In vivo rates of glucose utilization were estimated in red muscle and heart of rainbow trout using the radiolabeled glucose analogue, [14C]-2-deoxyglucose ([14C]-2-DG). The 2-DG ‘lumped constant’ (LC), representing the uptake ratio between 2-DG and glucose for each tissue, was determined in vitro using tissue slices. For both red muscle and heart, the LC was approximately 0.40. In addition, changes in circulatory glucose utilization induced in an isolated trout heart preparation had no effect on the LC. The LC was applied to calculations of in vivo rates of circulatory glucose disposal in muscle tissue of resting and swimming trout. Utilization was estimated at 0.87+/−0.15 and 5.31+/−1.04 nmol g-1 min-1 in red muscle and heart of resting fish, respectively. In trout swimming at 80 % Ucrit, glucose utilization in heart was unchanged compared to resting controls, while red muscle utilization increased by 28-fold. Estimates of the oxidative demand of heart and locomotory muscle of swimming trout indicate that glucose utilization accounted for less than 10 % of the energy production in both tissues. In trout heart, in vitro measures of glucose flux suggest an apparent excess capacity to use circulatory glucose. Preference for other intra- and extramuscular fuels may partially explain limited glucose utilization, but it remains unclear to what degree, and under what conditions, glucose utilization may be increased in vivo.

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