Studies on the control of lipogenesis: strain differences in hepatic metabolism

Detailed studies of hepatic metabolism of lipemic BHE and nonlipemic Wistar rats were conducted. Hepatic lipogenic capacity was varied through the use of starvation or meal feeding. Livers were clamped in precooled copper plates and used for the assay of glycolytic, gluconeogenic, and lipogenic metabolites. Redox and phosphorylation states were calculated. Mitochondrial metabolism was evaluated through studies of the oxygen consumption of isolated mitochondria and through the study of the activities of the alpha-glycerophosphate and malate aspartate shuttles and ATPase. BHE rats have higher phosphorylation states, higher redox ratios, and lower shuttle activities and oxygen consumption by isolated mitochondria than their Wistar cohorts. The differences in oxidative phosphorylation, redox and phosphorylation states, and in the various shuttle activities suggest that BHE liver cells are geared towards lipogenesis at the expense of oxidative phosphorylation. It appears that the activity of the shuttles is controlled in part by phosphorylation state which in turn appears to affect respiration. We theorize from these data that genetically determined differences in the structure and function of the mitochondrial membrane (and perhaps the cell membrane as well) may affect the communication (via metabolites and adenine nucleotides) between the cytosol and mitochondria. Subtle differences in the exchange of metabolites and/or adenine nucleotides across the mitochondrial membrane could thus explain the lipogenic tendency of the liver of the BHE rat and the subsequent development of maturity onset hyperlipemia and hyperglycemia in this strain of rat.