Effects of tryptophan supplementation on plasma tryptophan and related hormone levels in heifers and dairy cows

This study was conducted to investigate the effects of rumen-protected tryptophan (125 g tryptophan per day) in heifers and dairy cows. Blood samples from dairy cows and heifers were collected for 24h in 3-h intervals on the day before tryptophan supplementation, on day 2, 5 and 7 of tryptophan supplementation, and in heifers additionally on d 14 after tryptophan supplementation was ceased. Plasma tryptophan, melatonin, serotonin, and prolactin concentrations were determined. Tryptophan plasma concentrations on d 5 were augmented at day (11:00 h) and nighttime (02:00 h), (P<0.05) in response to tryptophan supplementation in heifers by 119% and in dairy cows by 47%, respectively, as compared with d 0. Melatonin increased (P<0.05) in response to tryptophan supplementation in heifers, but not in cows. The effect of tryptophan supplementation on plasma tryptophan and melatonin was reversible as demonstrated in heifers on d 14 after cessation of tryptophan supplementation. Serotonin and prolactin in plasma did not respond to tryptophan supplementation. However, milk yield during morning milking increased significantly in tryptophan supplemented cows on d 1, 3 and 4 as compared to the day before tryptophan supplementation. Additional blood samples were taken during afternoon milking in cows at 1-min intervals for the analyses of oxytocin and prolactin on the day before the start and on d 7 of tryptophan supplementation. Milk flow curves were recorded during milking. No effect of tryptophan supplementation on the milking related release of oxytocin and prolactin and on any characteristic of milk flow was observed. In conclusion, tryptophan supplementation caused increased plasma tryptophan in cows and heifers and plasma melatonin in heifers. However, plasma serotonin, prolactin and oxytocin release in cows remained unchanged by tryptophan supplementation. Milk yield at morning milking increased slightly and transiently in response to tryptophan supplementation.

Influence of amount fed on hormone concentrations and their relationship to mammary growth in heifers

Concentrations in blood serum of growth hormones, prolactin, insulin, and glucocorticoids were investigated in 22 pre- or postpubertal heifers fed restricted or ad libitum amounts of a ration. Gains for the respective feeding regimens were 613 and 1218 g daily. Growth hormone was elevated for prepubertal heifers on restricted feed, whereas there was no difference from feeding for postpubertal heifers. Concentrations of prolactin, insulin, and glucocorticoids were higher for heifers fed ad libitum than for those restricted. Mammary secretory tissue was positively correlated with blood serum concentrations of growth hormone, and extraparenchymal adipose tissue was negatively correlated with them. Prolactin was negatively related to mammary secretory tissue; however, this relationship disappeared after adjustment for concentrations of growth hormone, insulin, and glucocorticoids. Mammary tissue was not related to concentrations of insulin or glucocorticoids. The negative influence of excess feeding on mammary development of prepubertal heifers may be associated with a decrease in concentrations of growth hormone in blood.

Environmental and lactational variables affecting prolactin concentrations in bovine milk

Relationships were examined between prolactin concentrations in bovine milk and various environmental and lactational variables. Prolactin was quantified by radioimmunoassay in 1.316 milk samples from two experiments. Environmental temperatures preceding milking, stage of lactation, daily miik yield, and dominance rank of the cow were correlated significantly with milk prolactin concentration. Stepwise multiple regression analysis was used to determine, in order of importance, the variables having significant independent effects on milk prolactin. Ambient temperature extremes, high and low, exerted the greatest effects and were each associated with elevated concentrations of prolactin. In Experiment I,conducted throughout the year, the variable representing maximum temperature preceding milking accounted for 14.3% of the variation in milk prolactin. In Experiment II, conducted during late fall and early winter, the minimum temperature preceding milking accounted for 21.1% of the variation. Although earlier stages of lactation and larger daily milk yields were associated with higher prolactin concentrations, inclusion of lactation stage in the stepwise regression model tended to eliminate almost all the variance of prolactinpreviously associated with daily yield. Dominance measurements indicated that more submissive cows had higher milk prolactin. Individual cows tended to have characteristic prolactin, but this tendency was eliminated by statistical adjustment of the data for environmental and lactational variables.

Basal levels of prolactin in goat blood measured throughout a 24-h period by a rapid double antibody-solid phase radioimmunoassay

The development of a rapid, highly sensitive double antibody–solid phase (DASP) radioimmunoassay for ovine/caprine prolactin is described. The assay was applied to the measurement of basal levels of prolactin in the blood of 4 anoestrous lactating female, 4 anoestrous virgin female and 4 castrated male goats. For several hours after milking there was a chronic gradually decreasing release of prolactin in the lactating goats, such that the circulating levels of prolactin were, for 14–16 h of the day, at a much higher level than those in the virgin female or castrated male goats. The basal levels of prolactin in the castrated male goats (87–200ng/ml) were somewhat higher than those in the virgin females (47–77ng/ml). The effects of feeding and stress on prolactin levels are discussed.

Metabolite levels and enzyme activities in the bovine mammary gland at different stages of lactation. I. Metabolite levels related to energy production

Samples of the mammary gland were obtained from heifers, lactating and dry cows by a freeze-clamping technique. Amounts of lactate, pyruvate, malate, ATP, ADP and AMP were determined in the sample extracts and calculated per g fresh weight and per mg DNA. Pyruvate, lactate and AMP amounts showed only minor changes at different stages of lactation, whereas quantities of malate, ATP and ADP were significantly higher in cows giving high milk yields. Pyruvate levels showed seasonal variations parallel to changes in milk composition. NAD: NADH ratios in mammary cytoplasm showed no significant differences between different groups of cattle, whereas ATP: AMP ratios in mammary gland samples were highest in lactating animals.

Variations in the concentrations of free amino acids in the plasma of the dairy cow at parturition

The plasma levels of individual amino acids were studied in 6 dairy cows from 4 days before to 3 days after calving. During this sampling period, the concentrations of 13 amino acids showed significant changes. The levels of several amino acids were depressed markedly in the sample collected immediately before calving. Following parturition, the concentration of most amino acids gradually returned to values obtained 3 days before calving. The glutamine and alanine contents of the plasma rose to a peak value 1 day after calving and subsequently decreased. The mean concentrations of glycine and α-aminobutyric acid did not change before parturition but rose significantly thereafter. These observations are discussed in terms of amino-acid utilization for milk protein synthesis and gluconeogenesis at the onset of lactation. The changes in plasma amino acid levels appear to be synchronized with those reported for prolactin and progesterone in the 24 h before parturition. This may indicate an important influence of both hormones on the lactogenic process in the cow. The highly significant correlations obtained between the concentrations of 14 individual amino acids are discussed.