Transcriptome profiling of Streptococcus uberis-induced mastitis reveals fundamental differences between immune gene expression in the mammary gland and in a primary cell culture model.

Streptococcus uberis is a prevalent causative organism of mastitis and resides naturally in the environment of the dairy cow making prevention of the disease difficult. A bovine cDNA microarray comprising approximately 22,000 expressed sequence tags was used to evaluate the transcriptional changes that occur in the mammary gland after the onset of clinical Strep. uberis mastitis. Five lactating Friesian heifers were intramammary infused in an uninfected quarter with approximately 1,000 to 1,500 cfu of a wild-type strain of Strep. uberis. Microarray results showed that Strep. uberis mastitis led to the differential expression of more than 2,200 genes by greater than 1.5-fold compared with noninfected control quarters. The most highly upregulated genes were associated with the immune response, programmed cell death, and oxidative stress. Quantitative real-time reverse transcription PCR analysis confirmed the increase in mRNA expression of immune-related genes complement component 3, clusterin, IL-8, calgranulin C, IFN-gamma , IL-10, IL-1beta, IL-6, toll-like receptor-2, tumor necrosis factor-alpha, serum amyloid A3, lactoferrin, LPS-bonding protein, and oxidative stress-related genes metallothionein 1A and superoxide dimutase 2. In contrast, a decrease of mRNA levels was observed for the major milk protein genes. Bovine mammary epithelial cells in culture challenged with the same Strep. uberis strain used to induce clinical mastitis in the in vivo animal experiment did not cause a change in the mRNA levels of the immune-related genes. This suggests that the expression of immune-related genes by mammary epithelial cells may be initiated by host factors and not Strep. uberis. However, challenging epithelial cells with different Strep. uberis strains and Staphylococcus aureus resulted in an increase in the mRNA expression of a subset of the immune-related genes measured. In comparison, an Escherichia coli challenge caused an increase in the majority of immune-related genes measured. Results demonstrate the complexity of the bovine mammary gland immune response to an infecting pathogen and indicate that a coordinated response exists between the resident, recruited, and inducible immune factors.

cDNA microarray analysis reveals that antioxidant and immune genes are upregulated during involution of the bovine mammary gland.

We have used cDNA microarray analysis to identify genes that play a role in bovine mammary involution. Involution was induced by termination of milking, and alveolar tissue was collected from 48 nonpregnant Friesian cows in mid lactation sacrificed at 0, 6, 12, 18, 24, 36, 72, and 192 h (n = 6/group) postmilking. The most highly upregulated genes were those associated with oxidative stress. Quantitative real-time reverse-transcription PCR analysis confirmed that mRNA expression of spermidine/spermine N(1)-acetyltransferase was increased by 24 h, superoxide dismutase 2 and metallothionein 1A by 36 h, and glutathione peroxidase by 72 h postmilking. The mRNA expression of the host defense proteins lactoferrin and lingual antimicrobial peptide were increased by 192 h postmilking. A dramatic increase in the protein expression of lactoferrin by 192 h postmilking was also detected by Western analysis. Decreased mRNA expression of the milk protein genes alpha(S1)-, beta-, and kappa-casein, and alpha-lactalbumin were early events in the process of involution occurring within 24 to 36 h postmilking, whereas beta-lactoglobulin mRNA was decreased by 192 h postmilking. Decreases in alpha-lactalbumin and beta-lactoglobulin protein levels in alveolar tissue occurred by 24 and 192 h postmilking, respectively, and the cell survival factors beta1-integrin and focal adhesion kinase were decreased by 72 and 192 h postmilking, respectively. The results demonstrate that in the bovine mammary gland, decreased milk protein gene expression and cell survival signaling are associated with multiple protective responses to oxidative stress that occur before the induction of immune responses and mammary epithelial cell apoptosis during involution.

Stat5 phosphorylation status and DNA-binding activity in the bovine and murine mammary glands.

The transcription factors Stat5a and Stat5b are mediators of prolactin signalling in mammary epithelial cells, and are thought to play a role in lactogenesis. In cultured cells, activation of Stat5 activity through phosphorylation results in Stat5 binding to the promoters of at least some of the milk protein genes, thereby stimulating their transcription. However, the mammary biology of Stat5 differs between species, and the role of Stat5 in the bovine mammary gland is not fully understood. We have generated an antibody that specifically recognises the phosphorylated forms of Stat5a and Stat5b and used it to compare the levels of phosphorylated Stat5 with Stat5 DNA-binding activity in bovine and murine mammary tissue. Both Stat5 DNA-binding activity and phosphorylation status in the bovine mammary gland were at near-maximal levels at late pregnancy (27-35 days prior to calving), when at least three of the major milk proteins are not highly expressed. In addition, these studies revealed significant animal-to-animal variation in the level of Stat5 activity in both species. The results are consistent with a role in terminal differentiation of mammary epithelial cells. They also suggest that the stimulation of high-level expression of milk protein genes in the bovine mammary gland is not through activation of the prolactin receptor-Jak2-Stat5 pathway.

Effects of mammary engorgement and feed withdrawal on microvascular function in lactating goat mammary glands.

The responses of the mammary microvasculature in lactating goats (n = 8) during feed withdrawal (18-20 h) and mammary engorgement (26-28 h of milk accumulation) were compared using an indicator-dilution technique with FITC-albumin and [(14)C]sucrose as the intravascular and diffusible indicators, respectively. Feed withdrawal and mammary engorgement caused a 50-60% decrease in mammary arterial flow and in the permeability-surface area product (PS) values for sucrose. Only feed withdrawal increased the mean transit time [from 17.3 to 30.0 s, SE of the difference (SED) = 2.16, P < 0.01] of FITC-albumin, whereas only mammary engorgement reduced sucrose extraction (0.63 to 0.51, SED = 0.04, P < 0.05). Mammary engorgement also caused a substantial reduction in the sucrose-accessible extravascular space from 92 to 44 ml (SED = 15.2, P < 0.01). In a separate experiment using five goats, milking after mammary engorgement did not immediately restore arterial flow or sucrose extraction, indicating that the effect of milk accumulation was not mediated simply via increased intramammary pressure. In conclusion, feed withdrawal resulted in slower flow in the capillary bed but apparently no change in capillary recruitment, whereas mammary engorgement caused capillary derecruitment.

Metabolic effects of amylin in lactating goats.

The objective of this study was to investigate the role of amylin (a pancreatic hormone) in regulating metabolism in support of lactation. Rat amylin was infused (320 pmol.kg LW(-1).h(-1)) for 6 h via an external pudic (mammary) artery into six lactating goats. This dose of amylin led to a sixfold increase in plasma concentrations of amylin relative to baseline. Amylin infusion increased plasma concentrations (jugular) of glucose and NEFA up to 16 and 168%, respectively, relative to saline infusion. In contrast, plasma concentrations of Ca and PO4 during amylin infusion were reduced by 18 and 30%, respectively, relative to saline infusion. Plasma concentrations of IGF-I, insulin, and Mg were not different between the two treatments, although IGF-I concentrations in the amylin-infused group, 1 and 6 h postinfusion, were significantly higher than those in the saline-infused group. Similarly, amylin infusion failed to affect milk yield and major constituents of milk except protein; milk protein content decreased progressively until the end of amylin infusion and remained low thereafter. Amylin also had no effect on minerals in milk (Ca, PO4, Mg, Fe, Sr, S, K, or Na) except Zn, which was significantly decreased from 56.8+/-5.8 micromol/L at 0 h to 44.5+/-2.4 micromol/L at 6 h postinfusion. Mammary blood flow (measured with a transit-time blood flow probe) increased up to 26% during amylin infusion, although this effect lasted only for the first 3 h. In conclusion, amylin increased plasma concentrations of glucose and NEFA, and mammary blood flow, while decreasing plasma concentrations of Ca and PO4. Despite these metabolic changes, amylin infusion did not increase milk yield of lactating goats.

Alteration of the sodium to potassium ratio in milk and the effect on milk secretion in goats.

Saanen goats were used to determine the effect of the alteration of the intramammary Na to K ratio on milk secretion. Udders were infused via the teat with an isosmotic solution that was high in Na or K to increase or decrease, respectively, the intramammary Na to K ratio. Control glands received an isosmotic sucrose solution. To ensure that the results were not confounded by a decrease in milk secretion as a result of enhanced permeability of mammary tight junctions, the latter was monitored throughout the experiments. An increase in the Na to K ratio caused a significant transient reduction in milk secretion. Therefore, an increase in Na and a decrease in K in milk, commonly observed as a result of the leakiness of tight junctions, may at least partially explain the reduction in milk secretion when the permeability of tight junctions was increased. These experiments further showed that the adverse effects on secretion were not due to a high intracellular concentration per se but were related to a change in the Na to K ratio because a reduction in the ratio also lowered milk secretion. These data support the evidence for activity of Na(+)-K(+)-ATPase in the basolateral secretory cell membranes and passive movement of these ions across the apical cell membranes.

Elevated plasma cortisol reduces permeability of mammary tight junctions in the lactating bovine mammary epithelium.

Induction of tight junction permeability in the mammary epithelium decreases milk secretion, and in cows tight junctions become leaky after 17 h of milk accumulation. In vitro studies demonstrate the importance of glucocorticoids for the formation and maintenance of tight junctions. In this study we examined whether cortisol can prevent mammary tight junction permeability in the lactating gland in vivo, and inhibit the associated milk loss, using our milk-accumulation model to challenge tight junction patency. Following a 4-day control period Jersey cows were subjected to a 24-h period in which they were milked twice at 0700 and 1500 h (TM;n=6), once at 0700 h (OM;n=7), or once and treated with ACTH (40 IU per 2 h, starting after 14 h of milk accumulation) to increase endogenous cortisol levels (OM+ACTH;n=7). Frequent blood samples for cortisol, lactose and glucose analyses were taken via indwelling jugular catheters. ACTH treatment resulted in a sustained elevation of systemic cortisol concentrations. Plasma lactose, an indicator of tight junction leakiness, was not changed in TM cows, but began to increase rapidly at 17 h of milk accumulation in OM cows. Treatment with ACTH prevented the increase in plasma lactose, although levels were slightly, but not significantly, higher than in TM cows, indicating that elevated plasma cortisol reduced mammary tight junction leakiness. Milk yield was reduced by 12% in both once-milked groups, despite cortisol preventing tight junction leakiness. However, the milk loss in the latter group may not be related to leaky tight junctions, but be due to a reduction in milk precursor uptake by the mammary gland. Consistent with this notion was a 34% increase in plasma glucose levels in OM+ACTH cows only.

Partitioning of milk accumulation between cisternal and alveolar compartments of the bovine udder: relationship to production loss during once daily milking.

Experiments were undertaken to validate a method (using adrenaline injection) for determination of the size of cisternal and alveolar compartments in the udder, to use this method to determine the pattern of milk accumulation in the udder over time and to determine the relationship between the size of the alveolar and cisternal compartments and tolerance of once daily milking. Cows received intrajugular injections of adrenaline (3 mg) immediately before milking, to block milk ejection and allow harvesting of the cisternal milk fraction. This was followed by removal of the alveolar fraction 30 min later after intrajugular oxytocin (5 i.u.) injection. Results obtained were similar to those obtained by catheter drainage. The alveolar compartment was 90% full at 16 h post milking while the cisternal compartment filled more slowly and was only 70% full at 24 h post milking. At full capacity (measured at 40 h), the volumes of milk contained in the cisternal and alveolar compartments were similar. In a further experiment involving identical twin cows, it was shown that the greater the degree of filling of the cisternal compartment at 24 h, the lower was the production loss on once daily milking. This suggests that the freedom of the alveoli to drain was an important factor in the production loss on once daily milking. Although there were significant correlations within twin sets for milk yield and the size of udder compartments, the relationship within twin sets for yield loss on once daily milking was not statistically significant.

Time course of milk accumulation-induced opening of mammary tight junctions, and blood clearance of milk components.

Eight cows in early lactation were used to study the effect of milk accumulation on the state of mammary tight junctions and to examine alpha-lactalbumin as an indicator of tight junction permeability in vivo. During three successive periods, the cows were milked twice (4 days), once (6 days), and twice daily (4 days). Plasma lactose, alpha-lactalbumin, and milk sodium concentrations were used as indicators of tight junction permeability. Furthermore, four cows were used to study the clearance of lactose and alpha-lactalbumin from the blood. Milk yield during once-daily milking decreased by 15.4% (P < 0.001). All indicators of mammary tight junction patency increased (P < 0.05) transiently during once-daily milking and indicated that tight junctions opened after approximately 18 h. Plasma alpha-lactalbumin and lactose were highly correlated (r = 0.82, P < 0.001), indicating the suitability of plasma alpha-lactalbumin as an indicator of tight junction status in vivo. Clearance of alpha-lactalbumin and lactose from the blood was best described by a biexponential model. Elimination half-lives for lactose and alpha-lactalbumin were 44 and 40 min, respectively. This study showed that milk stasis during early established lactation induces tight junctions to switch to a leaky state after approximately 18 h and to revert to the closed state shortly after milking.

Local secretion of nitric oxide and the control of mammary blood flow.

Our objective was to test the hypothesis that local production of the vasorelaxant nitric oxide could regulate mammary blood flow. In four lactating Saanen goats, the response of mammary blood flow to intraarterial infusion of the nitric oxide donor diethylamine NONOate and the inhibitor of nitric oxide synthesis N omega-nitro-arginine was measured. Diethylamine NONOate induced a rapid and sustained increase of mammary blood flow in the infused gland only, suggesting a direct effect on vasculature of the mammary gland. In contrast, infusion of N omega-nitro-arginine decreased mammary blood flow by up to 35%, and the coinfusion of arginine, the nitric oxide precursor, with N omega-nitro-arginine markedly reduced its ability to decrease mammary blood flow. The distribution of nitric oxide synthase was investigated in cryosections of caprine and bovine mammary tissue by histochemical staining for NADPH-diaphorase activity and by immunocytochemistry using specific antibodies against two nitric oxide synthase isoforms. Both techniques revealed nitric oxide synthase in the vascular endothelium and secretory epithelium of the two species. Only antibodies against nitric oxide synthase-III showed specific staining. These results suggest that the mammary gland produces and responds to nitric oxide and, further, raise the possibility that the epithelium may control its own blood supply by secreting nitric oxide.

Regulation of blood flow in the mammary microvasculature.

Although milk yield of cows and goats is known to be closely related to the total flow of blood through the udder, a number of studies suggest that milk yield can vary independently. No studies have attempted to measure the proportion of total flow that is nutritive. Within the mammary gland, capillary networks form a basket-like architecture surrounding each alveolus. Notably, flow in individual capillaries is not constant and varies among capillaries. Capillary flow (measured by intravital microscopy) was decreased by oxytocin, which generally increased total flow in the mammary artery, suggesting that the proportion of total flow that is nutritive can vary. In addition to classic metabolic regulators (e.g., carbon dioxide and oxygen) of tissue blood flow, the mammary gland produces a number of vasodilatory compounds, including parathyroid hormone-related protein, insulin-like growth factor-I, prostacyclin, nitric oxide, and endothelin. All of these compounds have been shown to alter mammary blood flow. Mammary tissue also contains kallikrein and angiotensin-converting enzyme, which convert circulating kinins and angiotensin, respectively, into potent vasoactive compounds. A number of these compounds are produced by epithelial cells themselves, providing a mechanism for the functioning epithelium to control its own blood supply and, hence, nutrient flow for milk synthesis. In this review, we examine the nature of the mammary microcirculation, its behavior under different conditions, and some of the regulatory features of the mammary microvasculature.

An improved method for the routine biopsy of bovine mammary tissue.

Eight primiparous cows in midlactation were used to determine a method for the mammary biopsy of standing cows in full lactation. Cows were mildly sedated; therefore, preoperative feed withdrawal was not necessary. A core of secretory tissue (0.75 to 1 g) was extracted using a rotating stainless steel cannula with a retractable blade at the cutting edge. Postoperative recovery was rapid, taking only 15 min per cow, and the method was reliable and efficient. The presence of secretory tissue was verified by histology and in situ hybridization with alpha s1-casein and alpha-lactalbumin probes. The capsular end of the core contained more connective tissue, and the parenchyma showed heterogeneous expression of alpha s1-casein and alpha-lactalbumin. Despite some postoperative bleeding, milk yield and composition in the biopsied gland were affected only transiently. Yield recovered by 3.5 d after biopsy, and composition recovered by 6.5 d after biopsy. Yield and composition of milk from the control glands were not affected by the procedure. Biopsy sites healed rapidly and without infection. No clinical mastitis was observed in any of the biopsied cows throughout the remainder of the lactation.

Continuous versus single drainage of milk from the bovine mammary gland during a 24 hour period.

Five primiparous and five multiparous cows were used to determine if mammary cisternal storage of milk during a 24 h period of milk accumulation limited milk secretion. In addition, we investigated if there is a parity effect for the capacity of the mammary cisternal compartment to hold a 24 h accumulation of milk secretion, and studied the movement of milk from the alveolar compartment into the cisternal compartment. All cows were fitted with catheters in all teats in order to collect cisternal and alveolar milk fractions separately. For a 24 h period of milk accumulation, the milk was drained once (after 24 h) from one side of the udder (OD), and continuously from the other side of the udder (CD). There was no significant parity effect for cisternal, alveolar and total milk volumes at 24 h. Therefore, data from primi- and multiparous cows were pooled for subsequent analyses. Cisternal milk volume from CD glands was higher than that from OD glands (P < 0.01), indicating that cisternal storage of milk in the mammary gland may be limiting to milk secretion during 24 h milking intervals. Alveolar volumes did not differ between OD and CD, but, as a result of the higher cisternal milk volume, total milk volume was highest in the CD glands (P = 0.05). Movement of milk from the alveolar into the cisternal compartment was intermittent. Moreover, analyses of the slopes of individual milk accumulation profiles of the first 6 h of accumulation revealed that the cisternal compartment starts filling immediately following milking, although the rate of filling is relatively low until 7-8 h postmilking.

EGTA-induced disruption of epithelial cell tight junctions in the lactating caprine mammary gland.

The suitability of the Ca2+ chelator ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) to induce disruption of mammary tight junctions (TJ) and its effect on milk secretion were investigated in six goats. EGTA was administered via the teat of one gland as an isosmotic (300 mosmol/l) K-EGTA solution (68 mM EGTA), whereas the control gland received an isosmotic sucrose solution. Lactose, Na, K, and Cl in milk, blood lactose, and the presence of Evans blue (EB) in mammary lymph were used as indicators of TJ disruption. EGTA caused transient (approximately 60 h) changes (P < 0.05) in the concentration of lactose, K, Na, and Cl in milk, consistent with loss of TJ integrity. This was confirmed by a rapid (< 1 h) increase (P < 0.05) in blood lactose levels. Moreover, EB appeared in lymph < 1 h after EGTA+EB treatment. Milk secretion declined unilaterally by 15% (P < 0.05) after EGTA and did not return to baseline until approximately 60 h after EGTA. EGTA caused a unilateral, temporary (first 7 h) increase in mammary blood flow. This study shows that a rapid temporary disruption of mammary TJ can be successfully induced in vivo and that such disruption compromises milk secretion.

Effect of milking frequency and somatotropin on the activity of plasminogen activator, plasminogen, and plasmin in bovine milk.

Six pairs of identical twin cows during late lactation (213 d) were used to study the effect of milking frequency (twice vs. once daily) and bST during once daily milking on the activity of plasminogen activator, plasminogen, and plasmin in milk. Less frequent milking increased the activity of plasminogen, plasmin, and plasminogen activator in milk. The ratio of plasminogen to plasmin, a measure that is independent of milk volume, decreased during less frequent milking, suggesting that at least part of the increase in activity of plasmin was due to the accelerated conversion of plasminogen to plasmin. Changes in the activity of plasminogen and plasmin in milk were positively correlated with increases in the concentrations of milk BSA and plasma lactose, both of which are indicators of disruption of tight junctions between mammary epithelial cells, indicating that paracellular leakage may have contributed to increased protease activity in milk during less frequent milking. No correlation existed between changes in plasminogen activator and indicators of tight junction disruption, suggesting that increased activity of plasminogen activator in milk was not due to leakage across the mammary epithelium, but rather to increased local production in the mammary gland. Administration of bST during once daily milking did not significantly affect milk protease activity.

Effect of once daily milking and concurrent somatotropin on mammary tight junction permeability and yield of cows.

Six pairs of monozygous Friesian twin cows during late lactation were used to assess the effect of once daily milking and bST treatment on yields and tight junction permeability of mammary epithelial cells. During the first 7 d, all cows were milked twice daily; on d 8 through d 21, all cows were milked once daily, but one cow of each twin pair was treated daily with 20 mg of bST on d 13 through 21; and, finally, during d 22 through 28, all cows were again milked twice daily. Once daily milking, a common management practice in New Zealand, resulted in a small (7%) but significant decrease in milk yield. Treatment with bST increased milk yield by 19%, thereby exceeding the milk yield loss from once daily milking. The integrity of mammary tight junctions was assessed indirectly by measuring concentrations of plasma lactose and milk BSA. Once daily milking temporarily disrupted tight junction integrity, based on a 4- to 5-fold increase in plasma lactose and a 42 to 55% increase in the concentrations of milk BSA. In the present study, bST did not affect the permeability of mammary tight junctions.

Increased mammary blood flow in the lactating goat induced by parathyroid hormone-related protein.

Human synthetic parathyroid hormone-related protein (PTHrP) increased mammary blood flow (MBF) following close-arterial infusion via the external pudic artery in goats during mid-lactation. MBF increased 74 +/- 8% within 30 min of the start of continuous infusion of PTHrP compared with 10 +/- 3% in controls. MBF decreased by 90 min, however, and was not different from control values for the remainder of the infusion. The increase in plasma concentrations of calcium and decrease in phosphate during PTHrP suggests that this was not due to altered activity of PTHrP, but may relate to downregulation of response or production of counter-regulatory vasoconstrictive agents within the gland. This problem was alleviated when PTHrP was infused in a pulsatile fashion. An average 14-40% increase in MBF was achieved over 6 h, but this did not alter the rate of milk secretion, suggesting that mammary hyperaemia is not sufficient by itself to increase milk yield in the normally lactating goat. MBF increased in a dose-dependent fashion, although the lowest dose used to give a detectable response was approximately 40-fold higher than the concentration normally present in the mammary venous circulation. Thus, endogenous PTHrP may not be an important regulator of MBF during lactation in the goat.

Effects of close-arterial (external pudic) infusion of insulin-like growth factor-II on milk yield and mammary blood flow in lactating goats.

Five lactating goats were infused, via an external pudic arterial catheter, directly into the mammary gland with 0.9% (w/v) NaCl (20 ml/h), recombinant human insulin-like growth factor-I (IGF-I; 80 nmol/h), recombinant human IGF-II (133 nmol/h) or IGF-I and IGF-II combined. The infusion was for 6 h and milk yield was determined every 2 h. The ratio of milk yield in the infused relative to the non-infused gland was changed only slightly by saline (2%), but increased to 9% (P < 0.05) in response to IGF-I and 8% (P < 0.05) in response to IGF-II. When combined, both peptides increased this ratio by 6%. These effects were elicited within 2-4 h of the beginning of infusion. Mammary blood flow increased 50-80% (P < 0.05) during all IGF infusions, but only 28% during saline treatment. Plasma insulin decreased 50% (P < 0.01) during the infusion of IGF-I alone or in combination with IGF-II and 25% in response to IGF-II alone. Whereas plasma glucose increased by approximately 10% during infusion of IGF-I alone or with IGF-II, it was not altered by infusion of IGF-II only. The rapidity and unilateral nature of the milk-yield response to IGF-I and IGF-II is consistent with their acting directly on mammary tissue itself. Thus, the present results demonstrate similar local and systemic actions induced by intramammary infusion of IGF-II and IGF-I, although the magnitude of the response to IGF-II tends to be less than that to IGF-I.

Mammary epithelial cell tight junction integrity and mammary blood flow during an extended milking interval in goats.

The timing and relation of changes in mammary epithelial cell tight junction integrity and mammary blood flow during a 36-h milking interval were studied in six lactating Saanen goats. An increase in lactose concentration in plasma, a decrease in transepithelial potential difference, and changes in ionic milk composition were used to indicate tight junction patency. After 36 h of milk accumulation, mammary tight junctions had become disrupted. Further analyses indicated that this disruption began after 21 h of milk accumulation and that mammary blood flow also started to decline after 21 h. The time when both events occurred was not significantly different from the time when milk secretion began to decline (19 h). Moreover, positive but nonsignificant correlations existed between these events. Mammary tight junctions became disrupted when milk secretion declined, suggesting that impairment of mammary tight junction integrity is associated with decreased milk secretion during an extended milking interval. The decline in mammary blood flow may be the result of a negative feedback response to a reduced demand for metabolites, which is due to a reduced rate of milk secretion.

Effect of colostrum intake on alpha-lactalbumin concentrations in serum of calves.

Seven Friesian calves were fed colostrum for four days beginning within 24 hours of birth, and milk thereafter. The concentration of alpha-lactalbumin in serum was measured by specific radioimmunoassay and compared to IgG assayed by electroimmunodiffusion. Serum concentrations of alpha-lactalbumin peaked at 387 +/- 85 ng ml-1 within eight hours of initial intake of colostrum, declining to 12 +/- 3 ng ml-1 by day 6. IgG rose steadily to 17 mg ml-1 by 48 hours of birth and remained relatively constant thereafter. The temporal pattern of alpha-lactalbumin in serum following colostrum intake confirms previous studies suggesting reduced absorption of colostral proteins between 24 and 36 hours. The presence of variable amounts of alpha-lactalbumin in serum even after 17 days, however, indicates limited transfer of milk-derived proteins across the gut at this time. The data further show that cessation of maximal gut transfer does not relate to molecular weight of transferred protein.