The estimation of rates of utilization of glucose and ketone bodies in the brain of the suckling rat using compartmental analysis of isotopic data

Accelerative exchange diffusion kinetics of glucose between blood and brain and its relation to transport during anoxia

An earlier study showed that unidirectional glucose transport from blood to brain decreases during perfusion with anoxic blood (Betz, A. L., Gilboe, D. D. and Drewes, L. R. (1974) Brain Res. 67, 307-316). Brain glucose levels also decrease during anoxia. Therefore, the present study was designed to investigate whether the decreased transport might be the result of decreased accelerative exchange diffusion when brain glucose levels are low. The rate of undirectional transport into brain (v) of D-[6-3H]glucose was studied in 22 isolated, perfused dog brains by means of an indicator dilution technique using 22Na as the intravascular reference. The kinetics of transport were determined over a range of blood glucose concentrations (S1) at each of five different brain glucose levels (S2). The existence of accelerative exchange diffusion for glucose was indicated by a decrease in the intercept (increase of apparent V) of a double reciprocal plot (1/v versus 1/S1) as S2 increased. This phenomenon is consistent with a model for facilitated diffusion in which the mobility of the loaded carrier is greater than that of the unloaded carrier. Although the data predict a decrease in glucose transport during anoxia, the predicted decrease (5%) is less than the observed decrease (35%). It is concluded that the simple mobile-carrier model for facilitated diffusion cannot, by itself, describe all properties of blood-brain glucose transport.

Conversion of [U-14C]threonine into 14C-labelled amino acids in the brain of thiamin-deficient rats

In confirmation of the findings of Gaitonde et al. (1974), a decrease in the brain concentration of threonine and serine, and an increase in glycine, were observed in rats maintained on a thiamin-deficient diet. Similar changes were found in the blood, and the concentration of several other amino acids in the blood decreased significantly. There was a correlation between the concentrations of threonine, serine, aspartate and asparagine in the brain and blood. In experiments in which [U-14C]threonine was injected into rats most of the radioactivity in the brain and blood of control rats was, as expected, in threonine in the acid soluble metabolites. In contrast, a considerable proportion of radioactivity was also found in other amino acids, namely glutamate, glutamine, aspartate, gamma-aminobutyrate and alanine, in the brain of thiamin-deficient rats. [U-14C]Threonine was also converted into 14C-labelled lactate and glucose, but the extent of this conversion was severalfold higher in thiamin-deficient than in control rats. This finding gave evidence of the stimulation in thiamin-deficient rats of the catabolism of [U-14C]threonine to [14C]lactate by the aminoacetone pathway catalysed by threonine dehydrogenase, and into succinate via propionate by the alpha-oxobutyrate pathway catalysed by threonine dehydratase (deaminase). The measurement of specific radioactivities of glutamate, aspartate and glutamine after injection of [U-14C]threonine, indicated a stimulation of the activities of threonine dehydrogenase and threonine dehydratase (deaminase) in the brain of thiamin-deficient rats. The specific radioactivities of glutamate, asparatate and glutamine int he brain were consistent with an alteration in the metabolism of threonine, mainly in the 'large' compartment of the brain of thiamin-deficient rats. The measurement of relative specific radioactivity of proteins after injection of [U-14C]threonine indicated a marked decrease in the synthesis of proteins, mainly in the liver of thiamin-deficient rats.