PYRUVATE-DEHYDROGENASE ACTIVITIES AND RATES OF LIPOGENESIS DURING THE FED-TO-STARVED TRANSITION IN LIVER AND BROWN ADIPOSE-TISSUE OF THE RAT

The percentages of pyruvate dehydrogenase complex (PDH) in the active form (PDH(a)) in two lipogenic tissues (liver and brown adipose tissue) in the fed state were 12.0% and 13.4% respectively. After acute (0.5 h) insulin treatment, PDH(a) activities had increased by 77% in liver and by 234% in brown fat. Significant decreases in PDH(a) activities were observed in both tissues by 5 h after the removal of food. The patterns of decline in PDH(a) activities in the two lipogenic tissues were similar in that the major decreases in activities were observed within the first 7 h of starvation. The significant decreases in PDH(a) activities observed after starvation for 6 h were accompanied by decreased rates of lipogenesis. Hepatic and brown-fat PDH(a) activities after acute (30 min) exposure to exogenous insulin were less in 6 h-starved than in fed rats, but the absolute increases in PDH(a) activities over the 30 min exposure period were similar in fed and 6 h-starved rats. Increases in PDH(a) activities were paralleled by increases in lipid synthesis in both tissues. Re-activation of PDH in response to insulin treatment or chow re-feeding after 48 h starvation occurred more rapidly in brown adipose than in liver. The results are discussed in relation to the importance of the activity of the PDH complex as a determinant of the total rate of lipogenesis during the fed-to-starved transition and after insulin challenge or re-feeding.