The effects of vincristine on milk secretion and milk composition in the goat.

The effects of intramammary injection of vincristine on milk secretion were studied in goats. Vincristine inhibited milk secretion in a dose-dependent manner, with 1 mg decreasing milk yield to approximately 40% of previous yield. The inhibition was reversible with a full recovery of milk yield within a few days. During inhibition, there were significant changes in milk composition; Na concentration increased and K and lactose concentrations decreased, while Cl concentration was unaffected. Fat, protein, citrate and Ca concentrations all increased, particularly as milk yield was recovering. The concentrations in milk of the two lactose precursors, glucose and UDP-galactose, both increased. During inhibition and recovery, the secretion rates of three major components of milk, lactose, protein and fat, were affected to a different degree, with lactose secretion being inhibited to the greatest extent. The results are discussed in relation to the action of vincristine on milk secretion. It is proposed that vincristine's actions include a specific inhibition of lactose synthesis.