Effects of progesterone on glucose metabolism in isolated acini from mammary glands of lactating rats

Isolated acini from lactating rat mammary gland were incubated with glucose (5 mm) and progesterone. The steroid (0.1 mm) decreased glucose utilization and pyruvate accumulation, but increased the formation of lactate. The production of ¹⁴CO₂ and ¹⁴C-labeled lipid from 1-¹⁴C]glucose, and the incorporation of ³H₂O into lipid were also inhibited by progesterone. At lower concentrations of progesterone (0.01–0.025 mm) the only effects were an increased lactate], a decreased pyruvate], and a consequent rise in the lactate/pyruvate ratio. Addition of dichloroacetate, an activator of pyruvate dehydrogenase, did not reverse these effects and assays of active pyruvate dehydrogenase showed no inactivation by progesterone. The steroid did not affect pyruvate utilization but markedly inhibited the removal of lactate, suggesting that progesterone causes a decreased reoxidation of cytosolic NADH and thus alters the cytosolic redox state. The findings are discussed in relation to the physiological role of progesterone during pregnancy and lactation.