Effects of long-term administration of recombinant bovine somatotropin on the concentration of metabolites in milk in different stages of lactation in crossbred Holstein cattle

Effects of long-term treatment with recombinant bovine somatotropin (rbST) on concentrations of cellular metabolites in the milk of 87.5% crossbred Holstein cattle were performed. The peak milk yield of rbST-treated animals was 22% higher (P < 0.05) than that of the control animals in early lactation. The mammary glucose uptake of rbST-treated animals increased in early lactation, but decreased in mid and late lactation, while plasma glucose concentrations were not affected. Lactose and milk triacylglycerol secretion of rbST-treated animals significantly increased (P < 0.05) when compared with those of control animals in early lactation. The concentrations of milk glucose of rbST-treated animals significantly increased in early and mid-lactation (P < 0.05). The concentrations of milk galactose markedly increased (P < 0.05) whereas the concentrations of milk uridine 5'-diphosphoglucose (UDP-glucose) and UDP-galactose showed no significant changes as lactation advances in both groups. The concentrations of isocitrate, 2-oxoglutarate and citrate in milk from both groups showed no significant changes throughout experiment. The concentration of glucose-6-phosphate (G6P), glucose-1-phosphate and cyclic adenosine 3,5'monophosphate in milk from both groups markedly decreased as lactation advances exception in early lactation of rbST-treated animals, which G6P was not affected. These findings suggest that prolonged rbST treatment exerts its galactopoietic action at least in early lactation through both intramammary and extra-mammary changes. Increases in the concentrations of glucose and G6P in milk maintained the level of pretreatment in early lactation associated with increases in milk yields during rbST administration, reflect their concentrations in the cytosol or Golgi vesicles of mammary cells, which would be one of the factors regulating intermediary metabolites in the lactose biosynthetic pathway.