Tissue-specific metabolism during normothermy and daily torpor in deer mice (Peromyscus maniculatus)

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Abstract
Previous work with deer mice (Peromyscus maniculatus) has demonstrated that a significant acidosis occurs during daily torpor. In addition, carbohydrate levels are significantly lower, whereas fatty acid and ketone levels are significantly higher during torpor. The present study examined the effects of these in vivo acid‐base and metabolite adjustments on in vitro 14C‐glucose metabolism in tissues taken from normothermic and torpid deer mice. Glucose oxidation in liver and vastus lateralis taken from normothermic animals was reduced by a change in incubation temperature from 37°C to 25°C (liver, 0.44 to 0.23 μmoles/g.h; vastus, 0.66 to 0.25 μmoles/g.h), whereas heart from normothermic mice exhibited an increase from 0.57 to 0.99 μmoles/g.h at the lower temperature. Altering acid‐base conditions or metabolite levels had no effect on glucose metabolism in the heart or liver. However, both of these factors significantly influenced metabolism in vastus. Vastus taken from normothermic mice had an increased glucose oxidation rate under the more acidic torpor conditions (0.25 to 0.33 μmoles/g.h), whereas a reduction in oxidation occurred when incubated with torpor substrate concentrations (0.33 to 0.22 μmoles/g.h). In vastus taken from torpid mice, changing acid‐base state and metabolites to torpor levels resulted in a significant reduction in glucose oxidation (0.26 to 0.12 μmoles/g.h). Under most incubation conditions, glucose oxidation was significantly lower in tissues taken from torpid mice than in these tissues from normothermic mice, suggesting that adjustments in addition to changes in acid‐base and substrate parameters may be necessary to account for metabolic modifications during daily torpor.