We investigated the relationship between perfusate concentration of glucose and its utilization and lactate production derived from exogenous glucose and from metabolism of endogenous substrates. Isolated rat lungs were ventilated with 5% CO2 in air and perfused for 100 min with Krebs-Ringer bicarbonate buffer containing 3% bovine serum albumin, 10(-2) U/ml insulin, [U-14C]glucose and [5-3H]glucose. Glucose utilization, total lactate production, [14C]lactate production, and 3H2O production were measured. The apparent Km and Vmax for glucose utilization were 3.4 mM and 72.5 mumol/g dry wt per h, respectively. Lactate production from endogenous substrates, calculated as the difference between total and [14C]lactate, was 37.6 +/- 2.2 mumol/g dry wt (n = 36); it was unaffected by perfusate glucose concentration and by omission of insulin, but increased threefold with anoxia. Lactate production from 1.5 mM glucose was significantly less (P less than 0.02) with insulin omitted. Glycogen content was unchanged during perfusion without glucose. These results suggest that: 1) protein catabolism contributes to lung lactate production; 2) glucose utilization by lung is not maximal at resting physiological glucose concentrations; and 3) insulin is required at low glucose concentrations for maximal glycolytic rates.
Uptake of 2-deoxy-D-glucose (DG) was investigated with rat granular pneumocytes isolated in primary culture. Cells attached to flasks were incubated in Minimal Essential Medium usually containing 5 mM DG in place of glucose. Uptake of DG increased progressively with time of incubation and approached a plateau value of 35-40 mumol/10(6) cells at 60 min. Uptake increased as a function of external DG concentration with half-maximal uptake at approximately 2.0 mM DG. DG uptake was inhibited by the presence of glucose, alpha-methylglucoside, phlorizin, ouabain, or sodium-free medium. After 60 min incubation, approximately 20% of total intracellular DG was in the free form, and the calculated mean intracellular concentration of free DG (n = 4) was approximately twice the external concentration. Phosphatase activity was indicated by increase in free DG and efflux from cells after removal of external DG. In comparison with pneumocytes, uptake of DG by alveolar macrophages showed different kinetics, and intracellular free DG did not exceed the extracellular concentration. These findings indicate that type II cells take up DG by a sodium-dependent, carrier-mediated transport process that results in accumulation of free sugar against a concentration gradient.
Uptake of nonmetabolizable glucose analogues was investigated in isolated rat lungs ventilated with 5% CO2 in air and perfused with Krebs-Ringer bicarbonate medium. In some experiments, [5-3H]glucose, methyl(alpha-D-[U-14C]gluco)pyranoside (alpha-MG), 3-O-methyl-D-[U-14C]glucose (3-O-MG), or various inhibitors were added to the initial perfusate to determine glucose utilization by the rate of 3H2O production or to characterize the uptake of glucose analogues. [1,2-3H]polyethylene glycol (PEG) was used as an indicator of the extracellular water space and gave a mean value of 0.35 ml/g tissue; calculated mean intracellular H2O space was 0.48 ml/g tissue. Glucose utilization was 56.4 +/- 6.6 (mean +/- SE, n = 6) mumol . g dry wt-1 . h-1 and decreased by 61% with 3 mM phlorizin. After 1-2 h perfusion, intracellular alpha-MG concentration was 1.4-1.9 times the extracellular concentration. The mean tissue-to-medium ratio (T/M) for alpha-MG decreased by more than 30% in the presence of glucose (5.0 mM), phlorizin (0.5 mM), ouabain (0.5 mM), or the absence of external Na+. Intracellular 3-O-MG concentration did not exceed extracellular concentration during 2 h of perfusion and the mean T/M did not change with any of the inhibitors studied. The results indicate that the nonmetabolizable glucose analogue alpha-MG is accumulated against a concentration gradient by an active Na+-dependent transport process, whereas 3-O-MG is apparently taken up by a different mechanism.
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