Milk Fat Secretion Research Articles

Observations of the comparative anatomy and ultrastructure of mammary glands and on the fatty acids of the triglycerides in platypus and echidna milk fats

As far as gross structure is concerned the mammary glands of the monotremes consist of discrete lobules, in effect compound alveolar glands similar to those of the fully lactating marsupial. Ultrastructurally all mammary glands, prototherian, metatherian, and eutherian are identical, the alveoli consisting of a secretory epithelium invested by myoepithelium. The processes of formation and secretion of casein and milk fat are the same in the glands of all three subclasses of the Mammalia. In spite of close structural similarities the fatty acid complements of the milk triglycerides of the two monotremes, platypus and echidna, are different: echidna milk fat contains far more oleic acid than platypus milk does.

Perfusion of the isolated mammary gland of the goat.

Improvements to a previously reported technique for the perfusion of isolated goat mammary glands are described. Secretion of milk of relatively normal composition in vitro has been maintained at approximately 50% of the in vivo rate for about 12 hr. Factors found to be important for successful perfusion are a normal rate of mammary blood flow, the provision of adequate glucose, acetate and amino acids, and the inclusion in the circuit of an efficient oxygenator and dialyser.Contrary to earlier results successful perfusions have been performed with glands from anaesthetized and from shot animals, although removal of glands from the tranquillized animal under spinal anaesthesia is still preferred. There was a marked dependence of milk secretion rate on the rate of blood flow below a critical value, but at supra‐normal flows there was no correlation between these variables. Evidence has been obtained suggesting that kinins or similar substances may be formed during perfusion.The specific dependence of milk secretion on glucose is again stressed. Other carbohydrates sustained oxygen consumption similar to that shown with glucose, but by contrast did not support the secretion of water, a small amount of milk low in lactose but high in fat and protein being produced. Although milk fat secretion was dependent on the provision of acetate, the omission of β‐hydroxybutyrate, lactate and neutral fat (all shown to be absorbed by the gland in vivo) from the substrate mixture did not seriously impair milk fat secretion in vitro. Amino acids were shown to be necessary for normal rates of milk protein secretion, but the alteration of the substrate mixture to correspond to the amino acid absorption observed in vivo produced no significant change in milk secretion from that observed when amino acids were infused at rates corresponding to their occurrence in casein. The infusion of B vitamins and various hormones had no effect on milk secretion. Perfusions carried out with a conscious goat in circuit were no more successful than the completely in vitro system, but glands grafted onto recipient goats (even males) following complete removal from the donors produced substantially more milk than in vitro preparations.In some experiments milk [Na] and [Cl] increased during perfusion while [K] and [lactose] decreased, these trends being apparent in milk secreted before perfusion was begun and being probably due to the oxytocin used to produce milk ejection. In other experiments milk [Na] and [Cl] showed little change but [K] and [lactose] tended to rise.Possible factors limiting the performance of perfused organs are discussed.

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

SummaryThe 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.

Correlative secretion of protein, lactose and K + in milk of the goat.

AbstractRates of secretion of milk constituents (fat, protein, lactose, Na+ and K+) in the lactating goat were measured under normal circumstances and after injections of ouabain. In all experiments a close association was noted in the secretion rates for protein, lactose and K+. Under the influence of ouabain, the concentration of Na+ in the milk tended to rise and that of K+ to fall. The rate of milk fat secretion varied independently from the rates for the other constituents. It is reasonably assumed that the principal mechanism of milk protein secretion is by emptying of Golgi vesicles through the plasma membrane. The close correlation in rates for protein, lactose and K+ supports the contention that all three are assembled in Golgi vesicles and secreted by the same mechanism.

The relationship between plasma lipid composition and milk fat secretion in cows given diets containing soybean oil.

SummaryThe effects on the composition of the plasma lipids of the isocaloric replacement of starch in a low-fat concentrate mixture by 8 % soybean oil were investigated in a feeding experiment with 6 cows in mid-lactation. Two high-fat concentrate mixtures were given: in one, the soybean oil was incorporated into the diet by the inclusion of the appropriate amount of coarsely ground soybeans; in the other, the soybean oil was incorporated directly in the concentrate mixture. The concentrate mixtures were given with a high-roughage diet that supplied daily 5·5kg hay and 2·7kg of sugar-beet pulp.When the cows were given the 2 high-fat diets there were increases in the concentrations of total plasma fatty acids and in the concentrations of fatty acids circulating in the plasma as phospholipids and cholesteryl esters. There were increases in the concentrations of plasma unesterified fatty acids and triglyceride fatty acids when the cows were given the diet containing soybeans but there were decreases in the concentrations of these 2 fractions when the cows were given the diet containing soybean oil.The inclusion of soybeans in the diet increased the concentration of 18:0 in the plasma triglycerides and unesterified fatty acids whilst the inclusion of soybean oil in the diet increased the concentrations of 18:0 and 18:1 but decreased the concentration of 18:2 in the plasma unesterified fatty acids.When the cows were given the 2 high-fat diets there were decreases in the concentrations of 16:0 and 16:1 and increases in the concentration of 18:2 in the plasma cholesteryl esters. The inclusion of soybeans in the diet also reduced the concentrations of 18:1 and 20:3 in the plasma cholesteryl esters but increased the concentrations of 18:0 and 18:2 in the plasma phospholipids.The major fatty acid circulating in the plasma of the cows was 18:2 (46·4–55·3%) on all diets but only about 1% (0·40–1·04%) of the total plasma 18:2 occurred in the plasma triglycerides.The results are discussed in relation to the changes in the composition of the milk fatty acids produced by the cows when they were given the experimental diets.

The effects of increasing amounts of dietary coconut oil on milk-fat secretion in the cow.

Summary(1). A study is reported on the effects of 4 levels of coconut oil, added to a basal diet low in fat, on the secretion in cow's milk of fat and its component fatty acids. (2) A significant reduction in the yield of milk fat occurred at the highest level of supplementation. In terms of individual fatty acids the yields of lauric and myristic acids increased progressively with increased intake, maximum yields being obtained with the 7% level of coconut oil. Conversely the yields of caproic, caprylic, capric and palmitic acids progressively decreased with increased coconut oil intake. The yields of C18 acids were unchanged.

Section A. Physiology. Ruminant metabolism in relation to the synthesis and secretion of milk fat

Section A. Physiology. Ruminant metabolism in relation to the synthesis and secretion of milk fat

The relationships between dietary fatty acids, plasma lipid composition and milk fat secretion in the cow

SummaryThe effects of the isocaloric replacement of starch in a low-fat concentrate mixture by either 5 or 10% ‘stearic acid’ (85% pure) or by 10% ‘palmitic acid’ (85% pure) on the composition of the plasma lipids were investigated in a feeding experiment with 12 cows in mid-lactation. The concentrate mixtures were given with a high roughage diet that supplied daily 4·4 kg of hay and 2·7 kg of sugar-beet pulp. A study was made of the relationships between the compositions of the plasma and milk lipids.The inclusion of 10% ‘stearic acid’ or 10% ‘palmitic acid’ in the concentrate mixture increased the concentration of total plasma fatty acids. Irrespective of dietary treatment, about 40% of the total plasma fatty acids occurred in the cholesteryl ester fraction, 54% in the phospholipid fraction, 3% in the triglyceride fraction and 3% in the unesterified fatty acid fraction. There was a positive curvilinear relationship between the concentration of unesterified fatty acids in the plasma and the yield of total milk fatty acids.In the plasma triglycerides, the concentrations of 16:0 and 16:1 were decreased and the concentration of 18:0 was increased when the concentrate mixture contained ‘stearic acid’; the concentration of 16:0 was increased and the concentrations of 18:0, 18:1 and 18:2 were decreased when the concentrate mixture contained ‘palmitic acid’. Similar changes were observed in the compositions of the plasma unesterified fatty acids when the cows were given the different diets.In the plasma cholesteryl esters, the concentration of 16:0 was decreased and the concentrations of 18:3 and 20:4 were increased when the concentrate mixture contained stearic acid; the concentrations of 16:1, 18:3 and 20:4 were increased and the concentration of 18:2 was decreased when the diet was supplemented with palmitic acid. The addition of stearic acid to the diet increased the concentration of 18:0, 18:1 and 18:3 in the plasma phospholipids but decreased the concentrations of 16:0, 18:2, 20:3 and 20:4. When the diet contained palmitic acid the concentrations of 16:0, 16:1, 18:1 and 18:3 in the plasma phospholipids were increased but the concentrations of 18:0, 18:2 and 20:3 were decreased.The major fatty acid circulating in the plasma of the cows was 18:2, which accounted for about 45% of the total plasma fatty acids. Only about 0·7% of the total plasma 18:2 occurred in the plasma triglycerides.The results are discussed in relation to the changes in the composition of the milk fatty acids produced by the cows when they were given the experimental diets.

The effect of change from low-roughage to high-roughage diets on rumen fermentation, blood composition and milk fat secretion in the cow.

1. Changing cows from a diet with low hay and high flaked maize content, which depressed milk fat secretion, to a high-roughage diet resulted in increased milk fat secretion and in changes in the pattern of rumen fermentation and in the composition of the blood plasma.2. During the 1st week following the change in diet the proportions of propionic and valeric acids in the rumen decreased and the proportion of acetic acid increased. These changes in rumen volatile fatty acids were associated with decreases in the concentration of lactic acid, increases in pH and increases in the rate of breakdown of cotton threads in the ventral sac of the rumen. After 1 week, only minor fluctuations in the proportions of volatile fatty acids occurred from day to day.3. The percentage of fat in the milk increased over a period of 3 weeks following the change in diet and, with the exception of linoleic acid which decreased, the yields of all fatty acids in milk progressively increased over this period. The increase in yield of oleic acid was relatively much smaller than the increases in yield of saturated acids.4. The concentrations in blood plasma of acetic acid and β-hydroxybutyric acid increased in two cows and, in all cows, saturated triglyceride fatty acids increased, whereas the concentrations of the unsaturated triglyceride fatty acids decreased, during the 1st week after change to the high-roughage diet. The recovery in milk fat secretion is discussed in relation to the observed changes in rumen fermentation and blood composition.

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.

The effects of intravenous infusions of triglycerides on the secretion of milk fat in the cow.

1. Artificial emulsions of nine synthetic triglycerides ranging from tripropionin to triolein were given as continuous infusions into the jugular vein of lactating cows for periods of 2 days. The effects of these infusions on the composition of blood lipids and on the secretion of the component fatty acids in milk were examined.2. Tricaproin, tricaprylin, tripelargonin, tricaprin, trilaurin, trimyristin and triolein, in contrast to tripropionin and tributyrin, increased the yields in milk of the acid contained in the triglyceride. The increased yield of acid was positively correlated with chain length of the infused triglyceride, and with triglycerides above tricaprin the transfer of fatty acid to milk was sufficiently large to give consistently increased yields of total milk fat.3. The infusions increased the concentrations in blood plasma of cholesterol, phospholipid and in some experiments also of triglyceride. Concentrations of these lipids returned to normal by 2 days after the infusion. The fatty acid compositions of the plasma triglycerides were not affected by the infusion of triglycerides up to tripelargonin but with triglycerides longer than this the plasma triglycerides were altered in composition towards that of the infused triglyceride.4. Short- and intermediate-chain fatty acids of the infused triglycerides were elongated by the successive addition of two carbon units to give increased yields in milk of acids ranging up to C15and C16when they contained acids with an even and odd number of carbon atoms respectively. Possible interference with the conversion of stearic into oleic acid in the mammary gland owing to the infused emulsions is also discussed.

Effects of Sodium Bicarbonate, Magnesium Oxide, and Calcium Hydroxide on Milk Fat Secretion

Addition of 0.70% NaHCO3 plus 0.35% MgO, twice or three times that level to grain mixtures fed to cows on a high grain-restricted roughage ration, increased milk fat percentage and daily secretion. Increasing mineral levels produced a linear increase in fat percentage. The middle level fed had the greatest effect on daily secretion of milk fat, probably due to the lower grain consumption and milk production observed at the highest level. No differences in milk fat test, production, or grain consumption were noted when the higher mineral level (272g NaHCO3 plus 136g MgO) was administered directly to the rumen via a fistula or mixed with the grain consumed. Evidently, decreased grain intake when these minerals are fed is not due to the taste of the grain mixture. This level of minerals fed to cows on normal rations did not change milk fat percentage, but grain consumption was decreased. Magnesium oxide in three different calcined states and Ca(OH)2 were equally effective in maintaining fat percentage, but the hydroxide was less acceptable to cows.

The effect of increasing amounts of dietary red palm oil on milk fat secretion in the cow.

1. The effects, on the secretion in milk of fat and its component fatty acids, of supplementing a basal diet low in fat with red palm oil at three levels are reported.2. The secretion of total fat and of the fatty acids contained in the oil supplement was positively correlated with the dietary intakes, except for linoleic acid which was hydrogenated in the rumen before its absorption and secretion in milk.3. Evidence for the synthesisde novoof palmitic acid and for the possible depression of intramammaryde novosynthesis by high levels of dietary fat is discussed.

The effects of a series of saturated fatty acids in the diet on milk-fat secretion in the cow

SummaryThe effects of the isocaloric replacement of part of the dietary concentrate mixture by 5% lauric acid (99% pure) or by 10% of myristic (95% pure), palmitic (96% pure) or stearic (94% pure) on the yield and composition of milk fat was investigated in 2 feeding experiments with a total of 10 cows in mid-lactation. The concentrate mixtures were given with a high-roughage diet that supplied 9·1 or 7·7 kg hay/day in expts 1 and 2, respectively.In expt 1 the inclusion of myristic acid in the diet decreased the yields of milk and solids-not-fat (SNF), but increased the percentage of fat in the milk without altering the fat yield. When stearic or palmitic acid was included in the diet there was an increase in milk fat yield; palmitic acid caused the greater increase. In expt 2 the inclusion of lauric acid in the diet did not appear to affect the yields of milk or SNF but it caused large reductions in the percentage of fat in the milk and in the yield of fat.In expt 1 the inclusion of myristic acid increased the yields and percentages of 14:0, 14:1 and 16:1 in the milk fat and decreased the yields and percentages of the short-chain fatty acids (4:0–8:0), 16:0, 18:0 and 18:1. When palmitic acid was included in the diet there were increases in the yields and percentages of 16:0 and 16:1 in the milk fat and decreases in the yields and percentages of 10:0, 12:0, 14:0, 18:0 and 18:1. The inclusion of stearic acid in the diet increased the yields and percentages of 18:0 and 18:1 in the milk fat and decreased the yields and percentages of 12:0 and 16:0. In expt 2 the inclusion of lauric acid in the diet increased the yield and percentage of 12:0, and in 1 cow the content of 14:0 in the milk fat; there were reductions in the yields and percentages of all the other fatty acids in the milk fat. There was no evidence of any marked elongation of the carbon chains of 12:0 and 14:0 in the mammary gland to form 16:0 or 18:0.In expt 1 the incorporation of myristic acid in the diet decreased the concentration of total steam-volatile fatty acids in rumen liquor. The inclusion of either myristic, palmitic or stearic acid in the diet had little effect on the relative proportions of the individual volatile acids in the rumen liquor. In expt 2 the addition of lauric acid to the diet reduced the acetic acid:propionic acid ratio in the rumen liquor.

The effects of mono-unsaturated and saturated fatty acids in the diet on milk-fat secretion in the cow

SummaryThe effects of the addition of either 5 or 10% ‘oleic acid’ (78% pure) or 10% of a mixture of saturated fatty acids (64% palmitic acid, 31% stearic acid) to the dietary concentrate mixture on the yield and composition of the milk and milk fat and on the pattern of fermentation in the rumen were investigated in a feeding experiment with 8 cows in mid-lactation. The concentrate mixtures were given with a high-roughage diet that supplied 9·1 kg of hay/day.The addition of 5% ‘oleic acid’ to the concentrate mixture resulted in increased yields of milk and solids-not-fat (SNF); the percentage of fat in the milk was decreased but the yield of milk fat was unaltered. The addition of 10% ‘oleic acid’ to the concentrate mixture decreased both the yield and percentage of fat in the milk. In contrast, the concentrate mixture containing 10% of the mixture of saturated fatty acids increased the yield of milk fat.When the concentrate mixture containing 5% ‘oleic acid’ was given to the cows, the yields and percentages of the fatty acids from 4:0 to 16:0 (except 12:0) in the milk fat were decreased, but the yields and percentages of 18:0 and 18:1 were increased. Similar but more pronounced effects on the yields and percentages of the fatty acids from 4:0 to 16:0 (except 12:0) in the milk fat were observed when the cows were given the concentrate mixture containing 10% ‘oleic acid’, but under these dietary conditions the yield and percentage of only 18:1 in the milk fat were increased. The addition of the mixture of saturated fatty acids to the concentrate mixture decreased the percentages of the fatty acids from 4:0 to 14:0 (except 12:0) in the milk fat but decreased the yields of only 10:0 and 14:0; the yields and percentages of 16:0 and 18:1 were increased.When the cows were given the concentrate mixture containing 5% ‘oleic acid’ there was a small but significant decrease in the acetic acid:propionic acid ratio in the rumen liquor. A similar but more pronounced change in the acetic acid:propionic acid ratio in the rumen liquor was observed when the cows were given the concentrate mixture containing 10% ‘oleic acid’, but in this instance there was a significant reduction in the concentration of total volatile fatty acids in the rumen liquor. Apart from a small increase in the relative proportion of propionic acid, the addition of the mixture of saturated fatty acids to the concentrate mixture had no effect on the concentrations of volatile fatty acids in the rumen liquor.

Further studies on the effects of dietary cottonseed oil on milk-fat secretion in the cow

SummaryThe effects of the isocaloric replacement of part of the dietary concentrate mixture by cottonseed oil on the yield and composition of the milk fat and on the pattern of rumen fermentation was investigated in 2 feeding experiments with a total of 8 cows in mid-lactation. The concentrate mixtures were given with high- or low-roughage diets that supplied 9·1 or 1·8 kg of hay/day.In expt 1 the yield of milk fat was not altered by the addition of 10% cottonseed oil to the concentrate mixtures given either with the high- or with the low-levels of dietary roughage. On the low-roughage treatment, dietary cottonseed oil increased milk yield but reduced the fat content of the milk. The change from the high- to the low-roughage diets containing no cottonseed oil resulted in reductions in the yield and percentage of fat in the milk. In expt 2 the yields of milk and milk fat were similar irrespective of whether the concentrate mixture contained 5 or 10% cottonseed oil.In expt 1 the inclusion of 10% cottonseed oil in the concentrate mixture reduced the yields and percentages of the medium-chain fatty acids (12:0, 14:0 and 16:0) and increased the yields and percentages of the C18 fatty acids in the milk fat. In expt 2, when the concentrate mixture contained 5% cottonseed oil, the yields and percentages of all the fatty acids in the milk fat were similar to the values obtained when the concentrates contained 10% cottonseed oil. For any given concentrate mixture, the change from the high- to the low-roughage treatments in both expts 1 and 2 resulted in increases in the percentage of oleic acid in the milk fat. The highest concentration of trans-octadecenoic acid was observed in the milk fat of the cows when they were given the high-roughage diet with the concentrate mixture containing 10% cottonseed oil.In expt 2 the level of cottonseed oil in the concentrate mixture did not influence the pattern of rumen fermentation as measured by the concentrations of the various volatile fatty acids in the rumen liquor. However, the change from the high- to the low-roughage diets reduced the proportion of acetic and increased the proportions of propionic and n-valeric acids in the total volatile fatty acids in the rumen liquor.

Mammary and whole animal metabolism of glucose and fatty acids in fasting lactating goats.

1. Measurements were made of milk yield, mammary blood flow and mammary arteriovenous differences during the measurement of substrate entry rate by the isotope dilution method using [U-(14)C]glucose, acetate, palmitate, stearate or oleate in conscious lactating goats after 24 hr starvation.2. As previously reported, in fasting, milk yield fell to 40 +/- 3.4 (S.E.)%, lactose secretion to 31 +/- 3.4%, milk fat secretion to 81 +/- 6.7% and mammary blood flow fell to 53 +/- 7.5% of the values before fasting. Mammary O(2) uptake was only 45 +/- 5% of the mean value in fed animals and there were marked falls in the uptakes of glucose, acetate and triglycerides, a smaller fall in beta-hydroxybutyrate uptake, and a large increase in free fatty acid uptake.3. Glucose was found to enter the circulation of the fasting animal at 1-1.6 mg/min/kg body wt. (entry rate) and it gave rise to 3-5% of the total CO(2). The udder took up 10.7-16.1 mg/min/kg of tissue and 8-10% of mammary CO(2) was derived from glucose, although only 5-10% was oxidized. Mammary uptake accounted for 35-43% of the total glucose entering the circulation.4. In the whole animal acetate entry rate was 1-1.4 mg/min/kg and 9-10% of total CO(2) was derived from it. The udder used 0.8-2.4 mg/min/kg of tissue and 9-13% of mammary CO(2) was derived from acetate, 46-79% of that taken up being oxidized. Mammary uptake accounted for only 2-6% of the total acetate entry rate. Negligible quantities of isotope were found in milk fatty acids and there was a fall in the proportion of milk fatty acids of chain length up to C(14) which in fed animals are synthesized from acetate and beta-hydroxybutyrate.5. Palmitate, stearate and oleate entered the circulation as free fatty acids at 0.94-6.8 mg/min/kg and 6-9% of total CO(2) was derived from each. The udder took up 3.0-5.7 mg/min/kg of tissue and 4-8% of mammary CO(2) was derived from each acid. In the udder 8 and 5.5% of stearate and oleate were oxidized and 25% of palmitate. Mammary uptake of stearate was 31.5% of the total entry rate, palmitate 1%, and oleate 7.5%. Only long chain milk fatty acids were labelled.6. During fasting the mammary R.Q. was 0.85 +/- 0.045 compared with a value in fed animals of 1.24 +/- 0.02, when the udder is synthesizing fatty acids from acetate. The total mammary uptake of lipid precursors was only 74% of the rate of milk fat secretion and there was an 18% shrinkage in empty udder volume, suggesting the use of endogenous mammary tissue substrates.

The effects of dietary tallow and cottonseed oil on milk fat secretion in the cow

SummaryThe effects of the isocaloric replacement of part of the dietary concentrate mixture by either tallow or cottonseed oil on the yield and composition of the milk fat was investigated in 2 feeding experiments with a total of 26 cows in midlactation. The concentrates were given with high- or low-roughage diets that supplied either 9·1–9·5 or 1·8–3·2kg hay/day.In expt 1, the addition of cottonseed oil to the high-roughage diet increased the yield of milk fat during the first 8 days but decreased it during the last 4 days of a 28-day feeding period. In contrast, the inclusion of tallow in the high-roughage diet resulted in an increased yield of milk fat that was sustained throughout the period of 28 days. In expt 2, the intake of dietary fat was less than it was in expt 1 and the inclusion of tallow or cottonseed oil in either the high- or low-roughage diets had little effect on the yield of milk fat.The inclusion of either fat in the diet increased the yields and percentages of stearic and oleic acids and, in general, decreased the yields and percentages of the medium-chain fatty acids (10:0, 12:0 and 14:0) in the milk fat. The addition of tallow to the diet did not appear to alter the yields of the short-chain fatty acids (C4–C8, 4:0, 6:0 and 8:0) in the milk fat. When the low-roughage diets resulted in a decreased yield of milk fat, the secretion of all the fatty acids in the milk fat was reduced, but the reduction in the secretion of oleic acid was less than the reductions in the secretions of the other constituent fatty acids.

Dietary fat and milk fat secretion in the cow.

Dietary fat and milk fat secretion in the cow.

The role of the plasma membrane in the secretion of milk fat

On the basis of electron microscopic evidence, Bargmann & Knoop (1959) proposed that milk fat is secreted from the lactating cell by progressive envelopment of the fat droplet with plasma membrane at the apex of the cell. Ultimately the fully wrapped globule is pinched off from the cell. Support for this hypothesis is presented here in the form of biochemical data together with a rationale for the biophysics of the process. The biochemical evidence indicates that fat droplets within the cell are relatively devoid of certain membrane constituents, particularly phosphatidyl ethanolamine and carotenoids characteristically present in the surface coat of secreted milk fat globules. From a combination of compositional analyses and turnover studies with 14C-fatty acid, the logical origin of these components is the plasma membrane of the cell. The envelopment of the droplet appears adequately explained on the basis of London-van der Waals forces which are estimated to achieve an attraction force of one atmosphere when the separating distance between membrane and droplet is reduced to 20 Å. Milk proteins appear to be secreted by emptying of vacuoles through the plasma membrane. Membranes around such vacuoles may replenish the plasma membrane used in fat secretion, thus relating the two processes.