Milk synthesis and secretion rates in cows with milk composition changed by oxytocin

Supraphysiological doses of oxytocin were used to investigate the association among milk composition, secretion rates, and cell electrolyte concentrations. Groups of 8 Holstein cows were injected i.v. with 1, 10, 100, or 1000 IU oxytocin after a normal milking. Milk yield over the following 12-h period declined 5.5, 8.1, 17.0, and 43.7% respectively. Milk Na increased in proportion to oxytocin dose. Yield and concentration of lactose and protein declined with increasing oxytocin doses, but milk fat yield remained unchanged. To determine whether the decreased milk lactose secretion was caused by a decline in synthesis or leakage into blood plasma and subsequent renal clearance, milk and urine were collected for 24-h periods before and after 100 IU oxytocin. Urinary lactose accounted for 2.7 +/- 3.19 and 12.9 +/- 8.9% of the total collected on the days before and after oxytocin. This increase (46.7 g/d) was only one-fifth of the 25% decline in total lactose recovery, suggesting that reduced mammary lactose synthesis is primarily responsible for lower milk yield. Measurement of Na and K in biopsied mammary tissue suggests that a high Na:K ratio in milk will increase that ratio in secretory cells, which may be part of the mechanism for lower milk yield under circumstances that increase permeability of mammary tight junctions.