Skeletal muscle substrate utilization is altered by acute and acclimatory temperature in the American bullfrog (Lithobates catesbeiana)

Abstract

SUMMARYWe investigated the effect of acute and acclimatory temperature on the relative contribution of glucose and lactate to metabolism in resting sartorius muscle of the American bullfrog (Lithobates catesbeiana). We examined the fate of these metabolites in vitro by supplying radiolabeled [14C]glucose, [14C]lactate and[14C]palmitate to isolated muscle bundles from frogs (1) acutely exposed to incubation conditions of 5, 15 or 25°C, (2) acclimated for 2–6 weeks to 5 or 25°C or (3) acclimated for 2–6 weeks to 5 or 25°C and the muscles incubated at 15°C. Under all three temperature conditions tested, net rate of lactate metabolism exceeded that of glucose. Acute exposure to 5°C reduced net rate of glucose metabolism by 15×and net lactate metabolism by 10× as compared with 25°C-exposed tissues. Acclimation to 5°C favored glucose storage as glycogen and increased the proportion of lactate oxidized (versus stored or converted to glucose) when compared with 25°C-acclimated tissues. Net rates of storage of lactate as glycogen (glyconeogenesis) were significantly higher in muscles from 5°C-acclimated frogs during incubation at a common temperature of 15°C. These data suggest that lactate is the predominant fuel for resting skeletal muscle over this temperature range, and particularly so under cold conditions. Ready use of lactate as a substrate, and enhancement of glyconeogenic pathways in response to cold acclimation, could play a role in the tolerance of this species to seasonal temperature changes by promoting sequestration and storage of available substrate under cold conditions.