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We have previously constructed lines of heteroplasmic mice from two inbred strains (NZB/BinJ and BALB/c) to investigate the mechanisms of segregation of mtDNA sequence variants. Analysis of the segregation behaviour of mtDNA in several tissues showed that the NZB genotype was invariably selected in liver/kidney and the BALB genotype in blood/spleen. Segregation was not significant in post-mitotic tissues. Here we have investigated this novel pattern of mtDNA segregation in isolated hepatocytes to determine the mechanism of selection. Polarographic measurements of respiratory chain function showed no difference between mitochondria containing either 0 or 91-97% NZB mtDNAs on a BALB nuclear background. Single-cell PCR analysis of mtDNA in isolated hepatocytes demonstrated that most hepatocytes eventually fix the NZB genotype. The rate of selection was constant with time and independent of the initial genotype frequency. Based on a mtDNA replication rate of 9.4 days, NZB mtDNA has an approximately 14% selective advantage over BALB mtDNA; however, in vivo pulse labelling with BrdU demonstrated that this was not based on efficiency of replication. Surprisingly, when hepatocytes were cultured in vitro, the majority of independent colonies selected BALB mtDNA, even if they were nearly fixed for the NZB mtDNA genotype when initially plated. These data suggest that selection for NZB mtDNA in the liver of these mice is not based on respiratory chain function at the cellular or organellar level, or a simple replicative advantage, but on a factor(s) involved with mtDNA maintenance.

During the denning period, black bears (Ursus americanus) are capable of enduring several months without food. At the same time, female bears that are pregnant or lactating have an added metabolic stress. Based on laboratory studies, much of the energy required to support metabolism and lactation during denning in black bears comes from lipid reserves. These lipid reserves are mobilized and the most metabolically active lipid fraction in the blood are nonesterified fatty acids (NEFA). Therefore, we hypothesized that plasma NEFAs would be higher in denning relative to active bears and in lactating relative to non-lactating female bears. We further hypothesized that in bears with elevated plasma NEFA levels, other lipid-related parameters (e.g., ketone bodies, albumin, cholesterol, lipase) would also be elevated in the plasma. Denning bears had significantly increased NEFA levels in all classes (saturates, monoenes, and polyenes). A doubling of plasma NEFA levels and a 33% increase in albumin, the plasma fatty acid binding protein, in denning bears, resulted in NEFA/albumin ratios that were higher in denning bears (4:1) compared to those of active bears (3:1). Bears became relatively ketonemic with a 17-fold increase in D-beta-hydroxybutyrate levels during the denning period. Plasma cholesterol approximately doubled and lipase was ten-fold lower in denning relative to active bears. These findings indicate a strong correlation between plasma lipid metabolites and the denning period in a wild population of black bears.