. Then, gas exchange measurements were recorded individually by a mobile open-circuit indirect calorimetry system using a head box. Dry matter intake was different for both diets [1.83 ± 0.11 vs. 1.61 ± 0.08 (means ± SD), for O and RB, respectively]. Metabolizable energy intake and heat production were not significantly different between diets, with average values of 1,254 [standard error of the mean (SEM) = 110.0] and 640 (SEM = 21.0) kJ/kg of BW(0.75), respectively. Significant differences were found in milk fat content (5.3 and 6.9%, SEM = 0.36; for O and RB, respectively) and milk fatty acids: medium-chain fatty acids (17.17 vs. 12.90 g/100g, SEM = 0.969; for O and RB, respectively) and monounsaturated fatty acids (20.63 vs. 28.29 g/100g, SEM = 1.973; for O and RB, respectively). Enteric CH4 emission was lower for the RB diet (23.2 vs. 30.1g/d, SEM = 2.14; for O and RB, respectively), probably because of the higher lipid content in RB diets than O diets (11.7 vs. 4.1%, respectively). Lactating goats utilized RB without detrimental effects on energy metabolism. Higher milk fat and lower CH4 emissions were observed with the RB diet compared with the O diet.

The effects of intravenous infusions of insulin and of sodium succinate on milk secretion in the goat

The effects of intravenous infusion of insulin and of sodium succinate on milk yield and composition and on the yields of milk constituents was investigated in the goat. Depression of plasma glucose concentration to less than 40 mg/100 ml in response to infusion of insulin was associated with a decrease in milk volume and lactose yield, and increases in the contents of fat and casein and in the yield of fat. The increase in fat yield was the result of an increased output of C12–C18 fatty acids; the output of C4–C10 acids was slightly reduced. Infusion of succinate caused a reduction in the yields of milk and of lactose, protein and fat. There was no consistent change in milk composition.

The origin of the effects is discussed.

Section G. General. Nitrogen metabolism and the rumen

The fact that rumen micro-organisms have a modifying effect on the utilization of food nitrogen by ruminants has been recognized for very many years (references to early work are given in Hungate, 1966) but until about 1947, when this subject was reviewed by McNaught & Smith (1947), many of the processes involved were not well understood. Since that time a large amount of work has been done in this field and more recent reviews (Chalmers & Synge, 1954; Blackburn, 1965; Hungate, 1966; Waldo, 1968) present a much clearer picture. Most of the work covered by these authors will be mentioned only briefly in the present review and attention will be directed mainly to developments reported in the last few years. Few references to work published before about 1964 will be given as they may readily be found in later publications.

Use of maternal reserves as a lactation strategy in large mammals

The substrate demands of lactation must be met by increased dietary intake or by mobilization of nutrients from tissues. The capacity of animals to rely on stored nutrients depends to a large extent on body size; large animals have greater stores, relative to the demands of lactation, than do small animals. The substrate demands of lactation depend on the composition and amount of milk produced. Animals that fast or feed little during lactation are expected to produce milks low in sugar but high in fat, in order to minimize needs for gluconeogenesis while sustaining energy transfers to the young. The patterns of nutrient transfer are reviewed for four taxonomic groups that fast during part of or throughout lactation: sea lions and fur seals (Carnivora: Otariidae), bears (Carnivora: Ursidae), true seals (Carnivora: Phocidae) and baleen whales (Cetacea: Mysticeti). All these groups produce low-sugar high-fat milks, although the length of lactation, rate of milk production and growth of the young are variable. Milk protein concentrations also tend to be low, if considered in relation to milk energy content. Maternal reserves are heavily exploited for milk production in these taxa. The amounts of lipid transferred to the young represent about one-fifth to one-third of maternal lipid stores; the relative amount of the gross energy of the body transferred in the milk is similar. Some seals and bears also transfer up to 16-18 % of the maternal body protein via milk. Reliance on maternal reserves has allowed some large mammals to give birth and lactate at sites and times far removed from food resources.

Apolipoprotein B (apoB) concentrations in lipoproteins in cows

The concentrations of apolipoprotein B (apoB)-48 and apoB-100 in triglyceride-rich lipoproteins (TRL), intermediate density lipoprotein (IDL), and low density lipoprotein (LDL) fractions separated by gel permeation chromatography were determined in Holstein and Japanese black cows by enzyme-linked immunosorbent assay (ELISA). A significant correlation (p < 0.01) was observed between apoB-48 in TRL and plasma triglyceride (TG) levels in both Holstein and Japanese black cows. Additionally, apoB-48 in TRL and plasma TG levels in Holstein cows were significantly lower (p < 0.01) than those in Japanese black cows. These results suggested that TG derived from intestinal (exogenous) TRL rather than from liver (endogenous) TRL was the major source of milk fat.

Glucose 6-phosphatase activity in pregnant and lactating mammary glands of the mouse

Glucose 6-phosphatase activity was studied in the secretory epithelial cell and other cell types composing alveoli of the mammary gland (cytochemical study) and in the whole mammary gland (biochemical study) of pregnant and lactating mice. The reaction product for the enzyme activity was seen in the endoplasmic reticulum and nuclear envelope in secretory epithelial cells from all animals examined (days 7 and 14 of pregnancy, and days 0, 3, 10, and 20 of lactation. The amounts of the reaction product appeared scarce at day 7 of pregnancy, moderate at day 14 of pregnancy and day 0 of lactation, and abundant at days 3 and 10 of lactation. The reaction product, however, became generally scarce at day 20 of lactation. Biochemical activity was relatively low at days 7 and 14 of pregnancy and days 0 and 20 of lactation, while it was high at days 3 and 10 of lactation. The increased activity is probably related to functions of secretory epithelial cells in the lactating gland.

Uptake of substrates for milk-fat synthesis by lactating-rabbit mammary gland

1. A cannulation technique is described for measuring arteriovenous differences across the lactating-rabbit mammary gland. 2. Analysis of milk obtained before and after surgery shows no effect of cannulation on milk constituents. 3. Results of blood analysis show significant net changes in the concentrations of glucose, acetate, 3-hydroxybutrate, triacylglycerols and non-esterified fatty acids across the mammary gland. 4. The molar proportions of individual fatty acids in both the triacylglycerol and non-esterified fatty acid fractions did not alter between the arterial and venous samples. 5. The extraction rates are compared with those obtained from other species.

Acetate utilization by the isolated perfused guinea-pig mammary gland

Acetate uptake by isolated perfused guinea-pig mammary glands was approximately 1.0 mg g-1 h-1 when perfusate acetate concentrations were in the physiological range (5-11 mg/100 ml plasma). At perfusate concentrations below this range (mean 3.62 mg/100 ml plasma) the uptake was not significantly different, but in one experiment in which the mean acetate concentration was 14.5 mg/100 ml plasma the uptake was markedly elevated. Radioactivity from Na (1-14C) acetate was incorporated into CO2 and milk and tissue fat, being largely present in C16 and C18 fatty acids. The guinea-pig is atypical of non-ruminants and similar to ruminants in respect of the blood concentration and mammary utilization acetate.

Transport of dietary cholesterol into blood and milk of the goat

Position 4 labeled carbon-14 cholesterol was placed in abomasums (stomachs) of two lactating goats. Blood and milk samples were collected from the animals for 5 to 13 days. Specific activities of cholesterol and cholesteryl esters in various fractions of blood serum and milk were determined to reveal pathways by which dietary cholesterol enters milk. Results with the two animals showed similar trends. Within 24 h both cholesterol and cholesteryl esters of the three principal serum lipoproteins of the goat were labeled, and this labeling persisted in substantial degree for the 13-day experiment. Specific acities for cholesteryl esters in milk fat globules exhibited several remarkable attributes: they fluctuated in intensity with a 3-to 4-day cycle reaching a maximum at 7 to 8 days after tracer injection; they exceeded cholesteryl ester specific activity in the skim milk by an order of magnitude; and at their maximum they exceeded all specific activities for serum components. The results of this investigation exemplify the ease with which dietary cholesterol enters and crosses membranes in the animal body.

Stereospecific analysis of high melting triglycerides of bovine milk fat and their biosynthetic origin

Fatty acid composition, triglyceride molecular weight distributions, melting ranges, amount of cis and trans isomers, and stereospecific fatty acid distributions were determined for high melting triglyceride fractions and bovine milk fat free of them. Distributions of fatty acids on the high melting triglycerides differed from distributions of fatty acids in the rest of the milk fat triglycerides. The significance of these results is evaluated in relation to the biosynthetic origin of high melting triglycerides. Stereospecific distributions of these fatty acids support previous speculations that the monoglyceride pathway may play a significant role in biosynthesis of high melting triglyceride in bovine milk fat.

The effect of intravenous infusions of sterculic acid on milk fat synthesis

1. The effects of intraduodenal infusions of sterculic acid, a naturally occurring inhibitor of desaturase activity, on the yield and composition of milk were examined in a lactating goat.

2. Sterculic acid administration increased the percentage of stearic acid in milk fat, reflecting inhibition of mammary desaturase activity. Milk yield was not affected, but milk fat output decreased. Possible explanations are discussed.

3. No evidence was obtained for an alternative pathway of oleic acid synthesis from acetate.

Studies on the mode of uptake of blood triglycerides by the mammary gland of the lactating goat. The uptake and incorporation into milk fat and mammary lymph of labelled glycerol, fatty acids and triglycerides

1. The mode of uptake of the precursors of milk fat by the mammary gland of the lactating goat has been examined by infusing radioactive fatty acids, glucerol or doubly labelled triglycerides into the mammary artery or jugular vein of animals surgically prepared to permit samples of arterial and venous blood to be withdrawn without disturbance to the animal. 2. Acetate was taken up by the mammary gland and incorporated into milk fat. The decrease in the specific radioactivity of blood acetate across the gland was evidence of acetate production, but there was no significant release of labelled lipid from the mammary gland. 3. When labelled long-chain fatty acids or glycerol were infused into the lactating goat, there was extensive transfer of radioactivity into milk in spite of the absence of net uptake of substrate by the mammary gland. The decrease in the specific radioactivity of each substrate across the mammary gland, however, showed that both fatty acids and glycerol were simultaneously taken up and released by mammary tissue. 4. The infusion of chylomicra and triglyceride emulsions labelled with (3)H and (14)C revealed that both glycerol and fatty acids were released during triglyceride uptake by mammary tissue. Changes in the (3)H/(14)C ratio during the transfer of triglyceride from blood into milk showed that at least 80% of the triglyceride was hydrolysed during uptake, but the potential re-utilization of both products of hydrolysis for triglyceride synthesis in mammary tissue implied that only a minimum value could be obtained from the change in the ratio. 5. The time-course of the transfer of (3)H and (14)C into milk and lymph were closely similar after the infusion of [2-(3)H]glycerol tri[1-(14)C]oleate or of a mixture of [2-(3)H]glycerol and [1-(14)C]oleate. 6. The results were consistent with the hypothesis that plasma triglycerides are extensively or completely hydrolysed during mammary uptake.

Metabolism of cod-liver oil in relation to milk fat secretion

The effects of daily supplements of 300 g native and 2 hydrogenated cod-liver oils (CLO) to the diets of lactating cows have been examined. Native CLO supplements increased the proportion of propionate and decreased the proportion of acetate in the rumen and also decreased the yields in milk of fatty acids synthesized within the mammary gland and those derived from plasma triglycerides. These effects were reduced or eliminated when the hydrogenated oils were given instead of the native CLO. With all 3 supplements about 15% of the dietary C20and C22acids was secreted in milk.

Both native and hydrogenated CLO supplements increased the concentrations of the cholesteryl ester and phospholipid components of the α-lipoproteins in the blood plasma. These components contained most of the C20and C22acids of the CLO supplements that were incorporated into the blood plasma lipids. With the native CLO supplement about half of the C20and C22acids present in the blood plasma were polyunsaturated.

There were no differences in the effectiveness of high-density plasma lipoproteins as activators of triglyceride emulsions for lipoprotein lipase hydrolysis whether the lipoproteins were from cows receiving the native or the hydrogenated CLO supplements or the control diets. Significant rates of hydrolysis by lipoprotein lipase could not be demonstrated using as substrates low- or high-density plasma lipoproteins or activated emulsions of their extracted lipids.

Additional in vitro experiments using activated triglyceride emulsions as substrates for mammary lipoprotein lipase showed that replacement of a soybean oil emulsion by an emulsion of native CLO reduced the rate of hydrolysis considerably, whereas an emulsion of hydrogenated CLO was without effect unless it was present at levels in excess of 50%.

It is concluded that the change in rumen fermentation induced by the polyunsaturated fatty acids of CLO is implicated in the decreased intramammary synthesis of fatty acids, but that additional information is required before the reason for the reduced mammary uptake of plasma fatty acids can be established.

The effects of 2 methods of incorporating soybean oil into the diet on milk yield and composition in the cow

In a feeding experiment with 6 cows in mid-lactation a portion of the starch in the concentrate part of the diet was isocalorically replaced by 8% soybean oil, and the effects on the yield and composition of the milk were studied. The oil was incorporated into the diet either as coarsely ground soybeans or by direct addition of the oil itself. The concentrate mixtures were given with a high-roughage diet that supplied 5·5 kg hay and 2·7kg sugar-beet pulp/day.

When the soybean oil was included in the diet either in the form of soybeans or as the oil itself there was an increase in the yields of milk, solids-not-fat (SNF) and lactose, and an increase also in the percentage of lactose in the milk. The percentage of protein, however, was decreased.

When the oil was included in the form of soybeans there was an increase in the yield of fat but a decrease in the percentage of SNF in the milk.

When soybean oil was included in the diet, the yield of protein was increased but the yield of fat and the percentage of fat in the milk were both decreased.

With both methods of incorporating the oil there was an increase in the relative proportion of propionic acid and a decrease in that of butyric acid in the total volatile fatty acids of the rumen liquor. When the oil was added directly, but not when it was added as soybeans, there was a decrease in the proportion of acetic acid in the total volatile fatty acids. The implications of these findings are discussed.

The effects of intravenous infusions of cod-liver and soya-bean oils on the secretion of milk fat in the cow

1. The effects of intravenous infusions of emulsified cod-liver oil, Ethive (British Codliver Oils (Hull & Grimsby) Ltd; ethyl ester fraction of cod-liver oil rich in polyunsaturated acids) and soya-bean oil on the secretion of milk fat and the composition of blood lipids in the dairy cow were investigated.

2. Soya-bean oil increased the yield of total fat, by increasing the secretion of unsaturated C18 acids in milk and also increased the proportions of oleic and linoleic acids in the blood plasma triglycerides.

3. Cod-liver oil and Ethive both decreased the yield of total fat and the yields of C14 to C18 acids but had no effect on the yields of C4 to C12 acids in milk. Although the emulsions contained considerable quantities of C20 to C22 acids, which were incorporated into the blood plasma triglycerides, these acids were not secreted in the milk fat.

4. It is concluded that, if the polyunsaturated C20 and C22 acids in cod-liver oil are not hydrogenated in the rumen before their absorption from the digestive tract, they can act directly on the mammary gland, possibly through the inhibition of lipoprotein lipase, to reduce the secretion of long-chain acids in milk. In addition, cod-liver oil may have an indirect effect, through changed volatile fatty acid production in the rumen, on the secretion of short-chain acids in milk.