Regulation of fatty acid synthesis in lactating rat mammary gland in the fed to starved transition: asynchronous control of pyruvate dehydrogenase, phosphofructokinase and acetyl-CoA carboxylase.
1. Withdrawal of food from lactating rats produced a rapid and dramatic decrease in the uptake of glucose by the mammary gland and an inhibition of the rate of fatty acid synthesis that could not be explained alone by decreased substrate supply to the tissue. 2. Within the first 6 hr starvation, fatty acid synthesis and pyruvate dehydrogenase activity were inhibited by 87 and 80%, respectively, but acetyl-CoA carboxylase activity did not change significantly. 3. Between 6 and 24 hr starvation, total and expressed activities of acetyl-CoA carboxylase decreased by 62 and 55%, respectively. 4. The ratio of fructose-6-phosphate/fructose-1,6-bisphosphate concentration in mammary tissue increased 9-fold during the first 6 hr starvation, indicating an inhibition of 6-phosphofructo-1-kinase. However, the major inhibition of this enzyme occurred between 6 and 24 hr starvation when this metabolite ratio increased a further 160-fold in parallel with increased tissue citrate concentration. 5. The increase in citrate concentration between 6 and 24 hr starvation correlated with acetyl-CoA carboxylase inactivation and ketone body accumulation in the mammary gland. 6. This study confirms the asynchronous control of three important regulatory steps in the pathway of glucose utilization and fatty acid synthesis in the lactating rat mammary gland.