ASSESSMENT OF MAMMARY-GLAND METABOLISM IN THE SOW .3. CELLULAR METABOLITES IN THE MAMMARY SECRETION AND PLASMA FOLLOWING WEANING

The concentrations of lactose, glucose, glucose 6-phosphate, glucose 1 -phosphate, UDPglucose, UDPgalactose, UDP, UMP, inorganic phosphate, A DP and AMP (metabolites involved in the lactose synthesis pathway), an d cAMP, galactose and sodium were measured in the mammary secretion fr om four or five mammary glands on each of six sows during the first 5 d post weaning. The concentrations of lactase, glucose and galactose w ere also measured in plasma during this time. Following weaning, the r apid increase in the concentrations of glucose 6-phosphate and UDPgala ctose suggested that the rate of lactose synthesis was regulated by th e inhibition of hexokinase and/or lactase synthase, while the decrease in glucose and AMP indicated a subsequent decline in glucose and ATP utilization. The rapid increase in glucose 6-phosphate which plays a p ivotal role as a substrate for both lactose and de novo fatty acid syn thesis, and the rapid decrease in AMP which reflects ATP utilization, were good markers of decreased metabolic activity. These rapid changes in the metabolic activity of the mammary glands were not observed in a second weaning study when two piglets were removed from selected mam mary glands for periods up to 5 h during established lactation. Since concentrations of lactogenic hormones remain elevated following partia l weaning, but fall following total weaning (Rojkittikhun et al. 1991) , these differences in mammary gland metabolism indicate that endocrin e rather than autocrine mechanisms are controlling lactase and fat syn thesis during the initial stages of total weaning.