Effects of intramammary endotoxin infusion on milking-induced oxytocin release.

One overt sign of clinical coliform mastitis in dairy cows is the failure to eject milk normally or to "milk out" the udder. The effect, if any, of coliform mastitis on oxytocin release is unknown. Therefore, the objective of this study was to determine the effect of endotoxin mastitis on milking-induced release of oxytocin in lactating cows. Fifteen multiparous pregnant lactating Holstein cows were divided into three groups of 5 cows each. Cows in group 1 served as controls and received an intramammary infusion of sterile physiological saline. Cows in groups 2 and 3 received intramammary infusions of 12.5 and 25 micrograms of Escherichia coli endotoxin, respectively. Serum concentrations of oxytocin were measured by radioimmunoassay before, during, and after milkings commencing at 6 and 12 h after treatment. Rectal temperatures and milk SCC were monitored to follow the course of inflammation and to verify the biological activity of infused endotoxin. Endotoxin resulted in a 1.5- to 2-fold increase in milking-induced oxytocin release compared with that of control treatments. The effect was most prominent during the first 6 h after infusion and coincided with the peak pyretic response. This study shows that endotoxin-induced mastitis potentiates, rather than inhibits, milking-induced oxytocin release.