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Glucose utilization in fish muscle during sustained swimming and recovery from maximal swimming West, Timothy Gordon


The importance of circulatory glucose as a fuel for skeletal muscle and heart during sustained swimming and as a glycogenic substrate for skeletal muscle during exercise-recovery was examined. In vivo glucose utilization, using ¹⁴C-2 deox4yglucose (¹⁴C-2-DG), in swimming trout (Oncorhynchus mykiss) and carp (Cyprinus carpio) indicated similar rates of glucose utilization in red muscle of both species (20 - 30 nmol min ⁻¹g ⁻¹. Estimates of the energetic importance of glucose, assuming red muscle accounted for the bulk of the active oxygen consumption, suggest that glucose could be more important to the slower swimming carp - potentially accounting for 25 - 30 % of total energy provision. Glucose uptake in red muscle of the more active trout was estimated to contribute < 10 % of total substrate demands during exercise. Cardiac energetics was estimated to be largely independent of glucose utilization in trout. In vivo utilization of glucose in trout heart at rest (about 5 nmol min ⁻¹g ⁻¹) was considerably lower than in vitro determinations (120 nmol min ⁻¹g ⁻¹) and measurements of glucose utilization capacity (8300 nmol min ⁻¹g ⁻¹, in based on hexokinase activities). Nearly complete inhibition of glucose flux in vivo is likely the result of preference for alternative substrates for oxidation. In carp, cardiac glucose utilization was dependent on plasma glucose concentration, but was also estimated to be of minor energetic importance at plasma concentrations comparable to those observed in swimming trout (2 - 4 mmol L⁻¹). Reduced dependence of trout on glucose for swimming energetics agrees with the generalization that species with superior aerobic exercise capacities may be dependent on fat based fuels for muscle energetics. Plasma glucose concentration varied from 6 - 38 mmol L⁻¹ in trout during recovery from intense burst swimming. Hyperglycemia was not related specifically to exercise. Estimated glucose turnover rate, using 6-³H-glucose tracer, increased proportionally with plasma glucose availability. Similar concentration-dependent kinetics were observed in carp at rest and during sustained swimming. Regardless of the association between glucose turnover and plasma glucose level, glucose utilization (¹⁴C-2-DG phosphorylation) in trout white muscle accounted for < 10 % of the glycogen repletion after exercise. In trout red muscle, glucose utilization was dependent on plasma glucose concentration up to 10 - 12 mmol L⁻¹ and glycogenesis was estimated to account for 25 - 60 % of post-exercise glycogen repletion in this muscle type. Moderate post-exercise hyperglycemia sometimes seen in recovering salmonids seems pertinent to red muscle carbohydrate status, but white muscle recovery occurs independently of plasma glucose availability. These results provide evidence for the predominance of a glyconeogenic fate of white muscle lactate in trout. Investigation of lactate/glycogen recovery profiles in the fast swimming skipjack tuna (Katsuwonus pelamis) provides evidence that, despite the relatively high oxidative capacity of tuna white muscle and previous determinations of high circulatory fluxes of lactate, this species also seems to convert white muscle lactate to glycogen via intramuscular glyconeogenesis. A glyconeogenic fate for lactate conserves carbohydrate status and ensures relatively rapid replenishment of the principal carbon source in fish white muscle.

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