LIPID-METABOLISM IN ADIPOSE-TISSUE DURING LACTATION - A MODEL OF A METABOLIC CONTROL-SYSTEM

The flux of energy-yielding compounds through the pathways of lipogene sis, esterification into triglycerides and lipolysis in adipose tissue plays a pivotal role in supplying the demands of lactation and matern al health. The critical importance of these pathways is demonstrated b y the number of highly coordinated and redundant metabolic control ele ments that regulate the enzyme activity in these pathways, including p rotein and several steroid hormones, catecholamines, and blood concent rations of several nutrients. Control on these pathways is exerted by all of these elements during lactation. Insights have been gained rece ntly into the adaptations of these pathway reactions due to genetic pr opensity for milk production, stage of Lactation, and intake of energy -yielding components such as starch, cellulose and triglycerides. The rates of these pathways vary exponentially with the intakes of key sub strates and demands for milk precursors. The parameters of equations d escribing these pathways are not constant, but vary with genotype and with prolonged changes in nutritional and environmental conditions. Tw o major regulatory systems are critical to alterations of carbon flux during the entire lactational period. One is the interaction of growth hormone and insulin to control lipogenesis; the other is the counter- regulation by norepinephrine and insulin on cyclic AMP-initiated enzym e phosphorylation to regulate lipolysis. Examples of specific control points having a critical impact on lactational success and that are as sociated with genetic selection for milk production are the activities of acetyl-CoA carboxylase and hormone sensitive lipase. Further insig hts into the mechanisms of these adaptations will help us to improve t he efficiency of metabolic flux during lactation. However, the complex ity of the chemical interconversions of nutrients and of their regulat ion during lactation requires a coordinated effort to study both physi ological control mechanisms and to improve our quantitative understand ing of key parameters of lipid metabolism as influenced by a variety o f genetic and environmental situations.