Metabolic features of isolated rat lung cells. I. Factors controlling glucose utilization

Isolated rat lung cell suspensions were prepared by collagenase digestion of the lung stroma. These cells were functionally competent as judged, among other criteria, by their constant rates of oxygen uptake and glucose utilization. An important metabolic feature of these cells is that they display very high glycolytic rates. At least 60% of the glucose utilized was converted to lactate, regardless of the glucose concentration in the medium. The state of reduction of the nicotinamide system, as indicated by the lactate-to-pyruvate ratio, was normal, thus indicating that the high glycolytic fluxes are not related to poor oxygenation of the preparation. Utilization of glucose displayed Michaelis-Menten saturation type kinetics with a Vmax of 331 nmol/10(6) cells per h and an apparent Km of 2.4 mM. These values were not affected by the presence of ouabain (0.1 mM), mannoheptulose (5 mM), or insulin (1 mU/ml), whereas phloridzin produced a drastic inhibition of glucose utilzation showing an apparent Ki of 0.4 mM. The substitution of sodium by K+ or Li+ as the predominant cations in the incubation medium does not alter rates of glucose utilization. Optimal pH for glucose utilization was within the physiological range with a more pronounced inhibitory effect at alkaline pH's. The intracellular concentration glucose was found to be low. This finding, in conjunction with a Q10 (27-37 degrees C) for glucose utilization above 2.0 and the differential effects of D- and L-glucose on production, seems to indicate that a stereospecific glucose transport system exists in lung cells. Several findings point to glucose transport into the lung cells as a probable rate-limiting step for its metabolism:1) the activity of the glycolytic enzymes largely exceeded the observed rate of glucose utilization;2) the decrease in enzyme activity during starvation was not accompanied by a decreased glycolytic flux, suggesting that factors other than enzyme activity, perhaps the supply of fuel, are rate limiting in the overall process of glucose breakdown;3) fructose was able to increase lactate production in the presence of saturating concentrations of glucose. These additive effects of glucose and fructose seem to support the point of view that it is not the glycolytic machinery but the supply of fuel which is rate limiting for glucose utilization by isolated rat lung cells.