Reduction of lactational performance following bovine TNF-α administration to dairy cows

Twelve lactating Holstein cows were injected subcutaneously with recombinant bovine Tumor Necrosis Factor-α (rbTNF, 2.5 μg/kg/day) or saline (3 ml/head/day) at 12.00 h daily for 7 d (d 0 6). Milk yield was reduced by rbTNF injection from d 1 to 8. The milk fat content was increased on d 1-7 by rbTNF treatment, but milk protein content in the rbTNF group was decreased on d 5 and 7 as compared with that of the control group. The somatotropin (ST) response to ST-releasing hormone (0.25 μg/kg) was smaller in the rbTNF group than in the control. These results support the possibility that TNF is responsible for the changes in hormone secretion, milk production and composition observed during coliform mastitis.


INTRODUCTION
Coliform mastitis has adverse effects on the economics of milk production by reducing the quantity and quality of milk (Shuster et al., 1991).A recent study suggested that Tumor Necrosis Factor-α (TNF) plays a vital role in the pathophysiology associated with coliform mastitis and/or acute phase response (Hoeben et al., 2000).Bovine mammary macrophages secrete TNF in response to endotoxin (Pighetti and Sordillo, 1994), and the serum TNF level increases in coliform mastitis (Hoeben et al., 2000).Studies using endotoxin in lactating cows have been reported (Shuster et al., 1991), but the effects of exogenous TNF treatment on hormonal response, lactational performance and mastitic parameters have not yet been studied.Moreover, little information has been reported on somatotropin (ST), insulin-like growth factor-1 (IGF-1), cortisol, and thyroid hormones that are important regulators of mammary gland function.
The objective of the present study was to investigate some metabolic and hormonal changes and lactational performance during recombinant bovine TNF (rbTNF)-induced acute phase response in lactating cows in order to provide evidence that TNF is an in vivo mediator of mastitis-stimulated systemic changes.

MATERIAL AND METHODS
Twelve non-pregnant, lactating Holstein cows of 1st to 3rd parity between 10 and 14 wk after parturition were used in a crossover design in this study.They were milked twice daily at 06.00 and 16.00 h with a quarter milking machine.The diet consisted of, % dry matter basis: mixed concentrate 41.4, maize silage 22.0, lucerne hay cube 12.6, and orchardgrass hay 24.0.The cows were randomly divided into a rbTNF treatment group (n=6) and a saline treatment group (n=6) as control.The rbTNF (2.5 µg/kg/d) or saline (3 ml/head/d) was injected subcutaneously at 12.00 h daily for 7 consecutive days (d 0-6).On d 6 in the experiment period, we determined the effect of daily administration of rbTNF on ST-releasing hormone (STRH, 0.25 µg/kg) stimulated ST release.Milk fat and protein were measured with an infrared milk analyzer.The concentrations of plasma ST, cortisol, triiodothyronine (T 3 ), IGF-1, haptoglobin, and NEFA were measured.

RESULTS
Treatment with rbTNF decreased the milk yield (P<0.05) during the period between d 1 and 8 (Table 1).The milk fat percentage was elevated (P<0.05) at d 1 following rbTNF treatment and remained higher until d 7 in comparison with that in the control cows (Table 2).The protein content in milk was lower (P<0.05) in the rbTNF-treated cows than in the control cows on d 5 and 7 (Table 2).The plasma concentrations of cortisol, haptoglobin, and NEFA were increased (P < 0.05) by daily administration of rbTNF.The plasma T 3 and IGF-1 concentrations in the rbTNF-injected cows were lower (P<0.05)than those in the controls.The plasma ST concentration was lower (P<0.05) in the rbTNF group than in the control group from 15 to 120 min after STRH injection.

DISCUSSION
Daily treatment with rbTNF infl uenced lactational performance, and altered metabolites and hormone concentrations in lactating cows.In studies using the mastitis model to investigate the pathophysiological causes of reduced lactational performance during mastitis, milk yields in the stage of early lactation were depressed in quarters not receiving E. coli despite the absence of infl ammation in these quarters (Hoeben et al., 2000).The suppression of milk production was apparently mediated in part by systemic effects of intramammary E. coli infusion, and may be caused by changes in the concentrations of stimulatory or inhibitory hormones (e.g., ST, IGF-1, T 3 , and cortisol) (Hoeben et al., 2000).Data from the present study indicated that rbTNF induced systemic responses (e.g., acute phase response) associated with coliform mastitis in peak lactation, including a decrease in milk production, an increase of fat content and a decrease of protein content in milk.These changes in milk composition were similar to those observed following i.v.endotoxin treatment (Shuster et al., 1991).The reduction of plasma ST response to STRH in the rbTNF-treated cows is in agreement with the result obtained in our previous study (Kushibiki et al., 2000).It has been reported that TNF receptors are found in the bovine pituitary gland, and TNF inhibits STRHstimulated ST release in cultured bovine pituitary cells (Elsasser et al., 1991).

CONCLUSIONS
This study showed that exogenous rbTNF injection induced many of the systemic responses associated with coliform mastitis in the plasma and milk of lactating cows.These fi ndings support and confi rm the hypothesis that TNF, rather than endotoxin, is absorbed from the mammary gland and released into the circulation, and that TNF is responsible for the systemic changes observed during coliform mastitis.

Table 2 .
Percentages of fat and protein in milk of cows treated with saline (control, 3 ml/head) or rbTNF