INVIVO UTILIZATION OF GLUCOSE BY HEART AND LOCOMOTORY MUSCLES OF EXERCISING RAINBOW-TROUT (ONCORHYNCHUS-MYKISS)

In vivo rates of glucose utilization were estimated in red muscle and heart of rainbow trout using the radiolabeled glucose analogue, [C-14] -2-deoxyglucose ([C-14]-2-DG). The 2-DG 'lumped constant' (LC), repres enting 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 g lucose utilization induced in an isolated trout heart preparation had no effect on the LC. The LC was applied to calculations of in vivo rat es of circulatory glucose disposal in muscle tissue of resting and swi mming 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 % U(crit), glucose utilization in heart was un changed compared to resting controls, while red muscle utilization inc reased by 28-fold. Estimates of the oxidative demand of heart and loco motory muscle of swimming trout indicate that glucose utilization acco unted for less than 10 % of the energy production in both tissues. In trout heart, in vitro measures of glucose flux suggest an apparent exc ess capacity to use circulatory glucose. Preference for other intra- a nd extramuscular fuels may partially explain limited glucose utilizati on, but it remains unclear to what degree, and under what conditions, glucose utilization may be increased in vivo.