Metabolite Changes Associated with Initiation and Maintenance of Lactation in Rats and Cows

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Abstract
Metabolite levels in rat and cow mam- mary glands prepartum and at different stages of lactation were investigated, with a view toward determining whether changes in availabilities of energy sources and milk precursors to the mammary glands and changes in mammary metabolic regulatory systems are involved in effecting the ini- tiation of lactation and milk yield in these species. Levels of energy substrates and milk precursors in the mammary glands of both species appeared adequate pre- partum to support lactation. These ob- servations were considered consistent with previous suggestions that the changes in cellularity and enzyme activities that occur after parturition in the rat might be re- sponsible for initiation of lactation. Since no significant prepartum and postpartum differences in enzyme activities or avail- abilities of metabolites to the mammary gland have been observed in cows, the mechanisms involved in regulating the initiation of lactation in this species re- main unresolved. The metabolite data presented suggest that the hexokinase and P-fructokinase reactions are rate-limiting in rat and cow mammary glands and rep- resent potential regulatory sites in energy metabolism, as has been suggested for other tissues.